Use of Artificial Tree Hollows by Australian Birds and Bats

Use of articial tree hollows by Australian birds and bats

Ross L. GoldingayA,B and Jane R. StevensA

ASchool of Environmental Science and Management, Southern Cross University,PO Box 157, Lismore, NSW 2480, Australia.

BCorresponding author. Email: [email protected]

Abstract. Articial tree hollows (nest or roost boxes) may be of considerable importance to the conservation andmanagement of Australian hollow-using birds and microbats. This is suggested by recognition that the rate of collapse ofhollow-bearing trees may exceed replacement in some landscapes. We review the published literature to synthesise currentinformation on the use of articial hollows by Australian birds and bats, and to provide guidance to future research andmanagement. The use of articial hollows has been documented in some detail for 15 native bird and eight microbat species.A range of hollow designs has been employed but there is a limited understanding of favoured designs. General designs(e.g. front-entry plywood boxes) have been used extensively by some species and should continue to be used until moreeffective designs are identied. Species tend to use articial hollows that have entrance sizes just sufcient for their body size,and this should guide hollowdesign.Competitive interactionswith a range of non-target species (native and non-native)mayhave a pronounced inuence on articial hollow use andmust be considered in anymanagement program involving articialhollows.Wehighlight somedesign elements thatmay reduce interference bynon-target species. Temperature inside articialhollowsmay have a particular inuence on their use by bats due to the role ofmicroclimate in bat thermoregulation. Trials areneeded to investigate this factor and to inform general approaches to positioning of articial hollows. Several distinctmanagement uses exist for articial hollows, including assisting threatened species recovery, e.g. Kangaroo Island glossyblack-cockatoo (Calyptorhynchus lathami halmaturinus) and orange-bellied parrot (Neophema chrysogaster). Articialhollowsoffer an interim solution to hollow shortage but their full potentialwill only be realisedwhenpreferences for differentdesigns are better understood. This will require a commitment to monitoring and should be conducted in an adaptivemanagement context. Increased knowledge of the use of articial hollows by Australian birds and bats should be of globalrelevance to the management of hollow-using species because it provides an independent test of ideas and strengthens anygeneralisations.

Introduction

Species of wildlife throughout the world use tree hollows forprotectionwhen inactive and as breeding sites (e.g. Saunders et al.1982; Mackowski 1984; Newton 1994; Nowak 1999; Kunz andLumsden 2003; Zinner et al. 2003). In Australia, as many as 300native vertebrate species (birds, bats, arborealmarsupials, reptilesand frogs) use tree hollows (Gibbons and Lindenmayer 2002).This corresponds to a far greater proportion of wildlife speciesdependent on tree hollows for survival compared to that on othercontinents (see Saunders et al. 1982; Newton 1994; Gibbons andLindenmayer 2002). This prevalence of hollow dependenceamong Australian wildlife should result in land managers andresearchers leading the way internationally in understanding thehollow requirements of species and devising effectivemanagement programs for them. At present, this does notappear to be the case with widespread concern that theabundance of tree hollows has declined within manylandscapes in Australia (Joseph et al. 1991; Bennett et al.1994; Mawson and Long 1994; Lindenmayer et al. 1997; Pelland Tidemann 1997a; Gibbons et al. 2002; Heinsohn et al. 2003;Eyre 2005; Harper et al. 2005a; Courtney and Debus 2006).As a consequence,manyhollow-dependent species are now listed

as threatened (Gibbons and Lindenmayer 2002). Managementprograms in response to this appear to be in their infancy.

Articial hollows (nest and roost boxes) can potentiallysimulate the tree hollow environment for many hollow-usingspecies (Menkhorst 1984; Boyd and Stebbings 1989; Petty et al.1994; Smith and Agnew 2002; Harley 2004; Beyer andGoldingay 2006; Goldingay et al. 2007) and this is wellrecognised in Europe and North America (Stebbings andWalsh 1991; Newton 1994). Indeed, understanding factors thatmay inuence the use of articial hollows is an active area ofresearch in Europe and North America for birds (e.g. Pogue andSchnell 1994; Ardia et al. 2006) and bats (e.g. Kerth et al. 2001;Flaquer et al. 2006). Articial hollows have long been recognisedas an important research and management tool for Australianarboreal marsupials (Menkhorst 1984; Beyer and Goldingay2006). In contrast, the use of articial hollows by Australianbirds is poorly documented in the published literature despite thedeployment of articial hollows as a management tool for over adecade (e.g. Olsen 1996; Mooney and Pedler 2005). There isincreasing evidence that roost boxes will be used by Australianmicrobats (Golding 1979; Irvine and Bender 1995; Ward 2000;Smith and Agnew 2002) but the extent to which this can result in

CSIRO PUBLISHINGwww.publish.csiro.au/journals/wr Wildlife Research, 2009, 36, 8197

CSIRO 2009 10.1071/WR08064 1035-3712/09/020081

valuable research and management applications is not wellunderstood.

This paper reviews published information on the use ofarticial hollows by Australian birds and bats. The largenumber of hollow-using species in Australia means that anyincrease in knowledge and understanding can benet themanagement of a signicant component of biodiversity.Specically we aim to: (i) synthesise available publishedinformation on the use of articial hollows; (ii) examinedifferent applications of articial hollows; and (iii) identifygaps in our knowledge so as to stimulate future research. Thepresent review complements one by Beyer andGoldingay (2006)concerning the use of articial hollows by Australian arborealmarsupials. We make extensive reference to studies in thenorthern hemisphere because there is a substantial literaturefor that area that can greatly inform our understanding of therequirements and behaviour of Australian species. Furthermore,an increased understanding of Australian species can provideindependent tests of various hypotheses about the use of articialhollows, which will allow greater generalisation for anyhollow-using birds and bats.

Use of articial hollows by Australian birds and microbats

To review the literature on the use of articial hollows byAustralian birds and bats, we conducted searches of the Webof Science as well as the Australian journals Australian Journalof Zoology, Emu and Wildlife Research, using webpage searchtools and keywords relating to articial hollows. The journalAustralian Mammalogy was searched manually. Theses wereonly included when commonly referred to by published studies.Some studies and most books on this topic have documentedspecies observed using articial hollows but have provided fewdetails of such use. We restrict our attention to studies thatdescribe more than one observation of nest and roost box useby birds or bats, and inwhich some details of the articial hollowsused were described.Where possible we have collated data on allspecies referred to in studies, regardless of whether one species orwildlife group was targeted.

Only 27 studies have been published during the last 32 yearson the use of nest boxes by birds, with 14 published during19962000 (Fig. 1). This included 15 native and two introducedspecies. Nest box use has been described for seven parrot species,three passerines, one nocturnal bird and two waterfowl species.The use of articial hollows by microbats has fared much morepoorly with just ve studies covering eight species published inthe last 32 years (Fig. 1).

