Author: Vincent Muehter
Reviewers: Stephen Rothstein, Jamie Smith, Frank Gill
Photograph from Breeding Bird Survey, Patuxent Wildlife Research Center
The Brown-headed Cowbird (Molothrus ater) is North America's most notorious brood parasite. Instead of building their own nests, incubating their own eggs and raising their own nestlings, Brown-headed Cowbirds have a different breeding strategy. Cowbird females use other bird species as hosts -- laying their eggs in the nests of other bird species and relying on these hosts to incubate and raise their chicks. Scientists have now recorded that Brown-headed Cowbirds have parasitized over 220 host species, ranging from the Black-capped Vireo and Wood Thrush to the Blue-winged Teal and Red-headed Woodpecker. While not all hosts make good foster parents -- a number of species reject cowbird eggs -- cowbird chicks have been successfully reared by over 150 host species, with songbirds comprising the majority of hosts.
Brown-headed Cowbirds occupy most of North America south of the Arctic, but this large range has occurred only recently and is the result of human-induced factors. Prior to the arrival of Europeans in North America, cowbirds were nomadic, following the large herds of bison that roamed across the Great Plains. Cowbirds foraged on the insects and prairie grass seed stirred up by bison as they made their seasonal migrations north and south. When bison were nearly extirpated from the North American landscape and replaced by cattle, sheep and goats, cowbirds adapted and began to associate with livestock. In the last century, Brown-headed Cowbirds have experienced massive range expansions and population explosions as forests have been opened to make way for agricultural and suburban landscapes. Shadowing their livestock-associates, cowbirds occur in regions where bison did not previously exist and forage in the grain crops, feed lots and grain silos often found nearby livestock.
In recent decades, many land managers, conservationists and citizens have argued that parasitism by Brown-headed Cowbirds is a major threat to North American songbird populations and that cowbird parasitism is responsible for the range-wide population declines currently shown in a number of songbird species, particularly neotropical migrants. In response, various federal and local government non-game agencies have initiated programs to control cowbird numbers over the past 25 years.
Herein, Audubon Science reviews the most current evidence on the extent, if any, to which cowbirds harm bird populations in North America in order to ensure that Audubon policies regarding cowbird control programs are based on sound science. This white paper summarizes the results and take-home messages of the recent conference, Research and Management of the Brown-Headed Cowbird in Western and Eastern Landscapes, organized by Partners in Flight on 23-25 October 1997, Sacramento, California. At this conference, over 75 ornithologists, ecologists and land managers presented their most recent work covering a broad array of important topics, including the management of cowbird and host populations, ecological correlates of habitat types and parasitism and the evolution of host-parasite interactions. Vincent Muehter, Associate Director of Bird Conservation and Jesse Grantham, Executive Director of Mississippi State Office attended on behalf of Audubon.
- Cowbird populations are declining across the continent. Despite recent range extensions into Florida and some local areas of increase, the Breeding Bird Survey shows that cowbirds declined about 1% per year between 1966-1996. This dispels the widely held notion by the public, and even scientific community, that cowbirds are increasing.
- Rates of nest parasitism vary locally: when rates are high, parasitism may harm local populations of some species. The extent of parasitism varies with local land-use practices, habitat, and with the abundance, breeding behavior and conservation status of different host species. Some of the variation in parasitism rates is due to unknown factors.
- Black-capped Vireo: 90% of nests parasitized in open canyon woodlands
on Fort Hood, Texas (Eckrich et al.).
- Bell's Vireo and Yellow-breasted Chat: 80-90% of nests parasitized in desert riparian habitat in the lower Colorado River valley (Averill et al.).
- Lazuli Buntings: 50% of nests parasitized in shrubland prairie habitat in western Montana (Greene and Muehter).
- Host species often renest and are able to make-up reproductive success lost to parasitism. Contrary to previous notions, there is often little net loss in host reproductive success over an entire breeding season.
