Brown-headed cowbirds are a species of bird that are native to North America. However, there has been some debate around whether they are truly native to all parts of the United States or if their range has expanded through human changes to the landscape. To understand the answer, we need to look at the historical range of brown-headed cowbirds and how it has shifted over time.
Historical Range of Brown-headed Cowbirds
Brown-headed cowbirds have a long evolutionary history in North America. Their ancestors diverged from tanagers around 5 million years ago and were originally associated with bison herds on the Great Plains. The historical range of brown-headed cowbirds prior to European settlement stretched from southern Canada through the Great Plains and American Southwest. They were absent, however, from the eastern forests. This distribution closely matched the range of their preferred habitat – shortgrass prairie grazed by bison and other ungulates.
During the 19th century, brown-headed cowbirds began expanding their range eastward into the deciduous forests of the Midwest and Northeast. Several factors enabled this range expansion. The clearing of forests for agriculture created openings similar to prairie. The extirpation of wolves and other large predators allowed cowbird populations to increase. The introduction of livestock, especially cattle, provided familiar hosts and habitat. And the construction of railroads and roads provided corridors for dispersal.
By the early 1900s, brown-headed cowbirds could be found through most of the eastern United States. Today, they occupy the majority of North America below the boreal forests of Canada and Alaska. The one major exception is south Florida, where habitat alteration has reduced cowbird populations.
Cowbirds as Native Versus Introduced
Given this history, there is debate around whether brown-headed cowbirds should be considered native versus introduced in parts of their range. In a strict sense, they are native to central North America where their ancestors evolved alongside bison. In the eastern forests, they are more recent arrivals. So are they native or introduced there?
Those arguing cowbirds are native point out that the species spread east in response to human changes using its own mobility. They were not physically introduced by people in the way house sparrows or European starlings were. So their expanded range represents a natural shift enabled by anthropogenic habitat change.
On the other hand, those viewing cowbirds as introduced note they only entered eastern forests once these were opened and altered by Euro-American settlers. Without human clearing, hunting, agriculture, and sprawl, cowbirds would likely have remained restricted to the Great Plains. So their presence results more from human actions than natural dispersal abilities.
Which viewpoint one takes often depends on how narrowly we define ???native.??? A strict definition suggests cowbirds should only be considered native to their pre-settlement range on the Great Plains. A broader definition acknowledges that species ranges can shift over time in response to environmental changes both natural and human-caused. Their status may fall somewhere between strictly native and introduced depending on the time scale considered.
Impacts of Brown-headed Cowbirds
Part of the debate around cowbirds??? status relates to their impacts on other species. As nest parasites, brown-headed cowbirds lay their eggs in the nests of other birds. The cowbird chick then outcompetes the host???s chicks for food and resources. This reproductive strategy evolved on the Great Plains where periodic disturbances from grazing bison and fire limited nesting opportunities. In the eastern forests, however, cowbirds gained new hosts na??ve to brood parasitism. Their presence has contributed to declines in migratory songbirds like wood thrushes, cerulean warblers, and hooded warblers among others.
Viewing cowbirds as introduced invaders emphasizes these harmful impacts on eastern bird species. Seeing them as native gives a different perspective, suggesting shortgrass prairie birds like dickcissels and grasshopper sparrows may have co-evolved adaptations to tolerate parasitism. Ultimately, the debate reflects differences in prioritizing either conservation of regional diversity or ecological integrity of communities.
Management of Cowbirds
The question of cowbirds??? native status also plays into management decisions. Introduced species are often targeted for control or eradication. For native species, the goal is usually coexistence and balance. Cowbird management plans reflect this split perspective.
In areas where cowbirds are considered introduced, management often focuses on trapping and removing individuals during the breeding season. The goal is reducing parasitism rates on vulnerable hosts. Where deemed native, management may aim more for habitat improvements to support stable coexistence of cowbirds and host species.
There are good arguments on both sides of this issue. Seeing cowbirds as native emphasizes stability of the broader ecological community, while an introduced perspective prioritizes conservation of declining host species. Reality likely lies somewhere in between. With careful management, both cowbirds and their host species may be able to persist across a changing North American landscape.
Conclusion
In summary, brown-headed cowbirds have a complex history in North America. While native to the Great Plains, their range expanded east in the 19th century facilitated by human changes to the landscape. There are good arguments on both sides of considering them native versus introduced in eastern forests. Their status may fall along a gradient depending on how narrowly we define ???native???. This debate influences management priorities, with an introduced perspective favoring cowbird control and a native one focusing on habitat management. With thoughtful planning, we may be able to support both cowbird and host species populations across their modern overlapping ranges.
| Time Period | Cowbird Range |
|---|---|
| Pre-1800s | Great Plains and American Southwest |
| Early 1800s | Began expanding east with forest clearing and livestock grazing |
| 1900 | Occupied most of eastern U.S. below Canada |
| Today | Nearly all of North America except boreal forests and south Florida |
In this table, we summarize the shifting range of brown-headed cowbirds over the past few centuries as they spread from their original native range on the Great Plains across eastern North America.
References
Here are some references cited and sources for additional information on brown-headed cowbird history and ecology:
- Lowther, P. E. (1993). Brown-headed cowbird (Molothrus ater). In Birds of North America Online (A. Poole, Ed.). Cornell Lab of Ornithology. https://birdsna.org/Species-Account/bna/species/bnhcow
- Lowther, P. E. (2018). Lists of victims and hosts of the parasitic cowbirds (Molothrus). https://www.sapc.si.edu/index.php/parasi/hosts-of-parasitic-cowbirds
- Chace, J.F. et al. (2005). Cowbird (Molothrus spp.) ecology: a review of factors influencing distribution and abundance of cowbirds across spatial scales. Ornithological Monographs, 57, 45-70.
- Kostecke, R. M. & Smith, L. M. (2020). Brown-headed Cowbird (Molothrus ater), version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology. https://doi.org/10.2173/bow.bnhcow.01
