NC-PIF Fact Sheet

Cowbirds vs. Songbirds….and the winner is?
by James Sasser and  Mark Johns

America’s best known brood parasite is the Brown-headed Cowbird (Molothrus ater).  This bird lays its eggs in the nests of many different bird species, leaving its young to be raised by others.  Originally, cowbirds were called “buffalo birds” and were limited to short-grass plains.  There they followed herds of buffalo (Bison bison), feeding on the insects stirred up by the movement of the herds.  The Brown-headed Cowbird has since dispersed widely over other parts of North America as European settlement opened forests and helped create the agricultural and suburban landscapes we have today.

Probably no other bird today elicits such loathing as the Brown-headed Cowbird.  Many think the bird should be on the ‘10 most-wanted’ list at local post offices and courthouses.  This is interesting, since cowbirds are just doing what they have always done.  Humans have created the situations that allow cowbirds to prosper, and many more opportunities now exist for cowbirds to easily lay in other birds nests. In short, people have created “cowbird heaven” in many areas of the country.

Cowbirds were initially at home with the expansive herds of buffaloes (and later even cattle) that once roamed the prairies and mostly treeless plains of the American west.  Following the herds caused cowbirds to become a very nomadic species of open country.  This made it tough for cowbirds to establish territories, build a nest, lay or incubate eggs and feed young.  What was a mom on the go to do?  Simple, just get a nanny.  Unfortunately the so-called nanny is a victim of breaking and entering, but with a twist.

Basically, cowbirds evolved to become a nest parasite to be able to successfully reproduce.  Female cowbirds watch for other birds building their nests or sitting on nests.  They are very good at finding the nesting site of other birds.  Sometimes cowbirds actually “stalk” other birds and follow them back to their nests.  They also sit up high above the nesting activity of most birds and wait and watch, as well as maneuver through different layers of vegetation (or on the ground) looking for nests.  To summarize: cowbirds are good at finding bird nests!  Much better than people are at finding nests.

Once they locate another bird’s nest, the really interesting stuff starts to happen.  They wait and watch, and when the other female bird leaves the nest to water, feed or defecate, the cowbird springs to action.  It enters the nest and can do a couple of different things.  The cowbird can remove an egg from the nest and lay one of its own, just quickly lay one of its own eggs, or just evict an egg from the nest.  Cowbirds have also been documented killing nestlings.  This behavior can continue for several days.  The goal of the cowbird is to get as many of its eggs in the nest as possible, and evict as many of the other bird’s eggs as possible.  Most birds do not recognize that changes have occurred in their nest and continue to lay and/or incubate as normal.  Female cowbirds can lay many more eggs per season than most birds. Many lay 40 or more eggs each season.  Keep in mind that each female cowbird is laying eggs in more than one nest at a time during the breeding season, often several each summer.

Young cowbirds have several advantages.  They hatch before the host bird’s young do, with an average incubation period of about 12 days.  Cowbirds also hatch out larger than the small songbird hosts, and grows at a faster rate.  This enables the young cowbirds to often “hog” all the food brought to the nest, as they can simply push their way into position to get fed more often.  The host parents are often strained to try and feed the huge cowbird young and their own brood.  Often the host’s young starve, or are even pushed out of the nest by the aggressive cowbird youngsters.  Over 220 bird species have been reported as being parasitized (cowbird victims), and almost 150 species have actually raised (cowbird hosts) cowbird young.

  Some birds are victimized quite often by cowbirds.  Some of the most common cowbird “hosts” include: Yellow Warbler, Song Sparrow, Red-eyed Vireo, Chipping Sparrow, Eastern Towhee, Ovenbird, Common Yellowthroat, American Redstart, Indigo Bunting, Kentucky Warbler and Field Sparrow.  The Brown Thrasher is one of the few larger birds commonly parasitized.  Certain birds seem to recognize cowbird eggs and commonly remove them from their nests like American Robins and Gray Catbirds.  Yellow Warblers have been documented building new nests on top of cowbird eggs, and Eastern Phoebes will often desert a nest that cowbirds parasitize.