The most commonly encountered species in articialhollows, in terms of the number of individuals or boxesoccupied and the number of studies in which they featured,were the crimson rosella (Platycercus elegans), chestnut teal(Anas castanea), common myna (Acridotheres tristis), Gouldswattled bat (Chalinolobus gouldi) and the large forest bat(Vespadelus darlingtoni) (Table 1). Welcome swallows(Hirundo neoxena) used at least 800 plastic drum nest boxesin Western Australia but no further details were provided(Norman and Riggert 1977). The chestnut teal (Anas castanea)accounted for 363 boxes occupied in Victoria (Norman andRiggert 1977). This and one other study (Briggs 1991) are the

only ones that have targeted waterfowl despite the use of treehollows for nesting by 10 Australian species (Saunders et al.1982; Marchant and Higgins 1990) and the common use of nestboxes to studywaterfowl in theUSA (seeEadie et al. 1998; Evanset al. 2002). Sample sizes in Australian studies exceeded 50(either boxes occupied or individuals encountered) for just sixspecies (Table 1), though some studies did not document thenumber of individuals. The endangered orange-bellied parrot(Neophema chrysogaster) and Kangaroo Island glossy black-cockatoo (Calyptorhynchus lathami halmaturinus) have usednest boxes repeatedly but the details of this are not welldocumented (Mooney and Pedler 2005; Orange-bellied ParrotRecovery Team 2006).

Factors that affect the use of articial hollows

Several factors will inuence the use of nest and roost boxes.Of particular importance are hollow design and placement,natural hollow availability, and competition with other species.Breeding patternswill produce seasonal patterns of use, and thesemay differ across taxa. Temperature is also a key factor and mayproduce a seasonal response in the timing of use. These factorshave not been well studied and represent signicant gaps in ourunderstanding of articial hollow use.

Hollow design and placement

The main design elements that may inuence the use of articialhollows are entrance size, hollow volume, hollow depth belowentrance and wall thickness. A great amount of general literatureis available recommending detailed box designs for birds andbats (McCulloch and Thomas 1986; Stebbings and Walsh 1991;Grant 1997; Franks and Franks 2003). However, manyrecommendations appear to have been developed in an ad hocway. Understanding whether species show preferences forparticular design elements and placement positions is centralto all research andmanagement applications of articial hollows.Despite this only a few published studies have provided a choiceof hollow types or design elements for birds and bats in Australia(see section Research applications).

In the absence of preference studies (e.g. Lumsden 1989;Radunzel et al. 1997) it is likely that future studies will simplyfollow designs used elsewhere and be unable to state whether low

0

2

4

6

8

10

12

14

16

197680 198185 198690 199195 199600 200105 2006+

Year of publication

Num

ber o

f stu

dies

MicrobatsBirds-ManagementBirds-Research

Fig. 1. The number of studies on the use of articial hollows by birds andmicrobats published in 5-year periods since 1976.

82 Wildlife Research R. L. Goldingay and J. Stevens

Tab

le1.

Cha

racteristics

ofarticial

hollo

wsused

bydifferen

tspeciesof

bird

andba

tFrequency

ofuseofvariablecategories:X

X=frequentlyused

(>20%ofboxesu

sedatasiteorinastudy);X

=know

ntohave

been

used;

hasnotbeenused

orabsentfrom

studies.Notallvariablecategorieswere

presentw

herespeciesused

nestboxes.Sam

plesizeshave

been

pooled

acrossstudies.Som

estudiesmay

notgiveprecisedataso

>isused

toindicatethetrue

valueexceedsthatshow

n.Species

body

sizes,

Simpson

andDay

(1993),C

hurchill(1998).R

eferences:1,Norman

andRiggert(1977);2,M

illedge

(1978);3,G

olding

(1979);4,C

alderetal.(1983);5,Menkhorst(1984);6,Q

uinandBaker-G

abb(1993);

7,Irvine

andBender(1995),R

.BenderandR.Irvine

(unpubl.data),R.Bender(unpubl.data);8,

Trainor

(1995a,1995b);9,

Emison

(1996);10,Olsen

(1996);11,Pedler(1996);12,P

ellandTidem

ann

(1997a,1

997b);13,K

rebs

(1998,

1999),Krebs

etal.(1999);14,G

leeson

(1999);15,H

oman

(1999);16,W

ard(2000);17,S

mith

andAgnew

(2002);18,H

arperetal.(2005b);19,O

range-bellied

Parrot

RecoveryTeam

(2006);20,G

oldingay

(2007)

Box

variable

Australian

owlet-nightjar

NorfolkIsland

boobookowl

White-throated

treecreeper

Greyshrike-thrush

Striatedpardalote

Crimsonrosella

Easternrosella

Rainbowlorikeet

Red-rumped

parrot

Glossy

black-cockatoo

Red-tailed

black-cockatoo

Galah

Orange-bellied

parrot

Chestnutteal

Turquoiseparrot

Commonstarling

Commonmyna

Gouldswattled

bat

Goulds

long-earedbat

Easternfalse

pipistrelle

Lesser

long-earedbat

Largeforestbat

Southernforestbat

Chocolate

wattledbat

White-striped

freetailbat

Bodysize

birds(cm),

bats(g)

2430

1524

1235

3028

2748

6336

2148

2022

2512

921

77

58

38

Entrancediam

eter

12

X

X

XX

XX

XX

Entrancelocatio

nSlit

below

XX

XX

X

XX

XX

Front/to

pXX

XX

XXX

XX

XX

XXX

X

XX

X

XX

X

XX

XX

X

Spout

XX

XX

X

Volum

e(m

3)

Small(

0.03)

XX

XXX

X

XX

XX

XX

XX

X

X

XX

X

Heightaboveground

6m

X

XX

X

XX

X

X

X

XX

X

Aspect

North

X

XX

X

XX

X

XX

XX

South

X

XX

X

X

XX

XX

East

X

X

X

XX

X

West

X

X

Sam

plesizes

Individuals

24>2

21

4>8

5

5

>3

0>3

602

>250

50

56>1

00>2

>1>5

Boxes

occupied

24>2

213

4>9

58

64

415

228

>360

212

8130

243

10

10

References

3,4,

5,8

103,4,

5,8

52

3,4, 5,

12,13

8,12,

14,15

178

119

8,18

191

612,14,

1512,15,

183,4,7

17,

203,4,

163,4,

7,16

7,16

7,16

77

Use of articial tree hollows by Australian birds and bats Wildlife Research 83

usage was due to the design or local habitat factors. Studiesreporting use of articial hollows by bats are a good example ofthis. Golding (1979) used 22 sawn-timber boxes and 143 adaptedlog hollows. Both designs were of similar volume and had a6.5 cm diameter circular front entrance. He reported use of 23boxes by 260 bats of ve species but did not report whether onedesignwas favoured.Menkhorst (1984) reportednousebybats of240 front-entry boxes with circular entrances of 515 cmdiameter. Irvine and Bender (1995) recorded 34 bats in 5 of 10boxes with a slit-entry at the base. Ward (2000) used a small boxwith a slit-entry under the lid and reported 73 captures of fourspecies of bats. Smith and Agnew (2002) found that 17 of 48wedge-shaped boxes with a basal slit-entry were used by bats attwo sites. It is impossible to determine from these studiesspanning a broad latitudinal range whether one design ordesign element (e.g. slit v. circular entry) might be favoured.Given the inuence of temperature on roost selection by bats (seebelow), it is also likely that box temperature has played a role inthese results.