- Mandarte Island, British Columbia: 56% of Song Sparrows nests were parasitized, yet there was only a 5% reduction nest success over the entire breeding season when compared to a control site (Smith, pers. comm.).
- Sacramento River watershed: 83% of Lazuli Bunting nests were parasitized, yet these nests were no less successful than nonparasitized nests (Nur et al.).
- Southwest Colorado: 36% of Yellow Warbler nests were parasitized, but parasitized nests were as successful as nonparasitized nests (Ortega et al.).
- Hosts with short breeding periods and those that begin the season raising cowbirds may not have enough time to renest.
- Cowbird parasitism probably is not responsible for the continent-wide declines of many North American songbird species. Conservationists and the public tend to overestimate the significance of parasitism as a major cause of declining songbird populations. A recent study failed to show population-level effects of cowbird parasitism on host species and refuted key predictions about impact of parasitism. This research was based on the reasonable premises that (1) host populations should decrease in areas where cowbirds are increasing, and increase in areas where cowbirds decreasing (Breeding Bird Survey data); and (2) heavily parasitized hosts should decrease while less commonly used hosts should increase or maintain stable populations (Ontario Nest Record data) (Mitchell and Rothstein).
- Rates of parasitism depend on the proximity of cowbird feeding sites to host breeding sites. Cowbirds search for hosts near woodland edges and feed in agricultural/residential spots like grazing yards, grain silos, pack stations and home bird feeders. The closer such cowbird foraging areas are to host breeding habitats, the more likely hosts will suffer cowbird parasitism (Halterman and Laymon, Tewksbury et al.).
- Rates of parasitism sometimes correlate poorly with numbers of cowbirds counted in an area. Managers should differentiate between data showing cowbird numbers and distributions (as is obtained through point-counts) and data showing actual rates of parasitism (as is obtained through nesting ecology studies) (Geupel et al., Halterman and Laymon).
- Cowbirds are managed through lethal control -- trapping and killing of adults, removal of cowbird eggs from host nests, and the shooting of cowbirds at roosting sites. Trapping is seen as the most efficient tool for removing large numbers of cowbirds.
- Cowbird control is controversial. Land managers promote control, but most scientists support control only to restore local populations of high priority species (sensu Threatened and Endangered species). Scientists suggest there is little evidence that cowbirds have population-level impacts on hosts. Further, they point out that control has limited impact on reducing parasitism on a broad geographic scale. Traps must be located where cowbirds congregate, like in feeding areas, and trapping appears to reduce parasitism mostly near such trap locations. The farther hosts are from the traps, the less likely they will benefit from control.
- Cowbird control is expensive.
- Bell's Vireo: over $665K/yr (partially funded by permanent endowment. 225 traps /yr, ~ 3,000 cowbirds killed/trap).
- Kirtland's Warbler: $90K/yr (funded by USFWS).
- Black-capped Vireo: $45K/yr (funding source unknown).
- Cowbird control programs have proceeded without a general framework, with little coordination between programs, or between the land management and scientific communities. There has been little attempt to regularly measure progress and financial costs since control programs began three decades ago. Many agencies have been involved in control initiatives -- USFWS (permitting and funding), USDA (trapping and killing), BRD (research), BLM (habitat management), Bureau of Reclamation (funding). The absence of a central authority has fueled "red-tape" and made assessments of costs and long-term effectiveness difficult (Hahn).
- Cowbird control can reduce rates of parasitism on a local scale:
- Black-capped Vireo: rate of parasitized nests down from 90% in 1987
to 22% in 1996 (Eckrich et al.).
- Southwestern Willow Flycatcher: rate of parasitized nests down from 64% in 1993 to 11% in 1996, and host nest success increased from 20% to 61% during same period (Enos et al.).
- Black-capped Vireo: rate of parasitized nests reduced from 90% in 1987 to <25% in 1996, and host nest success increased from 3.0% to >40% during same period (Weinberg et al.)