  Many of the species that commonly “reject” cowbirds have evolved with cowbirds over long periods of time in the western part of North America, like the American Robin, Gray Catbird, Cedar Waxwing and Blue Jay.  For many smaller migratory species like warblers, tanagers and vireos, exposure to cowbirds has been a fairly recent phenomenon in the east.  For most of these birds, cowbirds are just one more problem that complicates their reproductive strategies in North America.

  A few things have contributed to the problems our native songbirds are having with cowbirds today.  For one thing, we eliminated the large buffalo herds of the west, and also cleared huge tracts of forests in the east to replace them with large open areas for homes, crops and pastures for the exotic “buffalo”, the cow.  The Brown-headed Cowbird has adapted quite well to this new open habitat, complete in some cases with cattle, but also often with agricultural lands nearby for easy feeding.  Cowbirds even feed easily in grass lawns in suburban situations.  In addition, the vast forests of some parts of the east that buffered forest songbirds are now gone, or highly fragmented for easy cowbird access.  Add cowbird parasitism to go along with migration barriers like buildings and towers, feral and free-ranging cats and other predators, deforestation in the tropics, and loss of habitat to suburban sprawl here in eastern North America, and problems abound for migratory songbirds.  Many researchers are actually now convinced that predators are the major source of nesting mortality for most open-cup nesting birds.

  Solutions to cowbird are not easy.  Live trapping or poisoning winter flocks is expensive and likely kills the wrong individuals.  Most cowbirds migrate between breeding and wintering grounds, with wintering populations dispersing widely across the U.S. and Canada to breed.  Researchers have found that only a minute percentage of birds in any winter flock would breed in the same location.  Winter control, therefore, would not target the individuals that breed in that area.  In addition, other bird species can be killed with winter trapping or poisoning of cowbirds.

  Small scale habitat management where cowbirds are determined to be a problem may include reduction of long linear forest edges, removal of tall perch snags or trees used by cowbirds, and placing wildlife food plots near other openings like roads or power line right-of-ways.  In areas with high fragmentation of forest or grassland habitat due to agriculture, limiting the number of artificial forest openings may be necessary.

  In North Carolina, cowbirds are very common in parts of the Coastal Plain and Piedmont, but seem to be less of a problem in many parts of the more forested mountain areas of the state.  Higher elevations especially seem to have less cowbird problems.  Fragmentation of habitats to create a mosaic of openings along with feeding areas seems to benefit cowbirds.  Long term, comprehensive land management with small-scale options may be our best hope of controlling cowbird problems.  In the fight of the 21st century, Brown-headed Cowbirds vs. Songbirds, we will determine who wins.  We should consider our options carefully, using the best scientific information we have to make management decisions that will benefit migratory songbirds.

QUICK COWBIRD FACTS

-Over 220 different bird species have been parasitized (cowbird hosts) and about 150 different species have actually raised cowbird young (cowbird hosts)

-Cowbirds find nest in 3 main ways: perch and watch, walk on the ground, and search and short flushing flights in vegetation

-Cowbirds lay up to 7 eggs in one day and can live up to 10-15 years (most don’t)

-Estimates of cowbird numbers range up to 60 million birds

-Females may lay up to 40 eggs per season, and can lay almost daily through the breeding season

-Song development by males likely in flocks at winter roosts

-Forage on the ground, rarely in vegetation

-Main foods taken are seeds and arthropods (about 75% seeds and 25% arthropods) like beetles and grasshoppers

-In migration and winter cowbirds are often part of mixed-species flocks containing Red-winged Blackbirds, Common Grackles, European Starlings and even American Robins

Suggested Readings for More Information on Cowbirds

Airola, D.A. 1986. Brown-headed Cowbird Parasitism and Habitat Disturbance in the Sierra Nevada. Journal of Wildlife Management. 50(4): 571-575.

Brittingham, M.C. and S.A. Temple. 1996. Vegetation around Parasitized and Non-Parasitized Nests Within Deciduous Forest. J. of Field Ornithology, 67(3): 406-413.