We have summarised information on the range in box designelements used by bird and bat species (Table 1), which provides abroad indication of preference in the absence of direct testing. Thematerials that boxes are made from have varied substantiallyamong studies. Hollows have been made from woodenammunition boxes and plastic drums for use by waterfowl(Norman and Riggert 1977), salvaged tree hollows placed intrees for turquoise parrots (Quin andBaker-Gabb 1993) or in treesand on powerpoles for red-tailed black-cockatoos(Calyptorhynchus banksii graptogyne) (Emison 1996; Fig. 2),polyvinyl chloride (PVC) pipe for use by glossy black-cockatoosonKangaroo Island (Pedler 1996) (Fig. 3), andplywoodboxes foreastern rosellas (Fig. 4) and crimson rosellas (Krebs 1998) and forbats (Fig. 5). In Britain, plastic drums have been used extensivelyby barn owls (Petty et al. 1994). Bat boxes have been made frommarine ply (Smith and Agnew 2002) or from pine (Irvine andBender 1995).

No detailed studies have been conducted in Australia toexamine the inuence of roost box design on bat occupancy.A variety of bat box designs have been used overseas, and often inthe context of providing alternative roost sites to bats displacedfrom roof spaces (Neilson and Fenton 1994). Brittingham andWilliams (2000) tested a vertical and a horizontal box of the samedimensions and found a preference for the horizontal box. Theyalso suggested that boxes needed to experience high internaltemperatures. Flaquer et al. (2006) compared single- and double-compartment bat boxes installed on houses, posts and trees. Theyfound greater use of the double-compartment box, and boxesattached to houses and posts, and recorded the highest occupancyby bats of any studies using bat boxes. Their single-compartmentboxwas similar to that used by Irvine and Bender (1995) but witha 1.5-cm rather than a 3-cm slit opening at the base.

Entrance size is a key design element, possibly enabling boxesto be designed to target species and it is one element that is easilyvaried. Most species generally prefer a minimum entrance size toallow access (i.e. one close to bodywidth) but avoid predation bylarger species. Menkhorst (1984) found that crimson rosellaspreferred one entrance size (8 cm) compared to three others.Several studies found that the Australian owlet-nightjar(Aegotheles cristatus) occupied a range of entrance diameters,

though it was more frequently encountered in boxes withentrances of 58 cm diameter (Table 1) (Fig. 6). Entrance sizemay be important to exclude some predatory species. Krebs(1998) implicated pied currawongs (Strepera graculina) inhigh levels of predation on crimson rosella chicks that ceasedwhen she used a 10 cm-long PVC tube to create a spout entrance.

Most studies (Table 1) have found that bats generally use small(20mm were found occupied by competinglarger mammals such as the sugar glider (Petaurus breviceps),introduced black rat (Rattus rattus) and common ring-tailedpossum (Pseudocheirus peregrinus) (R. Bender and R. Irvine,unpubl. data) or by feathertail gliders (Acrobates pygmaeus),sugar gliders and squirrel gliders (Petaurus norfolcensis)(Smith and Agnew 2002; Goldingay et al. 2007).

(a)

(b)

Fig. 2. (a) An articial hollow installed for red-tailed black-cockatoos inwestern Victoria. Metal ashing was attached near the base of the pole toexclude access by brushtail possums. (b) This articial hollow consisted of asalvaged tree hollow attached to a powerpole at a height of approximately12m. Photos: R. Goldingay.


Volume and depth are thought to inuence the suitability ofarticial hollows to birds and bats (Trainor 1995a) butinformation on this is limited. These parameters of naturalhollows inuence selection of hollow sites (Saunders et al.1982; Gibbons et al. 2002), and are known to affect thebreeding success of birds (Newton 1994). Nest box depth mayprovide security from predators (Trainor 1995a).

The insulating ability of nest boxes is determined by the typeand thickness ofmaterials used. Insulation is thought to affect nest

box use (Calder et al. 1983; Menkhorst 1984), leading to naturalhollows being favoured when available. Menkhorst (1984)postulated this to explain the relative absence of nest box useby the Australian owlet-nightjar and white-throated tree-creeper(Cormobates leucophaea) at one site where natural hollows wereabundant compared to another with low abundance where boxeswere used. Trainor (1995a) hypothesised that low insulatingproperties may affect the breeding success of birds. Commonmynas showed no preference for insulating properties (25-mmrough-sawn pine or 12-mm plywood) in one study (Harper et al.2005b). The insulating ability of nest boxes may be signicant tothe requirements of bats, which are thought to use hollows thatfacilitate thermoregulation (Gibbons and Lindenmayer 2002;Lourenco and Palmeirim 2004). R. Bender (unpubl. data)suggested no greater use of 10 roost boxes with a thick wall(45mm) designed for winter roosting by bats compared to otherboxes (19mm), though the extent to which this changed themicroclimate inside these boxes is unknown.

Aspect is likely to inuence temperature changes withinnest and roost boxes (Stebbings and Walsh 1991; Brittinghamand Williams 2000; Ardia et al. 2006). Its inuence may differbetween birds and bats due to the thermoregulatory requirementsof bats (see below). No preference for nest box aspect has beendetected in any Australian study (Table 1), though few specieshavebeenassessed and sample sizeshavebeensmall (Calder et al.1983; Menkhorst 1984; Smith and Agnew 2002). Furthermore,the actual position of boxes on trees may not have provided anadequate test of aspect (Menkhorst 1984). Stebbings and Walsh

(a) (b)

Fig. 3. (a) A polyvinyl chloride articial hollow installed for glossy black-cockatoos on Kangaroo Island. (b) The same designof articial hollow (~90 cm tall, 30 cmdiameter; entrance16 20 cm) installed for red-tailed black-cockatoos inwesternVictoriaon a powerpole. Photos: (a) E. Sobey; (b) R. Goldingay.

Fig. 4. Nestling eastern rosellas in a plywoodnest box.Photo:R.Goldingay.


(1991) recommended choosing aspects for bat boxes that allowthe sun to fall directly on the box for part of the day, as well asproviding boxes with various aspects in case overheating shouldoccur. Preference trials are needed to resolve this issue. Placementshould also consider the surrounding habitat because somespecies may be inuenced by habitat variables in their choiceof suitable nest boxes (e.g. Willner et al. 1983; Pogue andSchnell 1994).

