Effects of Human Disturbance on Colonial Species, Particularly Gulls (1981)

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Effectsof HumanDisturbance on Colonial Species,ParticularlyGullsJOANNABURGER

Department of Biology and Center for Coastal and Environmental Studies,Rutgers University, New Brunswick, New Jersey 08903 USAColonial birds are particularly vulner-able to human disturbance because of the

large concentrations of birds nesting inclose proximity. Vulnerability varies de-pending on species, nest location, and thetype of disturbance (see Manuwal 1978).Nearness to human activities is one keyfactor, as the potential for disturbance ismuch greater regardless of a species' in-trinsic vulnerability. Mere distance is notsufficient to assess the potential for dis-turbance because the surrounding habitataffects human (or predator) access. Birdsnesting a half mile from a town on anisland surrounded by deep, rough watersare surely less vulnerable than thosesurrounded by solid ground. Similarly, nest-ing habitat is crucial: nests on cliffs are lessaccessible than are nests on flat ground,nests in trees are less accessible than areground nests, and burrows are less accessiblethan are surface-areas. Ground-nestingspecies such as gulls, terns, and some herons,egrets, ibises, and cormorants are thus par-ticularly vulnerable to human disturbancebecause they are accessible and often nestin close proximity to human activities.The types of human activity that affectcolonial bird populations are varied,ranging from overall destruction to directintervention such as entering colonies,collecting eggs, and killing adults, eggs, orchicks. The response of colonial birds tohuman activities varies markedly, but nospecies can withstand the direct effects ofkilling adults, eggs, or chicks for sustainedperiods of time. Assessing the effects ofhuman disturbance on nesting colonial birdsis a difficult task because the observer cannever completely remove himself. Theproblem is further complicated by the de-creased accuracy of success measurementswith fewer egg or chick censuses. Observa-tion of reproductive success from afar isusually impossible in the egg stage, and iscomplicated in the chick phase by the

presence of vegetation and rocky or uneventerrain.Reproductive success can be loweredeither directly by causing desertion of nests,eggs or young, by adults, or indirectly bycausing thermal stress, predation, andcannibalism. In this paper I discuss thedirect and indirect ways human activitiesaffect colonial birds, particularly Larids, andI report on some of the sub-lethal effectsof human disturbance including behaviorscontributing to the lowered reproductivesuccess. I present results on the behavioralresponses of gulls, terns, and skimmers tohuman disturbance including decreased eggand chick attendance, shifts in the mate in-

cubating, movements and entanglements ofchicks, higher brood sizes, and greateraggressive interactions.GENERAL METHODS

The results presented in this paperwere gathered in several gull, tern andskimmer colonies from 1970 to 1980 in-cluding Islajo, Carvel, and Clam Islands(New Jersey), Cedar Beach and JamaicaBay Refuge (New York), Isles of Shoals(Maine), Agassiz National Wildlife Refuge(Minnesota), and at Murphy (Argentina).The specific methods employed to observethe effects of human disturbance varied andwill be described in the appropriatesections.

RESULTSNest Attendance

When disturbed frequently, gulls canrespond either by habituating (i.e. remain-ing at the nest) or by decreasing nest at-tendance. While studying Herring Gulls(Larus argentatus) on Clam Island, NewJersey in 1977 (see Burger 1979 for de-scription of Clam Island) I compared adultbehavior in one section where my assistants

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HUMAN DISTURBANCE OF GULLSwalked through the colony for 30 min. everyother day (= disturbed) with a secondsection in front of my hide where no onewalked among the birds. These data weretaken in an area with bushes, where gullscould not see the intruder until he ap-proached, but they could see other gullsflying above the head of the intruder. Inthe disturbed section the gulls flew when myassistants were 10-12 m from the plot,whereas in the undisturbed section the gullsdid not fly when the assistants approached(but walked by) as close as 5 m. The gullsin the undisturbed section had apparentlyhabituated to the nearby presence ofhumans (who never came into their area),while the birds in the disturbed section re-sponded quickly to an approaching human.Although the birds in the undisturbedsection did not actually see the intruders,they did see the cloud of mobbing gullsabove the intruder's head, and apparentlyhabituated to this mobbing behavior.Disrupted Incubation