- No research has tested the effectiveness of large-scale control on the wintering grounds, where cowbirds congregate in large roosts. However, most scientists suggest that winter control is unnecessary since it could harm non-target species that flock with cowbirds, and there is little evidence that cowbirds have population-level impacts on hosts. A few scientists and land managers suggest that winter control is an option that should be tested for its long-term effectiveness at reducing parasitism continent-wide, provided that such an undertaking is coordinated by an overarching body.
- Cowbird control programs on Endangered Species have had some success in meeting their ultimate goal: increasing local host populations. In the Least Bell's Vireo, populations increased following cowbird control and efforts to improve breeding habitat. In the Kirtland's Warbler, cowbird control and habitat restoration combined to bring populations in Michigan back from 200 breeding pairs in 1972 to about 400 breeding pairs in 1998 (Mike DeCapita, USFWS, pers. comm.). However, four years of cowbird control have not helped restore Willow Flycatcher populations in California, suggesting that habitat, not cowbirds, is the key limiting factor (Rothstein pers. comm.).
- Scientists suggest that cowbird control is a short-term solution that ignores the real problem of habitat degradation as a result of agriculture, grazing and development. They cite studies showing limited geographical reach of control and those showing no long-term benefit without indefinite support. Scientists caution against diverting limited human and financial resources to cowbirds and neglecting the root causes of why species are at-risk. Scientists, however, support limited control to help restore local populations of Threatened or Endangered species.
- Scientists advocate protection and restoration of host breeding habitat, and improvements in grazing and agricultural practices.
- Cowbird control initiatives should answer three practical questions: (1) Does control fix the root causes driving high rates of parasitism? (2) Do the benefits outweigh the inevitable long-term financial costs? (3) Is control an effective "stop-gap" measure to keep an endangered species viable until root problems can be corrected?
- Cowbirds occur most often in agricultural/residential landscapes near open woodlands. Cowbirds frequent woodland edges created when deforestation leads to a mosaic of trees and open brush/grassland. In the west, cowbirds strongly prefer riparian deciduous woodlands near agricultural/residential landscapes.
- Cowbirds rarely occur near continuous forests, deciduous or coniferous.
- Large, contiguous forests sustain lower rates of parasitism than fragmented forests. This is because cowbirds (1) scan for hosts at forest edges, rarely in forest interiors; and (2) fragmented forests have proportionally more edge than contiguous forests, creating small woodlots that are easy for cowbirds to penetrate.
- In mixed landscapes, cowbirds are more common at woodland edges than in prairies, grasslands and shrubsteppes. Cowbirds can be common in treeless, grassland habitats and actually reach their peak abundance in such habitats in the northern Great Plains. However, recent data suggests that grassland habitats tend to have fewer accepting host species than forests, so parasitism rates tend to be lower than in forests. Grassland habitats also lack perch sites that female cowbirds use to scan for nesting hosts. (Peer et al., Van der Haegen and Walker).
- COMING SOON
the National Cowbird Advisory Council�s Home Page
- Ecology and Management of Cowbirds. In Studies in the Conservation
of North American Passerine Birds (J.N.M. Smith, T. Cook. S.I.
Rothstein, S.G. Sealy, S.K. Robinson, eds.). This will be the most comprehensive
work on cowbirds and is result of the 1993 North American Research Group.
Contact: Sharon L. Casteel, Assistant Editor, University of Texas Press.
- Conference Proceedings. Research and Management of the Brown-headed
Cowbird in Western Landscapes. 1999. ML Morrison, LS Hall, SK Robinson,
SI Rothstein, DC Hahn, and TD Rich. Studies in Avian Biology 18: 1-312.
Cooper Ornithological Society [$25.00 from COS, c/o Western Foundation
of Vertebrate Zoology, 439 Calle San Pablo, Camarillo, CA 93010]
- Laymon, S.A. 1987. Brown-headed Cowbirds in California: historical
perspectives and management opportunities in riparian habitats. West.