Few studies have tested whether bird or bat species show apreference for the height at which boxes are placed in trees. Moststudies placed boxes at heights of 26m, presumably becausearticial hollows placed at lowheightswill be quicker and safer to

monitor. These relatively low heights may have limited the use ofboxes but for the species detected there is little evidence as yet thatgreater heights will be preferred (Table 1). Stebbings and Walsh(1991) suggested that some bat species in Great Britain preferboxes to be at least 5mhigh. The height atwhich boxes are placedmay inuence rates of predation (Milledge 1978; Menkhorst1984) but this remains to be tested. Lumsden et al. (2002a)found that a large proportion of tree roosts of lesser long-earedbats were 5mhigh.However, the latter species accounted for 91%ofbats present in boxes placed 46m high in Organ Pipes NationalPark (Irvine and Bender 1995; R. Bender, unpubl. data).

(a)

(b)

Fig. 5. (a) Awedge-shaped articial hollow for bats (26 20 18 cm; 1.5-cm slit-entry). (b) Goulds wattled bats in a wedge-shaped bat box. Photos: (a) R. Goldingay; (b) M. Grimson.

(a)

(b)

Fig. 6. (a)Anarticial hollowdesigned for rosellas (45 20 15 cm; 6.5-cmentrance).No research has addressed the need forperches on these hollows. (b) An owlet-nightjar roosting in this articial hollow. Photos: R. Goldingay.


The actual spacing of boxes may inuence their frequency ofuse. Nest boxes located too close together may be subject tointraspecic competition and nesting failure of birds (see Krebs1998). Among North American waterfowl, nest boxes placed athigh density can lead to high levels of conspecic broodparasitism (multiple females depositing eggs into one clutch),which can have adverse population effects (Eadie et al. 1998).Studies of tree hollow use in white-tailed black-cockatoos(Calyptorhynchus latirostris) and Major Mitchells cockatoos(Cacatua leadbeateri) suggest that conspecic interactions leadto a wide spacing of nest trees (Saunders et al. 1982).

Given the lack of proper testing of elements of hollowdesign and placement, there is a great need for research on thistopic. The effective use of articial hollows in research andmanagement will depend on sound knowledge of hollowdesign and placement.

Hollow availability

The availability of natural hollows is commonly assumed to havea strong inuence on the frequency of use of articial hollows(e.g. Newton 1994). However, few studies in Australia haveconsidered this issue in detail with many studies having beenconducted where it was apparent that a dearth of natural hollowswas precluding use of an area (e.g. Irvine and Bender 1995) orhindering reproduction (e.g. Orange-bellied Parrot RecoveryTeam 2006). Three studies in Victoria (Ambrose 1982; Calderet al. 1983; Menkhorst 1984) each installed boxes at severallocationswhere natural hollow abundancewas either lowor high.In all studies boxes were used more frequently by Australianowlet-nightjars where natural hollows were scarce. White-throated tree-creepers and crimson rosellas showed higheruse where natural hollows were scarce in two studies but nodifference in one study (crimson rosella) or higher use wherenatural hollows were abundant (white-throated tree-creeper).Ambrose (1982) found box use by barn owls (Tyto alba) to bemore frequent in forest with few hollows whereas boobook owls(Ninox novaeseelandiae) showed no difference between siteswith few or many hollows. Menkhorst (1984) found that greyshrike-thrushes (Colluricincla harmonica) only used boxeswhere natural hollows were abundant.

Few studies have been conducted of bats in this context.Golding (1979) found that 8% of boxes were used by thelesser long-eared bat and Goulds wattled bat where hollowswere abundant, compared to 3% where no hollows occurred and43% where hollows were in low abundance. Ambrose (1982)reported a single bat detection in a boxwhereasMenkhorst (1984)reported none. Smith andAgnew (2002) found thatGoulds long-eared bat used boxes frequently at two sites with no hollows andthere was little or no use where natural hollowswere either in lowor high abundance. Goldingay et al. (2007) observed low use ofbat boxes across ve locations that varied in abundance of naturalhollows including the two sites where Smith and Agnew (2002)reported frequent use. For both birds and bats, it appears that sitefactors (habitat, competitors) may have a strong inuence on boxuse that may mask a species response to natural hollowabundance and that properly replicated studies are required tobetter understand whether natural hollow abundance inuencesuse of articial hollows. Due to the mobility of birds and bats it is

likely that they can readily colonise areas devoid of naturalhollows if nest and roost boxes are installed.

Seasonal patterns related to breeding

Many species of hollow-using birds only use hollows forbreeding, so their breeding patterns will determine when nestboxes are used. This may require frequent checking of articialhollows during the year to accurately document use as well as todetermine the adequacy of the articial hollows provided forbreeding. The crimson rosella, white-throated tree-creeper(Calder et al. 1983; Menkhorst 1984), grey shrike-thrush(Menkhorst 1984), eastern rosella (Platycercus eximius) andred-rumped parrot (Psephotus haematonotus) (Trainor 1995a)used nest boxes in the spring and summer periods for breeding,althoughnest site preparation (hole chewing andnest building) bythe crimson rosella (Calder et al. 1983; Pell andTidemann 1997a;Krebs 1998) and white-throated tree-creeper (Calder et al. 1983)was observed during winter. In Victoria, crimson rosellas usedarticial hollows until chicks were edged in mid-January, aperiod of 56 days following egg laying (Golding 1979). Orange-bellied parrots laid eggs betweenNovember and January, and hadan incubation period of 24 days and a nestling period of 35 days(Orange-bellied Parrot Recovery Team 2006). Glossy black-cockatoos on Kangaroo Island laid eggs from late January tolate July, and had an incubation period of 30 days and a nestlingperiod of 90 days (Garnett et al. 1999). Red-tailed black-cockatoos laid eggs during September to December, and hadan incubation period of 30 days and a nestling period of 87 days(Commonwealth of Australia 2006). Maned or wood ducks(Chenonetta jubata) bred in nest boxes near Canberra betweenJuly and December with a 34-day incubation period (Briggs1991). The nocturnal Australian owlet-nightjar used nest boxesprimarily in spring and summer for breeding, but as a diurnal roostsite throughout the year (Calder et al. 1983; Menkhorst 1984;Trainor 1995a). Thewhite-throated tree-creeper was found to usenest boxes for roosting all year except during summer (Ambrose1982). The boobook owl and barn owl used nest boxes duringwinter and spring, and less frequently during summer (Ambrose1982). The abovedata indicate that seasonal patterns of use can behighly variable among bird species and consequently the designof monitoring programs will need to accommodate this.

