Brown-headed Cowbirds and Willow Flycatchers

Natural History of CowbirdsNatural History of CowbirdsEffects on Willow FlycatcherEffects on Willow FlycatcherManagement IssuesManagement Issues

BROWN-HEADED COWBIRD(Molothrus ater)

BROWN-HEADED COWBIRD(Molothrus ater)

Natural History of Cowbirds

BroodBrood--parasiteparasiteExpansion of DistributionExpansion of Distribution

Cowbird colonization of the Far West,Rothstein 1994

Natural History of Cowbirds

Migratory and ResidentMigratory and ResidentBreeding SeasonBreeding Season

Natural History of Cowbirds

Breeding SeasonBreeding SeasonEarly AM activityEarly AM activityNest parasitismNest parasitismProlific eggProlific egg--layerslayersParasitize openParasitize open--cup nesterscup nestersSeparate feeding and breeding areasSeparate feeding and breeding areas

Cowbird Abundance & Parasitism RatesKern River Population

0.00.20.40.60.81.0

0 0.5 1 1.5 2

Point Ct

Para

sitis

m R

ate

Do cowbirds affect Southwestern Willow Flycatchers?

Potential Effects of Parasitism on Willow Flycatchers

Nest abandonmentNest abandonmentEgg LossEgg LossNestling LossNestling LossPopulation effects?Population effects?

Parasitism and Desertion rates of parasitized Willow Flycatchers is highly VARIABLE in different regions.

RegionParasitism

rate (N)Desertion rate (N) Reference

California 68% (19) 57% (14) Harris 1991

California 63% (60) 45% (38) Whitfield 1990

New Mexico

22% (129) 35% (26) Stoleson & Finch 1999

Arizona 7% (2034) 36% (14) Paradzick et al. 1999

Potential Effects of Parasitism on Willow Flycatchers

Nest abandonmentNest abandonmentEgg LossEgg LossNestling LossNestling LossPopulation effects?Population effects?

How often flycatcher young are produced in parasitized nests

Arizona: 4 out of 31 nests (13%)Arizona: 4 out of 31 nests (13%)California: 18 out of 133 nests (14%)California: 18 out of 133 nests (14%)

Kus et al. 2006

Parasitism and SWFL Productivity

0.0 20.0 40.0 60.0 80.00.0

1.0

2.0

3.0

4.0

5.0

Num

ber F

ledg

lings

per

Pai

r

PENKERN

Kern

y = - 0.01x + 1.73

Percent of Nests Parasitized

Potential Effects of Parasitism on Willow Flycatchers

Nest abandonmentNest abandonmentEgg LossEgg LossNestling LossNestling LossPopulation effects?Population effects?

Management practices typically prescribed to help populations of endangered host species

Cowbird TrappingCowbird TrappingRemove cattleRemove cattle

Management practices typically prescribed to help populations of endangered host species

Cowbird TrappingCowbird TrappingRemove cattle or livestockRemove cattle or livestock

Experiments in Cowbird Removal Programs

WIFL populations and Cowbird Trapping-NO PATTERN

WIFL Pairs

0204060

1 2 3 4 5 6Num

ber o

f Pai

rs

Sites w/ trapping NO trapping

SWFL Population Size

(c)

1989 1992 1995 1998 2001 2004

Year

0

10

20

30

40

50

Num

ber o

f Ter

ritor

ies PEN

KERN

Cowbird Control Initiated

Southern CA Cowbird Trapping2004 (Famolaro summary, Sweetwater)

Counties: Riverside, Orange, San Counties: Riverside, Orange, San Bernardino and San DiegoBernardino and San Diego180 traps180 trapsAbout 5900 cowbirds trappedAbout 5900 cowbirds trappedAbout 730 EUST and HOSP trappedAbout 730 EUST and HOSP trappedAbout 18,350* About 18,350* NontargetNontarget species releasedspecies releasedAbout 200 About 200 NontargetNontarget ‘‘takentaken’’

Possible impacts of cowbird control:

Effective cowbird trapping removes the Effective cowbird trapping removes the selective pressure of parasitism necessary to selective pressure of parasitism necessary to promote the selection of promote the selection of antiparasiticantiparasiticdefense mechanismsdefense mechanisms

Economic, political, ethicalEconomic, political, ethical

Cowbird controlis effective in reducing parasitism and increasing annual productivityproduces population increases in somespeciesis effective only as long as suitable habitat is available

Should be thought of as a short-term tool for managing T&E species

Management practices typically prescribed to help populations of endangered host species

Cowbird TrappingCowbird TrappingRemove cattle or livestockRemove cattle or livestock

Cowbirds and Livestock

Cowbirds on Los Padres NF

2002 Results

with Cowbird ‘Chatter’ Playback

Not just livestock…In a centralIn a central--southern California forest, southern California forest, cowbirds are not associated with livestockcowbirds are not associated with livestockHowever, cowbirds ARE associated with However, cowbirds ARE associated with passerine birds first, and second, urban passerine birds first, and second, urban landland--use.use.The availability of host birds and cowbird The availability of host birds and cowbird feeding sites likely affect cowbird feeding sites likely affect cowbird distribution.distribution.

SUMMARYCowbird effects on willow Cowbird effects on willow

flycatchers are siteflycatchers are site--specific and specific and temporal.temporal.

Cowbird management Cowbird management recommendations must also be siterecommendations must also be site--specific and should be reviewed.specific and should be reviewed.

Cowbird control can help in the Cowbird control can help in the shortshort--term, but recovery of SWFL is term, but recovery of SWFL is *not* dependent on cowbird control.*not* dependent on cowbird control.