In most gulls the sexes spend equalamounts of time incubating eggs and brood-ing chicks (see Burger 1980a). Repeateddisturbance may disrupt this equal sharingof incubation and brooding activities if thesexes behave differently after a disturbance.In 1977 I watched 12 pairs of Herring Gullsfrom a blind (8-12 hr/day) during the in-cubating period. All birds were colormarked for individual identification. Thesebirds, in a disturbed section, were visitedevery other day for 30 min. during the incu-bation period. I saw 11 disturbances, whichprovided 132opportunities for the disturbedbirds to resume incubation after the dis-turbance. Males incubated after a dis-turbance more often than expected bychance (X2 = 13.2, df = 1, P < .01). Thatis, males incubating before a disturbanceusually resettled on nests, whereas whenfemales were disturbed while incubating,their mates (when present) resumed incuba-tion after the disturbance. Males did notallow females to incubate the eggs for sometime afterwards.

In Black Skimmers (Rynchops niger),both sexes are usually present during theday (Erwin 1977). While studying 11 nestsof skimmers at Cedar Beach, Long Island,

I found that males usually resumed incu-bating after a disturbance (x2=8.0, df=-1,P < .005, based on 36 exchanges). Pre-sumably, energy demands are similar forbirds incubating and for their mates stand-ing nearby. However, during the broodingphase, the non-brooding parent not onlyrests nearby, but chases intruders that landnear the nest, and periodically flies off toobtain food for the young. Thus, femalesdisplaced from brooding might be expend-ing more energy foraging and defendingnests than they would be doing in an un-disturbed colony.Aggressive Behavior

Human disturbance in gull colonies alsoincreases the rate of aggressive behaviortoward conspecifics as well as towardshuman intruders. I examined the changesin aggression rates of Herring Gulls towarda human intruder on Clam Island, NewJersey in 1978. I made observations eachweek from early April to June, from hidesin two habitats; grass (Spartina patens),and bushes (Iva sp.) As an intruder walkedsteadily through the area, I recorded thenumber of diving gulls, the mean heightof the dives above the intruder's head, thenumber of dives, and the number of attackson conspecifics in 1 min. samples (10 perhabitat). For both habitats the number ofdivers, dives, and conspecific attacks in-creased, while the mean height above theintruder's head decreased toward hatching(Fig. 1 and 2). For all measures, the re-

APRIL MAY JUNEFig. 1. Top: Mean height of Herring Gullsdiving at a human intruder in the bush habitat.Bottom: Number of Herring Gulls diving, thenumber of dives, and the number of conspecificattacks in bushes as a function of season.

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HUMAN DISTURBANCE OF GULLSBLACK-BACKED GULL

zLUCL.

HERRINGGULL

walk ground groundaerial aeriallong call chase fight chase fightgrasspullFig. 4. Behaviour of gulls toward gulls when ahuman was in the area checking nests (hatchedbar) compared to when no human was in the area(solid bar).

National Seashore, New York) I noted thatthe gulls seemed to respond differentiallyto the noises produced by supersonictransports compared to noises emitted byother kinds of aircraft. Thereafter, I re-corded the noise level, number of birdsflying over a prescribed section, and thenumber of fights/min. Whenever a plane