Few data are available for bats but if articial hollow usemirrors natural hollow use then the requirements of some speciesmaydiffer during the breedingperiod fromother timesof theyear,when females aggregate into maternity colonies when breeding(e.g. Churchill 1998). For example, females of the lesser long-eared bat preferred enclosed hollows with ssure entrances invery large-girth trees during breeding periods compared to whennot breeding (e.g. Lumsden et al. 2002a). Golding (1979) foundthat individual or small groups of Goulds wattled bat used roostboxes throughout most of the year but female-dominated groups(up to 39 bats) occurred in spring and for the lesser long-eared bat(10 bats) in December in a few boxes. Smith and Agnew (2002)observed small numbers (four to eight) of Goulds long-eared batin several months but one aggregation of 16 females with youngin October. R. Bender (unpubl. data) observed a summer peak inbreeding by Goulds wattled bats in boxes. The frequency withwhich patterns of use mimic those within natural hollows by


catering for non-breeding females as well as breedingaggregations should indicate the effectiveness of articialhollows. Studies that involve a choice of box size and locationare needed to better understand the potentially varied seasonalrequirements of bats.

Temperature

The inuence of temperature on hollow use will vary dependingon whether a species requires hollows throughout the year(bats, hollow-roosting birds) or just during the breedingseason (hollow-breeding birds). Temperature within hollowsmay be critical to the daily thermoregulation of individuals orin promoting faster rates of growth by developing young(e.g. Dawson et al. 2005). The temperature inside articialhollows can be an important factor in their use if hollows eitherexceed or do not reach a preferred thermal zone. There may be aseasonal element to this due to the poor insulative properties ofarticial hollows (Calder et al. 1983; Kerth et al. 2001), so theymay be ignored inwinterwhen temperatures are lowor ignored insummerwhen temperatures are high (R.Goldingay, unpubl. obs).This seasonalpattern is likely tovary latitudinallywithclimateandmay lead to variation in the preferred orientation of entrances tonests (e.g. Burton 2007) or the preferred position of an articialhollow (full sun v. shade). Furthermore, elevational gradients intemperaturemayalsohavean inuence.Therefore,wherewereferto studies from different geographic locations below, weacknowledge that some ndings may be location specic.

In Australia, the inuence of the thermal environment inarticial hollows has not been well studied. Norman andRiggert (1977) found that temperatures inside black plastic-drum nest boxes in southern Western Australia were an averageof 12Chotter than ambient. Temperatures in the boxes commonlyexceeded 35C in October but were not assessed later in the yearwhen they might have been much hotter. Menkhorst (1984)suggested that reduced survival of white-throated tree-creeperhatchlings in his study may have resulted from overheatingwithin nest boxes. Calder et al. (1983) have shown that aspect,wall thickness, entrance position and box depth can all haveprofound inuences on temperatures within articial hollows.Overly high summer temperatures may not be such a problemfor hollow-breeding birds that breed in spring, but the young ofsome species (e.g. crimson rosella, white-throated tree-creeper)may not edge until JanuaryFebruary (Calder et al. 1983). Somebird speciesmay choosewarmhollows in springdue to the positiveinuence that temperature can have on incubation andnestling development (e.g. Ardia et al. 2006). Consistentwith this was the observation that Bechsteins bats (Myotisbechsteinii) in Germany preferred sun-exposed boxes duringlactation whereas shaded boxes were preferred pre-lactation(Kerth et al. 2001).

Temperature is a critical issue for microbats in roost selection(Boyles 2007) and therefore roost box selection.Manymicrobatsare quite small in size (

for some target species. Competition may arise from native ornon-native species. The impacts of displacement by non-nativespecies is relatively well documented. The common myna (Pelland Tidemann 1997a; Homan 1999; Harper et al. 2005b),common starling (Sturnus vulgaris) (Ambrose 1982; Trainor1995a; Pell and Tidemann 1997a, 1997b) and introducedhoneybee (Ambrose 1982; Trainor 1995b; Emison 1996; Pelland Tidemann 1997a; Homan 1999; Harper et al. 2005b) are thespecies of most concern, though the black rat could also be aproblem at some locations. These species have the potential todeter native species from using nest boxes and reduce the numberof available hollows (Trainor 1995b; Pell and Tidemann 1997b;Smith and Agnew 2002). The common myna was found to out-compete several species of native parrot and the common starlingcan cause nest failure of native species (Ambrose 1982; Trainor1995a; Pell and Tidemann 1997b; Gleeson 1999; Orange-belliedParrot Recovery Team 2006). Harper et al. (2005b) found thatmynas occupied 45 of 120 nest boxes in Melbourne over a7-month period, and rebuilt their nests following removal.Although competition for nest sites with the starling has led tofew declines among cavity-nesting birds in the USA (Koenig2003), the longer-term effects of starlings and mynas onAustralian birds are unknown.

The invasion of both articial and natural hollows by feralhoneybees can be a signicant management issue (e.g. Trainor1995b; Wood and Wallis 1998). The recovery plans of theKangaroo Island glossy black-cockatoo and orange-belliedparrot have identied the need for regular maintenance to deterand remove feral honeybees from articial and natural hollows(Mooney andPedler 2005;Orange-bellied ParrotRecoveryTeam2006). Feral honeybee invasionhas been reported inmany studiesof nest boxes (e.g. Wood and Wallis 1998; Harper et al. 2005b;Goldingay et al. 2007) and needs to be documented in detail so itsincidence and treatment can be better understood. For example,the use of carpet (Franks and Franks 2003) and insecticide strips(Irvine andBender 1995; Soderquist et al. 1996) inside nest boxeshave been trialled to prevent honeybee infestations but fewdetailshave been reported.

Nest box design may mitigate the impact of pest species andreduce the need for costlymaintenance. Lumsden (1989) reportedthat starlings did not use nest boxes painted white inside. In apreliminary study by Homan (2000), the use of a bafe installedon the front of nest boxes successfully excluded the commonmyna without excluding native species. A slit entrance for batboxes has been suggested to exclude competition and predationby introduced birds (Smith and Agnew 2002). Small nest boxvolumemay reduce hive building by honeybees (Goldingay et al.2007). The inuence of pest species is relevant to the success andeconomic cost of articial hollows in management, and furtherinvestigation using an experimental approach is needed.

Some native hollow-using species will displace others. Boxesinstalled in Australia for use by bats may be used by arborealmammals (Smith andAgnew2002;Goldingay et al. 2007). Irvineand Bender (1995) reported that 2 of 10 bat boxes had nests ofsugar gliders but over time all of these boxes had sugar gliderspresent either often or occasionally (R. Bender and R. Irvine,unpubl. data). Small entrance sizewill not overcome this problembecause feathertail gliders will use entrances of 1.5 cm and theirnests can completely ll a box (Fig. 7a). Goldingay et al. (2007)

reported that 6 of 12 bat boxes at locations in south-eastQueensland had feathertail gliders or their nests and only fourboxes were used by bats. This may require a reliance on open-bottom roost boxes for bats that preclude the construction of a leafnest inside (Fig. 7b, c).