- Lowther, P.E. 1993. Brown-headed Cowbird (Molothrus ater ). In The
Birds of North America, No. 47 (A. Poole and F. Gill, Eds.).
Philadelphia: The Academy of Natural Sciences; Washington, D.C.: The
American Ornithologists' Union.
- Rothstein, S.I. 1994. The cowbird's invasion of the Far West: history,
causes and consequences experienced by host species. Studies in
Avian Biology 15:301-315.
In addition to top academic credentials, each expert below has a practical, science-based, perspective on how and when to manage cowbirds, and on the need to and effectiveness of controlling cowbirds.
References (all references from conference except where noted)
- Averill, A., Lynn, S., & Morrison, M. L. Cowbird parasitism of Bell's Vireos and Yellow-breasted Chats in a desert riparian landscape: Implications for cowbird management and riparian restoration.
- Cook, T. L., Koloszar, J. A. & Goernig, M. D. Management implications of Cowbird behavior and movement relative to the distribution of cattle.
- Eckrich, G.H. and T.E. Koloszar. Effective Management of Brown-headed Cowbirds on the Fort Hood Military Reservation.
- Enos, K. M., Whitfield, M. J., & Rowe, S. P. Is Brown-headed Cowbird removal an effective management tool for the recovery of the Endangered Southwestern Willow Flycatcher?
- Geupel, G. R., Nur, N. & Ballard, G. The impact of Brown-headed Cowbirds on riparian bird communities of the Central Valley of California.
- Goguen, C. B. & Mathews, N. E. Cowbird parasitism and behavior in a grazed and ungrazed lanscape in New Mexico.
- Greene, E. & Muehter, V. R. 1996. Lazuli Bunting species account. In The Birds of North America. American Ornithologists' Union and the Philadelphia Academy of Sciences.
- Griffith, J. T. & Griffith, J. C. Brown-headed cowbird control: The next level.
- Grzybowski, J. A. & Pease, C. M. Application of Pease-Grzybowski seasonal fecundity models in management of Black-capped Vireos.
- Hahn, D.C. Synthesis of directions in cowbird management.
- Halterman, M. D. & Laymon, S. A. The impacts of the parasitic Brown-headed Cowbird on Neotropical migrants in eight western National Parks.
- Mitchell, D. & S. I. Rothstein. The use of breeding bird survey data to assess cowbird effects on widespread host species.
- Naef, V. L. Brown-headed Cowbird distribution patterns in agricultural and contiguously-forested landscapes of the Skagit Valley, Washington State.
- Nur, N., Gardali, T. & Geupel, G. R. Modeling the impact of cowbirds on Lazuli Buntings: The source/sink equation in the Central Valley of California.
- Ortega, J. C., Ortega, C. P., Allerton, S., Backensto, S. A., Rapp, C. A. & Vorisek, S. The effect of Brown-headed Cowbird parasitism on Yellow Warblers in Southwest Colorado.
- Peer, B. D., Robinson, S. K. & Herket, J. R. Low frequency of observed cowbird parasitism on grassland hosts in Illinois: a result of egg rejection?
- Robinson, S. K., Brawn, J. D., Herket, J. L. & Morse, S. Habitat differences in levels of cowbird parasitism in Illinois.
- Smith, J. N. M. & Taitt, M. J. Cowbird removal sharply lowers nest failure rates in Song Sparrow.
- Staab, C.A. and M.L. Morrison. Host and nest selection by Brown-headed Cowbirds.
- Tewksbury, J. J., Redmond, T. R. & Wheller, J. Landscape context, species-habitat relationships and the effect of parasitism on regional host populations.
- Vander Haegen, W. M. & Walker, B. Parasitism by Brown-headed Cowbirds in shrubsteppe communities of eastern Washington.
- Weinberg, H. J., Hayden, T. J. & Cornelius, J. D. The effects of cowbird control on the nest success and nesting seasonality of the endangered Black-capped Vireo.