flew over the colony, and at 1 min. intervalsbeginning one minute after the plane hadpassed (Table 1). Fights involved physicalcontact, and usually lasted for 1-5 min. Idistinguished the supersonic transport(SST) from all other airplanes. These ob-servations showed that the noise levels werehigher, more birds flew overhead, and morefights occurred when the SST flew over thanwhen non-SST's flew over, or during periodswhen no air traffic occurred. My observa-tions were made during late incubationafter the gulls had had two to three monthsto habituate to the supersonic transportplanes (see Burger 1980b). During the chickstage, the passage of an SST often resultedin prolonged fights between adults, whosechicks were then exposed to attacks fromother neighbors or intruders not directlyinvolved in the territorial clashes. Severalresearchers have noted the effect of heli-copters on colonial birds when the aircraftflew too close to the colony (J. Rodgerspers comm, Kushlan 1979).These data indicate that when dis-turbed, gulls engage in more aggressionof a higher intensity. This aggression isdirected both at the intruder and towardsother gulls. Such interactions result in theireggs or young being unattended while theychased away another gull. Although theseaggressive interactions do not appear toalter territory size or shape, they nonethe-less involve an expenditure of energy andexpose the eggs or chicks to potential preda-tion.

TABLE 1. Behavior of nesting Herring Gulls when exposed to subsonic airplanes, supersonic trans-ports, and normal colony noises. Noise levels are in decibels on the A scale.

Non-SST'Normal Planes SST F2 pNoise Level (dB-A)Mean 77.0 + 3.7 91.8 + 3.6 108.2 ? 3.8 364.3 < 0.001Range 72-83 88- 101 101 -116Number of Birds Flying OverheadMean 11.7 + 3.3 11.0 _? 2.9 137 ? 40.5 173.6 < 0.001Range 8 -18 8 -16 36 - 220Number of Fights3Mean 0.1 ? 0.4 0.2 ? 0.3 6.3 + 1.6 121.3 < 0.001Range 0- 1 0-2 2 -7

lAll planes except supersonic transports (SST).2F value, analysis of variance3Involving physical contact.

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[Colonial WaterbirdsChick Mobility

Gulls are semiprecocial, and the youngcan stand and walk about as soon as theydry off after hatching. Thereafter chickmovement around and away from the nestvaries from species to species. In somespecies, such as Kittiwakes, Rissa tridactyla,(Cullen 1957), Brown-hooded Gull, L.maculipennis, and Franklin's Gulls, L.pipixcan (Burger 1974), the chicks do notnormally move from the nest until theyare almost ready to fly. Only when disturbedby humans will Franklin's and Brown-headed Gull chicks leave the nest to swim inthe water, where they often become en-tangled or lost in the vegetation. A singlewalk through an undisturbed Franklin'sGull colony resulted in two of 30 chicksdisappearing completely, and nine chicksending up on the wrong nest (they hadbeen banded previously within a day ofhatching). Several pairs ended up withmore chicks than they had before the dis-turbance. Similarly, supernormal broodswere noted in a Brown-hooded Gull colonyin Argentina disturbed only once duringthe chick phase (unpub. data). The creationof larger than normal broods places in-creased energy demands on the parents be-cause they must feed more young. InFranklin's Gulls, broods of over three rarelyfledged chicks, and when they did the chickswere so underweight that they died beforeleaving the vicinity of the colony (Burger1974). During heavy rain and hail storms,Franklin's Gull parents with more thanthree chicks were unable to keep thembrooded, and chicks died from exposure.

In some species the chicks remain nearthe nest on their parent's territories untilthey fledge (Kelp Gull, L. dominicanus,Herring Gull); while in others the parentsfrequently move broods from place to place(Ring-billed Gull, L. delawarensis, Evans1970, Conover and Miller 1978). In ground-nesting gulls the presence of humans ap-pears to increase the distance chicks runfrom the nest (Hunt 1972), and may resultin premature fledging. Although severalauthors have attributed decreases in repro-ductive success to this increased mobility, ithas not been carefully examined in gulls.Further, the effects of such mobility aredifficult to assess as displaced chicks either