Arboreal marsupials are likely to compete with birds as wellbut the extent of this is not well documented. Menkhorst (1984)provided circumstantial evidence that two bobucks (Trichosuruscaninus) discouraged the use of many nest boxes by othermammals and birds. Common brushtail possums (Trichosurusvulpecula) will kill nestlings present in articial hollows. OnKangaroo Island, this has required placing metal guards aroundtrees containing hollows used by glossy black-cockatoos toprevent possums gaining access (Garnett et al. 1999). Sugargliders compete with orange-bellied parrots for nest sites andmay even kill incubating birds (Orange-bellied Parrot RecoveryTeam2006).This requires specic research to identifymethods toreduce this impact. For example, squirrel gliders and sugar glidersmay favour rear-entry nest boxes (the entrance occurs on the backof thebox; seeBeyer andGoldingay2006) andbe less likely touseother front-entry box types (with the entrance on the front or sideof the box) when a choice is provided (Goldingay et al. 2007;R. Goldingay, unpubl. obs). Loeb and Hooper (1997) found thatthe provision of nest boxes reduced the use of natural cavitiesrequired by endangered red-cockaded woodpeckers (Picoidesborealis) by competing cavity users.

Crimson rosellas on Norfolk Island competed for nest boxeswith endangered Norfolk Island boobook owls (Olsen 1996) andfor natural hollows with endangered Norfolk Island greenparrots (Cyanoramphus novaezelandiae cookii) (Hill 2002).Galahs (Cacatua roseicapilla) have been implicated in theloss of eggs and little corellas (Cacatua pastinator)implicated in the loss of nestlings of the glossy black-cockatoo (Garnett et al. 1999). Deployment of decoy nestboxes may help to alleviate competition and provide easieraccess to these species for control.

Emison (1996) recorded yellow-tailed black-cockatoos(Calyptorhynchus funereus), long-billed corellas (Cacatuatenuirostris), maned ducks and owls using articial hollowserected for red-tailed black-cockatoos. The expansion in thegeographic ranges of common hollow-nesting parrots may putcompetitive pressure on other hollow-using native species.Clearly, competition from non-target species can have asubstantial inuence on the effectiveness of any articialhollow program. However, although the problem is wellacknowledged by many authors it remains poorly documentedand this will hamper attempts to reduce its impact. Therefore,research must be conducted to understand the magnitude of theproblem and how it might be managed.

Articial hollow applications

Articial hollows have been used in a variety ofways for researchand management. Beyer and Goldingay (2006) recognised forarboreal marsupials that nest boxes had three researchapplications (detection of species, ecological studies,investigation of nest-box design and placement) and threemanagement applications (threatened species recovery, speciesintroductions, strategic placement). These also apply to birds and


bats. In addition, we also recognise species establishmentthrough habitat enhancement and hollow-bearing tree offset.These research and management applications are discussed indetail below.

Research applications

Species detection

Nest boxes have been used as a survey tool to determine thedistribution and abundance of cryptic arboreal mammal species(Beyer and Goldingay 2006). Using articial hollows in this

way may not be of great importance for birds and bats, whichmay be detected more readily using conventional surveytechniques, particularly sonar detection for bats (but seeFlaquer et al. 2007). Homan (1999) installed 12 nest boxesto survey for small parrots but eight were quickly occupied bycommon starlings and common mynas. However, the value ofnest and roost boxes is that they can be left in place for longperiods and may provide insights not readily gained fromperiodic survey using other techniques. In Great Britain, theplacement of large numbers of bat boxes has producedsignicant range extensions for some species (Stebbings and

(a)(b)

(c)

Fig. 7. (a) A feathertail glider nest completely lled this wedge-shaped bat box. (b) An open-bottom designbat box (42 17 15 cm; front panel 32 cm). (c) Goulds long-eared bats in an open-bottom box. Photos: (a, b)R. Goldingay; (c) J. Lindsay.


Walsh 1991). Articial hollows could be deployed to identifyspecies that might respond to hollow provision, as a precursor toa management application.

Ecological research

Nest boxes have been used extensively to investigate thebreeding biology and life histories of hollow-using birds inNorth America and Europe (Koenig et al. 1992; Evans et al.2002). InAustralia, only a small numberof published studies haveused articial hollows to investigate the ecologyof birds andnonehave investigated the ecology of bats. Norman (1982) used nestboxes to study egg laying and incubation in the chestnut teal.Briggs (1991) used nest boxes to investigate intraspecic nestparasitism in maned ducks. Pell and Tidemann (1997a) used nestboxes to study factors that affect the breeding success of thecrimson rosella, and eastern rosella when in competition withintroduced hollow-using birds. Pell and Tidemann (1997b)investigated the ecology of the common myna using nestboxes. Nest boxes have allowed detailed investigation ofbreeding biology, patterns of food allocation in broods, andnestling growth and survival in the crimson rosella (Krebs1998, 1999, 2001; Krebs et al. 1999, 2002; Krebs andMcGrath 2000). Nest box use by the glossy black-cockatoohas allowed aspects of its breeding ecology to be described(Garnett et al. 1999). Baltz and Clark (1999) used nest boxesto investigate the inuence of conspecics on nest choice inbudgerigars (Melopsittacus undulates) in captivity. The abovestudies highlight the value of nest boxes as a research tool,in providing access to animals that may not be available anyother way.

Articial hollow preferences

The design of articial hollows can have a pronouncedinuence on their frequency of use (see above). Despite this,fewpreference studies have been conducted inAustralia.Normanand Riggert (1977) examined the use by waterfowl of eight nestbox types that differed in construction material and dimensions.This revealed greater use of a thick wooden box (ammunitionbox) but few of some types were installed and it appears thata direct choice of different box types was not provided.

Menkhorst (1984) provides the rst example of a properlyreplicated choice experiment in which combinations of fourentrance sizes, three height placements and two aspect typeswere used. He cautioned about the comparison of aspect becauseboxes were placed on the east or west side of a tree and likely tohave received approximately equal exposure to the sun. Heidentied a preference by crimson rosellas for entrance sizebut sample sizes for owlet-nightjars, white-throated tree-creepers and grey shrike-thrushes were too low to demonstrateany choice. No bats were detected in this study.

R. Bender and R. Irvine (unpubl. data) explored therelationship between entrance size and bat body size with asmall number of roost boxes. They suggested that Gouldswattled bat (10 g) preferred a slit entrance size >15mm,whereas smaller species, the large forest bat (7 g) and thesouthern forest bat (Vespadelus regulus) (5 g), preferred slitentrance sizes

success similar (and in some years greater) to that in naturalhollows (Mooney and Pedler 2005). This program could informthe use of articial hollows in other recovery programs.

