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Cowbirds: Villainous mobsters? Or falsely maligned native species?

  • winnics1
  • Jun 13
  • 14 min read

Updated: Jun 14

Brown-headed Cowbirds (Molothrus ater) are notorious for their breeding system called "brood parasitism"-- they lay their eggs in the nests of hundreds of other bird species, forcing those birds to feed their ravenous young. They've been painted as dastardly villains, but what are they really?

 

In 2019 I found myself getting really frustrated with Facebook comments advocating for removing cowbird eggs from host nests in Kansas, where I lived at the time researching cowbirds and their hosts. I produced a large Twitter thread communicating why this is a bad idea. In 2020 representatives of Kansas’s Wichita Audubon Society invited me to convert that thread into an hour-long seminar, which I’ve since presented to clubs around the country. When I posted a recent (2025) version of that presentation on YouTube I wanted to include the references presented in the talk in another format. Here is that YouTube presentation: https://youtu.be/6l4pnz4W51s?si=mKZX7Uo-nioXWmZz.

 

This post will summarize the above video and give additional reference links. Sections on this post include:

1.     What is brood parasitism?

2.     What makes cowbirds villainous mobsters?

3.     What makes cowbirds a falsely maligned native species?

4.     Should you remove cowbird eggs?

5.     Information about other cool brood parasites

6.     Cowbird research referenced in the video

 

Brown-headed Cowbird in Northeast Kansas
Brown-headed Cowbird in Northeast Kansas

 

1.     What is brood parasitism?

Brood parasitism is an evolved breeding strategy where an individual relies on others to raise its offspring. The parasite lays eggs in the nest of the host. This strategy appears in multiple types of animals, but because I am an ornithologist the video is focused on birds.

There are multiple types of brood parasitism in birds. The most common strategy is conspecific brood parasitism (“con-” means same, “-specific” means species); this is a breeding strategy where a bird dumps her eggs in another bird’s nest of the same species. This happens in many birds, possibly most bird species, but it is especially well-known in ducks. Why might a bird lay her egg in another neighbor’s nest? Well, predation is the number one risk to breeding birds; eggs and baby birds in nests are “sitting ducks” vulnerable to roaming predators. Conspecific brood parasitism allows laying birds to spread out that risk– they are putting their eggs in “more baskets,” so that if a predator gets their own nest, some of their genetic babies might survive in the neighbor’s nest. This is not a big problem for the host parents in ducks and other species whose babies hatch well-developed and don’t need to be directly fed by the parents. In fact, it may even be beneficial; if a predator targets a flock of ducklings filled with not only the parent’s genetic offspring but also her neighbors’, her young are statistically less likely to be eaten. However, for altricial birds, the kind that hatch dependent on their parents for food, extra babies mean having more mouths to feed, and caring for constantly-begging nestlings is a lot of work. Despite this, we still see conspecific brood parasitism in altricial songbird species, suggesting that the benefits may outweigh the risks. More information on conspecific brood parasitism: https://www2.nau.edu/~gaud/bio300b/cbp.htm.

Heterospecific brood parasitism (“hetero-” for different, “-specific” for species) is a type of brood parasitism where a bird lays her eggs in the nest of a different species. North American cuckoos, such as the Yellow-billed Cuckoo, make their own nest and raise their own babies, yet will also occasionally lay an egg in the nest of another species, like Northern Cardinals. This still spreads out the predation risk among the parent’s genetic offspring, but comes with added risk, as the host parents may be ill-equipped to raise a baby of a different species or better able to distinguish that baby from their own offspring. Link to research on N. American cuckoos: https://tinyurl.com/56axjn5h.

Obligate heterospecific brood parasitism describes species that have evolved to only lay their eggs in other species’ nests, never building their own nest and raising their own babies. This breeding strategy has evolved at least seven times in birds.