may quickly return to their nests, or maybe killed enroute back, or they may getlost.Most investigators who are making nestchecks pick up chicks to band or weighthem. I examined the effect of human in-trusion on chick mobility in Herring Gullsnesting on Clam Island and on nearbyIslajo (see Burger 1977) and Carvel Islands(New Jersey) in 1975, 1977 and 1978. I com-pared the distance banded chicks were lo-cated from the nest for chicks that were dis-turbed (and handled) daily with chicks dis-turbed weekly (Carvel and Clam Islands;similar bush habitat). All study plots werein the centers of colonies to eliminate edgeposition effects, and as there were nodifferences in chick sizes I assumed agedifferences were minimal (see Ryder 1980).Chicks handled daily moved farther fromtheir nests than those handled weekly (Fig.5), the largest change in distance movedoccurred after 10 days of age.In order to assess the effect of handlingI also compared the movements of chickspicked up daily with those of chicks thatwere walked by but were not picked up(Clam Island, bush habitat, 1978). In twocases I dropped dye on the chicks (for ex-perimental purposes), but this did not causethem to run. In both experimental pro-cedures, the intruder spent equal time ineach study plot; only the handling varied.The response of Herring Gull chicks variedaccording to the procedure used (Fig. 6).Repeated handling resulted in the chicks

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0-5 '6-10 '11-15 ' 16-20 21-25126-30'31-35AGE OF CHICKSFig. 5. Location of Herring Gull chicks relativeto their nest locations: Daily = chicks were pickedup and weighed daily (bush habitat); Weekly =

chicks were picked up and weighed weekly (bushhabitat); Open mat = chicks from an openSpartina mat were observed from afar, and werenot picked up; Bush = chicks from a bush habitatwere observed from afar, and were not picked up.

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HANDLED

HANDLEDI I I i I 14 8 12 16 20 24 28 32AGE OF CHICKS

Fig. 6. Distance Herring Gull chicks werefoun I from the nest in the bush habitat. Chickswere located and were then handled (picked upand weighed) or not handled.running farther from the nest, rather thanhabituating to the handling. Interestingly,chicks that were not handled did not runwhen approached until they were older than20 days, whereas handled chicks ran almostimmediately when approached by a human.Some of the chicks that were not handleddid not run until they were 30 days old.

To assess the influence of habitat onchick mobility when disturbed by humans,I compared the movements of chicks nest-ing in bushes with those nesting on acompletely open Spartina alterniflora mat(on Islajo Island, 1975). In both cases Imade all observations from blinds enteredfrom the rear, and I never walked throughthe study plot after the chicks were banded.All nests in the study plot were syn-chronous, having been initiated withinthree days of one another, so I could bandall chicks at once. After being in the hidefor one hour I recorded the position of allchicks every 10 min. for at least six hourseach day (Fig. 5). Under these conditionsthere were no differences in where chicksnormally stood with respect to the nest,and chicks usually remained within 2m ofthe nest. Figure 5 shows a comparison ofchicks completely undisturbed, with thosehandled daily, and weekly in the bushhabitat.

Clearly, the nature and type of dis-turbance is extremely important, and needsto be described and, if necessary, controlledin all studies of breeding biology, not onlythose dealing specifically with investigator

effects. Chicks that remain within 2m oftheir nests are unlikely to encounterterritorial aggression or outright cannibal-ism from their neighbors. Further, nestinggulls that are not disturbed remain closeto their chicks and are able to defend themfrom intruders. The difference in mobilitybetween handled and unhandled chicks(exposed to the same human intruder)suggests that the investigator must weighthe advantage to his study of handlingchicks against the costs to the bird popula-tion in terms of lowered reproductivesuccess.