Articial hollows consisting of hollows cut from fallen treeshave been used to supplement the breeding habitat of the south-eastern red-tailed black-cockatoo. A programwas trialled for thissubspecies and included erecting some nest boxes on 12m-highpower poles (Emison 1996). Nest boxes were quickly occupied,and over a 2-year period 30% of all articial hollows were used(Emison 1996). Approximately 60 articial hollows, comprisingboth PVCplumbing pipe and natural hollows cut from fallen trees(Figs 2, 3) have been installed on power poles in the last 5 yearsbut only salvaged natural hollows (at least seven) were used(R. Hill, pers. comm.). The recovery plan for this species hasrecognised that although the availability of natural nest hollows isnot currently limiting the population, dead nest trees arecollapsing at a rate of 47% per year, and this is likely to be aserious threat in years to come (Commonwealth of Australia2007). The recovery plan recommends for the situation to bemonitored and the articial hollow program expanded only ifsignicant nest tree losses are observed (Commonwealth ofAustralia 2006). Articial hollows made from fallen brancheswere installed to supplement the breeding habitat of the turquoiseparrot (Neophemapulchella) inChiltern Park, Victoria (Quin andBaker-Gabb 1993). Twowere used for nesting. Articial hollowsmade from salvaged tree hollows and erected in known breedingareas have been used in breeding by Norfolk Island green parrots(Hill 2002).

Nest boxes were installed in two studies in Tasmania totarget the endangered forty-spotted pardalote (Pardalotusquadragintus) but were only successful in enabling the striatedpardalote (Pardalotus striatus) to breed (Milledge 1978;Woinarski and Bulman 1985). Nest boxes are currently beingtrialled as part of the recovery plan for this species (ThreatenedSpecies Section 2006).

Roost boxes could play an important role in the recovery ofthreatened bats but existing studies are inadequate to guide thisapplication. The recovery plan for theChristmas Island pipistrelle(Pipistrellus murrayi) proposes to install roost boxes tosupplement the natural roosts of this species, which mayinclude beneath tree bark, under dead palm fronds, beneathtree canopies, and in tree hollows (Schulz and Lumsden 2004).The eastern false pipistrelle (Falsistrellus tasmaniensis), avulnerable species in New South Wales, has been recordedusing nest boxes in Victoria (Golding 1979; Ward 2000),suggesting there is potential to use roost boxes in the recoveryof this species.

Existing studies demonstrate the usefulness of articialhollows to threatened species recovery. However, there is aneed for further research to improve and expand current use.This application has potential for threatened bats but will dependon a dramatic increase in our understanding of preferred roostbox designs.

Species introductions

We recognise this application as distinct from threatenedspecies recovery because it may not always involve such speciesand the approach may be quite different. It involves installing

articial hollows at a location where a species is to beintroduced. The deployment of articial hollows to introducea species at a site has been documented for just one species.Several nest boxes were installed on Norfolk Island in 1987 forthe reintroduction of the endangered Norfolk Island boobookowl (Ninox novaeseelandiae undulata) (Olsen 1996). Malebirds from the closest extant subspecies, the New Zealandmorepork (Ninox novaeseelandiae novaeseelandiae), weretranslocated to the island. The nest boxes enabled successfulbreeding to occur.

Species establishment through habitat enhancement

Because birds and bats are highly mobile it is quite possibleto attract and establish species at a location by the installation ofarticial hollows. This is as a form of habitat enhancementwhere the decline or disappearance of hollow-using species isrecognised and articial hollows have been installed to preventthe permanent loss of biodiversity. Currently, the onlysuccessful example of this management application is that byIrvine and Bender (1995) for bats. Bat boxes were installed inOrgan Pipes National Park in Victoria in 1992 to facilitate theestablishment of bat populations within regenerating forest inthe park where habitat restoration had commenced in 1972(Irvine and Bender 1995). Bat boxes were not occupied for30 months, which is in stark contrast to within 3 months ofinstallation documented by Smith and Agnew (2002) in boxes inQueensland. The number of bats in Organ Pipes National Parkincreased from 15 per check in 199495 to >100 per check in200405 from 34 boxes (R. Bender, unpubl. data). Gouldswattled bat comprised 91% of records but a small number oflarge forest bats also consistently used the boxes. Boyd andStebbings (1989) reported a doubling over a 10-year period in apopulation of brown long-eared bats (Plecotus auritus)supported by roost boxes in managed forest in Great Britain.These observations suggest that the local bat population inOrgan Pipes National Park increased in size over time so theinitial delay may have been due in part to a small localpopulation.

Harper et al. (2005b) installed nest boxes as habitatenhancement (~3 per ha) for vertebrate fauna across remnantvegetation within the urban and suburban landscape ofMelbourne. Although nest box design favoured large arborealmammals (large entrance and volume), a small numberwere usedby rainbow lorikeets (six), an eastern rosella (one) and a galah(one). The provision of articial hollows may also allow hollow-using species lost from urban areas to recolonise. Other studiesthat install articial hollows as habitat enhancement have beenplanned and will target a range of birds and bats (B. Law andR. Kavanagh, pers. comm.; R. Goldingay, unpubl. data).

This management application has considerable merit but iscurrently hamperedby the lackofunderstandingof the factors thatinuence the use of articial hollows (see above). Many trials areneeded todeterminewhichhollowdesigns shouldbe installed, thenumber of eachdesign, themost effective placement, andwhetherparticular species have beneted from such habitat enhancement.Evidence that this application has been successful would be theon-going use and breeding in articial hollows (Petty et al. 1994),rather than only sporadic use by species. Such trials should be


conducted in an adaptivemanagement framework so that changescan be made as information accumulates on factors that improveeffectiveness.Monitoringwill be fundamental to such studies andneeds to be continued for 25 years to provide the best insights.

Hollow-bearing tree offset

Another management application that has been used by landmanagers (e.g. local government, road agencies, powercompanies) is to install articial hollows to compensate forhollow-bearing trees lost during authorised clearing (Fig. 8).This might occur either adjacent to the development site orpossibly away from the site if a landscape approach is taken tomanaging hollow-using species.We stress that the potential valueof articial hollows should not be used to justify the removal ofhollow-bearing trees.Themerit of this application is that birds andbats can be highly mobile and providing some replacementhollows may assist some species as an interim solution whileother longer-term measures are devised. If such a managementapplication is used there needs to be monitoring over a 25-yearperiod to document the outcome. Currently, information islacking to demonstrate the value of such a use of articialhollows to hollow-using species so there is an obvious needfor research on this application.

Strategic placement

This management application emphasises the speciclocations where articial hollows are placed. This might be aspecic location in a landscape suchaswithinwildlife corridorsorit might be a location where a specic objective is to be achievedsuch as to attract particular kinds of hollow-using species(Beyer and Goldingay 2006). Although it will be concernedwith establishing species in an area it primarily differs fromspecies establishment because fewer hollows will be used,their installation will be highly targeted and a more specicobjective will be stated.

In Europe there is increasing recognition of the value of nestboxes to attract hole-nesting birds as ameans of controlling insectpests in forests and orchards (Mols and Visser 2002). Smith and

Agnew (2002) hypothesised that small mammals, particularlybats, could control insects in farm forests and therefore provide ahealth benet to the trees. They installed bat boxes in smallhardwood plantations in south-east Queensland and had successin attracting a small number of bats to their sites but better successwith attracting feathertail gliders and squirrel gliders, which mayhave discouraged greater use of the boxes by the bats. Thework inOrgan Pipes National Park provides insight to the potential ofstrategic placement with regard to bats. In 200405, >100 batswere captured per round of box checks from 34 roostboxes scattered through 5 ha of regrowth forest (R. Bender,unpubl. data).

