The brown-headed cowbird (Molothrus ater) is a small songbird species found throughout much of North America. Unlike most songbirds, brown-headed cowbirds do not build their own nests. Instead, female cowbirds lay their eggs in the nests of other bird species, leaving the host parents to raise the cowbird chicks – a practice known as brood parasitism. This unusual nesting behavior has made the brown-headed cowbird a species of great interest to ornithologists and birders. But where exactly do these so-called “nest parasites” lay their eggs? The answer is more complex than one might expect.
Brown-headed cowbird breeding range
Brown-headed cowbirds have an extremely large breeding range that covers much of the lower 48 United States. Their breeding range stretches westward from the East Coast to the Pacific, and northward from Texas and the Gulf Coast up into Canada. Within this expansive range, brown-headed cowbirds can be found in a variety of open and semi-open habitats, including grasslands, pastures, agricultural fields, forest edges, and even suburban areas like parks and backyards. So in essence, female brown-headed cowbirds could potentially parasitize nests throughout this entire breeding range.
However, there are some parts of their breeding range where brown-headed cowbirds are more abundant and more reliant on brood parasitism than others. The core of their range centers around the Great Plains and the Midwest, where grasslands and agricultural areas provide ideal cowbird habitat. It is here where brood parasitism strategies are most common and where the majority of brown-headed cowbird nests can be found.
Preferred host species by region
While brown-headed cowbirds will use over 220 different bird species as hosts, they do seem to prefer laying their eggs in the nests of certain species depending on the region:
Great Plains/Midwest
In the grasslands of the Great Plains and Midwest, brown-headed cowbirds frequently target host species like:
– Dickcissels
– Red-winged blackbirds
– Common yellowthroats
– Yellow warblers
– Field sparrows
– Song sparrows
These are abundant bird species that build open, cup-shaped nests in grasslands and shrubby areas which cowbirds seem to favor in this habitat.
Eastern U.S.
In the eastern half of their range, common brown-headed cowbird hosts include:
– Gray catbirds
– Northern cardinals
– Red-eyed vireos
– Wood thrushes
– Ovenbirds
– American robins
These species are common in the forests, forest edges, thickets, and suburban areas that cowbirds inhabit in the East. The hooded, deep nests they build help conceal cowbird eggs.
Western U.S.
Out West, frequent brown-headed cowbird hosts are:
– Yellow-breasted chats
– Lucy’s warblers
– Black-throated gray warblers
– Lark sparrows
– Chipping sparrows
– Spotted towhees
Like in the Great Plains, these are mainly open country birds that nest low in shrubs and thickets, allowing cowbirds access.
Southern U.S./Gulf States
In the southern parts of their range, cowbirds target hosts like:
– Northern mockingbirds
– Brown thrashers
– Painted buntings
– Indigo buntings
– Rufous-crowned sparrows
– Various vireos
The warm open forests, brushy areas, and suburban parks of the South attract these cowbird hosts. Their cup nests are parasitized frequently.
Habitat preferences
Beyond specific host species, brown-headed cowbirds also select nests based on the surrounding habitat. Ideal cowbird habitat tends to have these features:
– Open or semi-open areas with sparse ground cover
– Presence of perching spots like fences or powerlines
– Low, shrubby vegetation
– Forest edges or openings
– Proximity to feeding sites like pastures or feedlots
These habitats allow cowbirds to locate and access host nests more easily. Dense, closed forests with heavy undergrowth tend to be less suitable, as it is harder for cowbirds to find and monitor host nests. However, as human alteration of the landscape creates more fragmented, edge-type habitat, cowbirds have expanded into more forested areas.
Ideal host nest features
Brown-headed cowbirds also preferentially choose host nests with certain physical characteristics:
– Cup-shaped structure with walls low enough for cowbird to peer over
– Lined with soft material like grasses or hair
– Medium sized: too small and cowbird egg may push out host eggs, too big could allow host to bury cowbird egg
– Early enough in nesting stage for cowbird to lay during host’s egg laying period
Nests with these features allow the cowbird egg and chick the highest chance of acceptance and survival.
Geographic exceptions
While brown-headed cowbirds will use a wide variety of hosts across North America, there are certain geographical restrictions on where they can parasitize nests:
Far northern range
In the northernmost parts of their range in Canada, brown-headed cowbirds are rare or absent as breeders. The shorter summers and lack of suitable cowbird habitat restrict parasite nesting activity in much of the Canadian Northwest and Alaska.
Florida
Although common in other parts of the southern U.S., brown-headed cowbird populations are very small and localized within Florida. The swampy everglades habitat lacks suitable host species for the cowbirds to parasitize.
Pacific Northwest
In parts of Washington, Oregon, and northern California, brown-headed cowbird abundance is also lower, likely because extensive coniferous forests again limit nest parasitism opportunities. However, where agriculture or habitat disturbance occurs, cowbird populations expand.
Southwestern deserts
Arid habitats like the chaparral of Arizona and New Mexico and the deserts of Nevada and southeastern California have sparser cowbird populations. But they still regularly parasitize drought-adapted species like Lucy’s warblers.
So while brown-headed cowbirds use a wide range of host species across North America, their nest parasitism behavior is largely concentrated in the grasslands, agricultural regions, and suburban areas of central and eastern North America. It is here where most cowbird nests can be found. But continued habitat changes may allow cowbirds to expand as nest parasites even further across the continent in the future.
Conclusion
In summary, brown-headed cowbirds do not build their own nests. Female cowbirds lay eggs in the nests of over 200 different host species across their breeding range in North America. However, certain habitats like grasslands and disturbed areas, along with host species like sparrows, blackbirds, and thrushes that build open, cup-shaped nests, are preferred. The highest densities of parasitized nests occur in the Great Plains and Midwest, but cowbirds will take advantage of suitable habitat and host nests nearly wherever they occur across the continent. Continued research helps reveal more about the complex nesting behaviors of these unique brood parasites.
Region | Typical Hosts | Favored Nest Features | Typical Habitats |
---|---|---|---|
Great Plains/Midwest | Dickcissels, Red-winged Blackbirds, Sparrows | Open cup nests, low in shrubs/thickets | Grasslands, pastures, agricultural areas |
Eastern U.S. | Thrashers, Catbirds, Cardinals, Vireos | Deep cup nests lined with soft material | Forest edges, thickets, suburbs |
Western U.S. | Sparrows, Warblers, Towhees | Open cup nests low in shrubs | Shrublands, chaparral, grasslands |
Southern U.S. | Mockingbirds, Buntings, Vireos | Cup nests lined with soft material | Open woodlands, forest edges, suburbs |
Conservation concerns
The brown-headed cowbird’s brood parasitic nesting habits can potentially impact host species. Cowbird eggs often outcompete host young, reducing reproductive success. Some endangered species like the Kirtland’s warbler have been significantly affected. Conservation plans have included cowbird trapping or shooting in sensitive areas. However, cowbird parasitism evolved naturally, and management must balance cowbird control with maintaining healthy, diverse ecosystems. Overall, the complex nesting behaviors of the brown-headed cowbird provide a fascinating look into the remarkable strategies bird species develop to propagate themselves across the varied landscapes of North America.