GENERAL DISCUSSIONIn addition to causing actual destructionof eggs, chicks, or adults, human dis-turbance can cause birds to abandon theirnest sites and colonies. In Black-crownedNight Herons (Nycticorax nycticorax),visits before or during egg-laying causedabandonment of nests and late-nesting birdsdid not settle in disturbed areas (Tremblayand Ellison 1979). Double-crested Cormor-ants (Phalacrocorax auritus) were similarlyaffected (Ellison and Cleary 1978). Suchabandonment can be permanent or tempor-ary. Permanent abandonment of nests seemsto occur more commonly during the egg-laying phase (Manuwal 1978).Gulls are particularly vulnerable tohuman disturbance prior to and during egg-laying. Conover and Miller (1978) foundthat when Ring-billed Gulls were firstsettling on the nesting island, they roostedon the water at night. During this stage, thegulls would not land on the colony ifhumans were present, and left for the re-mainder of the day if humans entered thecolony. Human disturbance during egg-laying resulted in nearby individuals flyingup, but the entire colony rose up whenadults were being captured. Continued cap-turing of adults during this stage causedcomplete desertion of the colony site. Gullswere less wary once incubation began. Inmy experience, Franklin's Gulls andLaughing Gulls L. atricilla, behave simi-larly. Repeated entering of blinds orcensusing of an area prior to egg-layingusually resulted in abandonment of thatnesting habitat (particularly if optimalhabitat were limited): such abandonmentsmay preclude nesting for that year.

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[Colonial WaterbirdsTemporary abandonment of nests andeggs, caused by disturbance often results inegg and chick loss because of thermal stress(Bartholomew et al. 1953, Bartholomew andDawson 1954, Vermeer 1963, Harris 1964,Drent 1967, Hunt 1972), predation (Kuryand Gochfeld 1975, Ellison and Cleary1978) and cannibalism (Harris 1964,Parsons 1971, Hunt and Hunt 1975, 1976).Most investigators working with gulls havenoted that cannibalism is the primary causeof mortality (see Parsons 1971) althoughcannibalism is not an important cause ofegg and chick mortality in Franklin's Gull(Burger 1974) and Glaucous-winged Gull,

L. glaucescens (Gillett et al. 1975). Thustemporary removal of adults from eggs andchicks can drastically reduce reproductivesuccess for most gulls.The effect of the experimenter on gulland tern colonies has been noted in passingby several authors (see Ashmole 1963,Kadlec and Drury 1968, Buckley andBuckley 1972) and most of them havefound deceased reproductive success withincreased human disturbance (but seeWeaver 1970). Hunt (1972) found thathatching success of Herring Gulls was lowerin disturbed areas (19-25%) compared toundisturbed areas (43-54%); a difference heattributed to thermal stress when eggs wereexposed. He did not find a difference inchick-rearing success. Similarly, Parsons(1975) found no difference in hatchingsuccess between an area visited twice dailyand one visited every other day. In Glau-cous-winged Gulls, however, chick deathswere higher in disturbed plots (27%) com-pared to undisturbed plots (11%) in 1973;and egg and chick deaths were higher indisturbed (31%) compared to undisturbed(8%) plots in 1972 (Gillett et alt. 1975). Inthese two studies the investigator effect wasnot the principle object of the study. Robertand Ralph (1975) conducted a controlledexperiment to examine the effect of the in-vestigator on Western Gulls, L. occidentalis.They had six types of disturbances rangingfrom being disturbed three times each day(30 min. each time) to being disturbed onlytwice during the entire season (called un-disturbed). They found that hatchingfailure was directly proportional to theamount of disturbance, whereas chickmortality was inversely related to the

amount of disturbance. They reasoned thatthe chicks that were disturbed more oftenhad become habituated to the investigatorand so did not run into nearby territories(where they could be killed or eaten). None-theless, when they compared the un-disturbed plot (visited twice in the season)with all disturbed plots, chick mortality wasless in the undisturbed plot. Althoughdisturbed gull chicks run from the nest ontheir own, and then attempt to return afterthe disturbance, adult terns lure their chicksfrom their nests when they are disturbed,and feed them in these new locations (seeVeen 1977).As is apparent from the above discus-sion, the effects of human disturbances aremany and varied. With several years ofdata on the effects of human disturbance Ifind that reproductive success is sometimeshigher and sometimes lower in completelyundisturbed study areas. Although it iseasy to demonstrate behavioral differencesin attentiveness, aggression rates, and chickmobility, the relationship between theseevents and reproductive success is less clear,and varies among species. Clearly, the in-

vestigator must evaluate how each pro-cedure affects the behavior and reproductivesuccess of the birds he is examining.