Carrie, N.R. 1999. Brown-headed Cowbird Parasitism of Wood Thrush Nests in Eastern Texas. J. of Field Ornithology, 70(2): 263-267.

Carter, M.D. 1986. The Parasitic Behavior of the Bronzed Cowbird in South Texas. The Condor 88: 11-25.

Coker, D.R. and D.E. Capen. 1995. Landscape-level habitat use by Brown-headed Cowbirds in Vermont. Journal of Wildlife Management 59: 631-637.

Condon, Marlene. 1996. Cowbirds Aren’t Evil. Living Bird. Spring 1996. pp. 8-9.

Coon, D.W., and K.A. Arnold. 1977. Origins of Brown-headed Cowbird populations wintering in central Texas. North American Bird Banding, (2): 7-11.

Cooper, S. 1990. North Carolina’s first shiny cowbird (Molothrus bonarienses), Chat 54: 28-34.

Curson, D.R, C.B. Goguen and N.E. Mathews. 2000. Long distance Commuting by Brown-headed Cowbirds in New Mexico, The Auk 117(3): 795-799.

Dolbeer, R.A. 1998. Population dynamics: the foundation of wildlife damage management for the twenty-first century. In Proceeedings of the 18th Vertebrate Pest Conference (R.O. Baker and A.C. Crabb. eds.) pp. 2-11. University of California, Davis.

Elliott, P.F. 1999. Killing of Host Nestlings by the Brown-headed Cowbird. J. of Field Ornithology, 70(1): 55-57.

Graham, F. 1998. Bad, Bad Birds. Audubon, Sept.-Oct. 1998. pp 104-108.

Harrison, Hal. 1991. Master Moocher. Birder’s World, Oct. 1991. pp 40-41.

Kattan, G.H. 1996. Growth and Provisioning of Shiny Cowbird and House Wren Host Nestlings. J. of Field Ornithology, 67(3): 434-441.

Kenaga, E. 1997. Brood Parasitism among Birds. Birding, Oct. 1997. pp. 393-401.

Lowther, P.E. 1984. Cowbird Nest Selection. The Wilson Bulletin. 96(1): 103-107.

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.

Mason, Paul. 1986. Brood Parasitism in a Host Generalist, the Shiny Cowbird: Quality of Different Species as Hosts. The Auk 103(1): 52-60.

Ortega, C.P. 1998. Cowbirds and Other Brood Parasites. University of Arizona Press.

Ortega, C.P. 2000. More on Cowbirds: Broad-scale Control Targets the Wrong Birds Birding, August, 362-364.

Parker, T.H. 1999. Responses of Bell’s Vireos to Brood Parasitism by the Brown-headed Cowbird in Kansas. The Wilson Bulletin, 111(4): 499-504.

Peer, B.D. and S.G. Sealy. 1999 Laying Time of the Bronzed Cowbird. The Wilson Bulletin, 111(1): 137-139.

Post, W., A. Cruz and D.B. McNair. 1992. The North American Invasion Pattern of the Shiny Cowbird. Journal of Field Ornithology, 61: 32-41.

Smith, J.N.M. 1994. Cowbirds: Conservation villains or convenient scapegoats? Birding 26: 257-259.

Stutchbury, B.J.M. 1997. Effects of Female Cowbird Removal on Reproductive Success of Hooded Warblers. The Wilson Bulletin, 109(1): 74-81.

Teather, K.A. and R.J. Robertson. 1986. Pair Bonds and Factors Influencing the Diversity of Mating Systems in Brown-Headed Cowbirds. The Condor 88: 63-69.

Thompson, F.R. III. 1994. Temporal and Spatial patterns of breeding Brown-headed Cowbirds in the midwestern United States. The Auk 111: 979-990.

Zimmerman, J.L. 1983. Cowbird Parasitism of Dickcissels in Different Habitats and at Different Nest Densities. The Wilson Bulletin, 95(1) 7-22.

Return to Fact Sheets