Articial hollows are being trialled in northern Australia toincrease barn owl and masked owl numbers for the control ofrodent pests in sugarcane crops (Gibbons and Lindenmayer2002). Studies on the strategic use of articial hollows inAustralia are in their infancy. Further research is needed forthis application.

Conclusion

In many landscapes across Australia the collapse of hollow-bearing trees has outpaced the recruitment of replacementhollows and future shortages in this resource are inevitable(Saunders 1979; Lindenmayer et al. 1990, 1997; Saunderset al. 2003; Courtney and Debus 2006; Commonwealth ofAustralia 2007; Beyer et al. 2008). The provision of articialhollows is likely to be the most appropriate interim solution tothis shortage but existing information on the use of articialhollows is too limited to enable this response to be effective andprogress in this eld has been slow. Applying our criteria ofrequiring more than a single record of articial hollow use andsome details of the hollows, we found information wasdocumented for just 15 of 114 hollow-using bird species and8 of 41 hollow-using microbat species.

The lack of information is most stark for microbat species.The deployment of roost boxes for bats in Australia is clearly inits infancy. Progress has in part been hampered by the relativelyinfrequent use of articial hollows by bats, which may be aconsequence of positioning roost boxes without regard forcreating suitable thermal environments. Selecting roosts toenable passive rewarming from daily torpor may be quitewidespread among Australian tree-roosting bats (Turbill et al.2003a, 2003b; Turbill 2006) and this will inuence theirability to use articial hollows. Correct positioning ofarticial hollows poses a challenge because deployment inforested habitats may preclude positioning boxes in highlyfavourable microsites due to shading from canopy cover.Some natural roosts of bats in mature forest may be locatedhigh in trees (e.g. Herr and Klomp 1999; Lumsden et al. 2002a)and have low canopy cover (e.g. Campbell et al. 2005) tominimise shading. However, given that bats may commute overlong distances where tree hollows are scarce (e.g. Lunney et al.1985, 1988; Lumsden et al. 2002b), it may be appropriate totarget edge and forest gap sites to place articial hollows wheresun exposure will be optimal.

A key deciency in deploying articial hollows inresearch and management of hollow-using birds and bats is inunderstanding the most effective designs to use. Research

Fig. 8. Bat boxes installed on powerpoles through a powerline easement inBrisbane. Some poles also have parrot boxes attached. These boxes wereinstalled after expansion of an existing powerline easement. Photo:R. McLean.


suggests that most species favour articial hollows withentrances just wide enough to enter. This enables avoidance oflarger competitors and possibly predators. Other elements ofdesign have not been properly investigated though guidancecan be provided by research on natural hollows, which showthat hollow depth is also important (e.g. Saunders et al. 1982;Gibbons et al. 2002). Some bird specieswill use articial hollowsmade from salvaged tree limbs but it is not well documentedwhether constructed hollows that emulate these have been triedadequately. Batsmay have different requirements during the year(breeding v. non-breeding periods) and this might require theprovision of several designs. So few studies have been conductedof bats using roost boxes in Australia that suitable designsremain largely unknown. There is an obvious need for manyeld experiments that compare different hollow designs orwhich vary specic design elements for birds and bats. Thisshould lead to better knowledge of designs preferred by differentspecies.

Another aspect that requires further investigation is theextent that competing species may pre-empt target speciesfrom articial hollows. Many studies have described potentialcompetition among hollow users but the seriousness of this tomanagement is not well documented. In some cases competitorsmay favour a specic box design (e.g. arboreal mammalsthat displace birds or bats) and exert less interference if theirhollow needs are catered for. This requires specic researchto address. Furthermore, there are various issues that relate tothe maintenance of articial hollows, such as the occupationby feral honeybees and the collapse of boxes (Beyer andGoldingay 2006). This also must be addressed by researchwith particular attention given to the cost of maintenance(see also Harley 2006).

One aspect of articial hollow placement that is not wellunderstood and in need of research is whether species show apreference for the height at which boxes are placed on trees. Thisis of considerable management relevance because this can havecost and safety implications in effectively employing articialhollows. Available information suggests that heights >5m arerarely needed (Table 1). The only examples where hollows havebeen placed very high is that of theKangaroo Island glossy black-cockatoo with hollows at ~16m above ground (Pedler 1996) andthe red-tailed black-cockatoo with hollows at 12m above ground(Emison 1996). It appears that articial hollows have been placedat an equivalent height to natural tree hollows (Pedler 1996). Suchhollows will require an enormous amount of time to check andmaintain. Given that red-tailed black-cockatoos have used treehollows at heights as low as 4.4m (mean height 7m) in WesternAustralia (Saunders et al. 1982), it is possible that very highhollows are not needed. Examples where bats select high roosts(i.e. >10m) may reect their preference for suitable thermalenvironments, which may be replicated by placing articialhollows in canopy gaps (see above).

Studies inAustralia on the useof articial hollowshave laggedbehind those conducted in Europe and North America (e.g. Boydand Stebbings 1989; Newton 1994; Petty et al. 1994; Pogue andSchnell 1994;BrittinghamandWilliams 2000;Kerth et al. 2001),which is surprising given that a greater proportion of wildlifespecies in Australia compared to other continents is dependent ontree hollows for survival (see Saunders et al. 1982; Newton 1994;

Gibbons and Lindenmayer 2002). The small number ofAustralian studies currently available is insufcient to allowmany generalisations. Articial hollows have enormouspotential to become an important management tool for hollow-using species in Australia but this can only be realised byconducting many additional studies. This needs to occuracross a range of species and landscapes to maximise ourunderstanding of interactions between species and differentenvironments. Furthermore, species of Australian birds andbats are likely to respond to similar factors in choosingarticial hollows as taxonomically equivalent species overseas(e.g. parrots, vespertilionid bats), so investigations can provideindependent tests of current hypotheses concerning inuences onuse, or of management applications. This will allow greatergeneralisation and can also lead to work conducted inAustralia informing research and management of hollow-usingspecies in other countries.

Acknowledgements

The comments of Brendan Taylor, Geoff Smith and two anonymous refereeshelped improve this paper. This paper has been informed by several nest boxprojects conducted in Brisbane and an articial hollow project conducted atBrunswick Heads, NSW.We thank Brisbane City Council for support of ourresearch inBrisbaneandAbigroup for assistancewith theproject atBrunswickHeads. Matthew Grimson and Geoff Smith are thanked for sharing ideas andeld work that have provided background to this paper. We thank RobertBender and Robert Irvine for their pioneering work with bat boxes in OrganPipes National Park.

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Manuscript received 1 May 2008, accepted 9 October 2008


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Use of Artificial Tree Hollows by Australian Birds and Bats

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