SUMMARYPeople affect nesting colonies of birdsin diverse ways. They can destroy the nests,eggs, or chicks or force birds to abandontheir nest sites and colonies. They can keepadults off their chicks and eggs, thus ex-

posing them to temperature stress or preda-tion. The above effects can be lethal toeggs or chicks, and result in immediate andobvious decreases in reproductive success.Many other effects of human disturbanceare less obvious, but nonetheless contributeto lowered overall breeding success. Sucheffects include; decreased incubation andchick attendance, shifts in the mate incubat-ing, earlier and distant movements of chicks,entanglement of chicks in vegetation, in-creased brood sizes (when several chicksenter the same nest), more frequent aggres-sive encounters, greater energy expendi-tures for territorial defense, and attractionof predators to nest sites.

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HUMAN DISTURBANCE OF GULLSACKNOWLEDGMENTS

Over the years I have had many fruitfuldiscussions on the effects of human dis-turbance on colonial birds with M. Goch-feld, F. G. Buckley, P. A. Buckley, M.Erwin, M. Fitch, J. Ryder, W. Southern, G.Shugart and C. Safina, and I now expressmy appreciation. I thank the followingorganizations and people for support whileworking on colonial birds: Frank M. Chap-man Fund of the American Museum ofNatural History (Franklin's Gull), Ameri-can Association of University Women(Brown-hooded Gull), Rutger's ResearchCouncil (Herring Gull, Laughing Gull),P. A. Buckley and the North Atlantic Re-gional Office of the National Park Service(Herring Gull, Contract #CX 1600-8-0007),The Center for Coastal and EnvironmentalStudies of Rutgers University (BlackSkimmer), and P. McGill-Harelstadt, S.Lewis, and the Shoals Marine Laboratory(Herring Gull, Great Black-backed Gull).I am grateful for the comments of the re-viewers, 0. Austin and J. Rodgers.

LITERATURE CITEDASHMOLE, N. P. 1963. The biology of the Wide-awake or Sooty Tern Sterna fuscata on AscensionIsland. Ibis 103B: 297-364.BARTHOLOMEW, G. A., W. R. DAWSON, && E. J. O'NEILL. 1953. A field study of tempera-ture regulation in young White Pelicans,Pelecanus erythrorhynchos Ecology 34: 554-560.BARTHOLOMEW, G. A., &W. R. DAWSON. 1954.Temperature regulation in young pelicans,herons and gulls. Ecology 35: 466-472.BUCKLEY, F. G., & P. A. BUCKLEY. 1972. Thebreeding biology of the Royal Terns Sterna

(Thalasseus) maxima maxima. Ibis 114: 344-359.BURGER, J. 1974. Breeding adaptations ofFranklin's Gull (Larus pipixcan) to a marshhabitat. Anim. Behav. 22: 521-567.BURGER, J. 1977. Nesting behavior of HerringGulls: Invasion into Spartia salt marsh areas ofNew Jersey. Condor 79: 162-169.BURGER, J. 1979. Colony size: A test for breedingsynchrony in Herring Gull (Larus argentatus)colonies. Auk 96:694-703.BURGER, J. 1980a. On becoming independent inHerring Gulls: Parent-young conflict. Am.Naturalist 117: 444-456.BURGER, J. 1980b. Behavioural responses ofHerring Gulls Larus argentatus to aircraft noises.Environ. Pollution. 24: 177-184.CONOVER, M. R., & D. E. MILLER. 1979. Re-action of Ring-billed Gulls to predators andhuman disturbance at their breeding colonies.Proc. 1978' Conf. Colonial Waterbird Group 2:41-47,

CULLEN, E. 1957. Adaptations in the Kittiwake tocliff-nesting. Ibis 99: 275-303.DRENT, R. H. 1967. Functional aspects of incuba-tion in the Herring Gull (Larus argentatusPont.). Behav. Suppl. 17: 1-132.ELLISON, L. N., & L. CLEARY. 1978. Effects ofhuman disturbance on breeding of Double-crested Cormorants. Auk 95: 510-517.ERWIN, R. M. 1977. Black Skimmer breedingecology and behavior. Auk 94: 709-717.EVANS, R. M. 1970. Imprinting and mobility inyoung Ring-billed Gulls, Larus delawarensis.Anim. Behav. Monogr. 3: 193-248.GILLETT, W. H., J. L. HAYWARD, JR., & J. F.STOUT. 1975. Effects of human activity on eggand chick mortality in a Glaucous-winged Gullcolony. Condor 77: 492-495.HARRIS, M. P. 1964. Aspects of the breedingbiology of the gulls Larus argentatus, L. fuscusand L. marinus. Ibis 106: 432-456.HUNT, G. L., JR. 1972. Influence of food distribu-tion and human disturbance on the reproduc-tive success of Herring Gulls. Ecology 53: 1051-1061.HUNT, G. L., JR., & M. W. HUNT. 1975. Re-productive ecology of the Western Gull: theimportance of nest spacing. Auk 92: 270-279.HUNT, G. L., JR., & M. W. HUNT. 1976. Gullchick survival: the significance of growth rates,timing of breeding, and territory size. Ecology55: 62-75.KADLEC, J. A., & W. H. DRURY. 1968. Structureof the New England Herring Gull population.Ecology 49: 644-676.KURY, C. R., & M. GOCHFELD. 1975. Humaninterference and gull predation in Cormorantcolonies. Biol. Conservation 8: 23-34.KUSHLAN, J. 1979. Effects of helicopter censuseson wading bird colonies. J. Wildl. Mgmt. 43:756-760.MANUWAL, D. A. 1978. Effect of man on marinebirds: a review. pp. 140-160 in Wildlife andPeople. Purdue Univ. Press. West Lafayette, IN.MCGILL, P. 1977. Breeding ecology and competi-tion between Great Black-backed and HerringGulls. Unpubl. Ph.D. Thesis, Cornell Univ.,Ithaca.

PARSONS, J. 1971. Cannibalism in Herring Gulls.Brit. Birds 64: 528-537.PARSON, J. 1975. Seasonal variation in the breed-ing success of the Herring Gull: An experimentalapproach to pre-fledgling success. J. Anim. Ecol.44: 553-573.ROBERT, H. C., & C. J. RALPH. 1975. Effects ofhuman disturbance on the breeding success ofgulls. Condor 77: 490-495.RYDER, J. 1980. The influence of age on thebreeding biology of colonial nesting birds. Pp.153-168 in Behavior of Marine Animals. (J.Burger, B. L. Olla, H. E. Winn, Eds.). Vol. 4.Marine Birds. Plenum Press, New York.TREMBLAY, J., & L. N. ELLISON. 1979. Effectsof human disturbance on breeding of Black-crowned Night Herons. Auk 96: 364-369.VEEN, J. 1977. Functional and causal aspects ofnest distribution in colonies of the SandwichTern (Sterna sandvicensis Lath.). Behav. Suppl.

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[Colonial Waterbirds20: 1-193.

VERMEER, K. 1963. The breeding ecology of theGlaucous-winged Gull (Larus glaucescens) onMandarte Island, B.C. Occ. Pap. Brit. ColumbiaProvincial Mus. No. 13: 1-104.

WEAVER, D. K. 1970. Parental behavior of theHerring Gull (Larus argentatus smithsonianus)and its effect on reproductive success. Unpubl.Ph.D. Thesis, University of Michigan, AnnArbor.

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Effects of Human Disturbance on Colonial Species, Particularly Gulls (1981)

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