 

 

Begging Brown-headed Cowbird chick (left) in a nest of Dickcissels, NE Kansas
Begging Brown-headed Cowbird chick (left) in a nest of Dickcissels, NE Kansas

2.     What makes cowbirds villainous mobsters?

Brown-headed Cowbirds (scientific name: Molothrus ater) earned their villainous reputation by being one such obligate heterospecific brood parasite. They are known as cowbirds because they follow cattle around, eating the insects that cows stomp up. Historically they were known as “bison birds” because they followed bison instead of cattle. It was once thought that cowbirds evolved brood parasitism so they could better follow bison moving around the prairie, which left cowbirds little time to build their own nest and raise their own babies. However, the cowbird group likely evolved brood parasitism in South America (without bison) and spread north to meet the bison (more information: https://www.birdnote.org/explore/field-notes/2015/05/cowbird-story-revisited). Furthermore, bison in the eastern Plains, where cowbirds are currently the most concentrated, may have moved less than previously thought (<50 km during the breeding season, scientific reference:  https://doi.org/10.1016/j.yqres.2009.12.001).

Rather than building their own nest and raising their own babies, cowbirds instead lay their eggs in the nests of 220+ host species. That doesn’t always work out for the baby cowbird, as the host parents may not be equipped to care for them. Cowbirds often parasitize House Finch nests, which almost never works out for the baby cowbirds, because House Finches do not feed their babies enough insects—the cowbirds ultimately starve to death.

Being a brood parasite requires some complicated planning. Female cowbirds must be able to find host nests and find them at the correct time, laying their eggs in the 2-6 day time period (depending on the host species) when that host is laying their eggs. If the cowbirds eggs are laid too late after the host parent starts incubating, the baby cowbirds would be outcompeted by the host offspring that hatch before they do. Our research suggests that cowbirds may adjust their embryonic and post-hatch growth to match the timing of their hosts’ nesting period, but the eggs still need to be laid in the host nest while the host is laying her own eggs (see Research on this website for PDF, or use this link: https://doi.org/10.1139/cjz-2020-0147),

How do cowbirds know if the nest is in the laying stage? They may be watching the host parents’ behavior, or they may poke a hole in one of the hosts’ eggs to gauge the embryos’ developmental stage. This kills the developing embryo and allows the cowbird to replace that egg with her own egg, especially useful when parasitizing hosts that can count the eggs in their nests.

If the host eggs are too far developed or the nestlings have already hatched, cowbirds may destroy the nest. This forces the host parent to try again, building another nest and laying new eggs. The cowbird female could then lay her eggs in the next nest. This behavior is called farming—the cowbirds are laying the seeds of their next parasitism attempt by ending the hosts’ current nesting attempt. Here is a cowbird destroying a Dickcissel nest, which we captured on camera in the prairies of Northeast Kansas:


If the cowbird manages to get an egg in the nest at the right time, the baby cowbird will grow up alongside its surviving host nestmates. Cowbirds require a lot of food from the host parents who are already at the most energy-intensive point in their year. The presence of cowbirds can also be costly for the host babies, as cowbirds are often larger and grow faster than their host nestmates. In extreme cases, cowbirds can outcompete the host babies, leading to their starvation, although I never witnessed that in these grassland birds (the historical hosts of the Brown-headed Cowbird). Even after they have grown enough to fledge the nest, cowbirds require care from their host parents, forcing them to spend energy that could otherwise be dedicated to their own babies or to future nests.

This cowbird parasitism is costly enough that hosts have evolved defenses. Yellow Warblers respond to the presence of cowbird eggs in their nests by building a “roof” over the egg, burying the cowbird egg (and any of their own eggs that were already laid) and laying new eggs over top. Other birds, such as the American Robin, can recognize cowbird eggs and throw them out of nests. You can watch the robins in action in this video:

Why haven’t all birds managed to evolve this egg rejection? I was taught that some species may be limited by their beak size and shape; they may simply be too small to remove a cowbird egg. That was the hypothesis for why Grasshopper Sparrows fail to remove cowbird eggs, but here is footage from our nest cameras in Northeast Kansas showing a sparrow male rolling a cowbird egg out of the nest. In this footage you can see a cowbird begging behind him in the nest, so he only removed the cowbird egg after it had failed to hatch:

Why might birds that are capable of removing cowbird eggs fail to do so? Perhaps they have not evolved the ability to distinguish their own eggs from cowbird eggs and can’t risk accidentally killing their own embryos. However, scientists have proposed an alternative explanation in the form of the “Mafia Hypothesis.” This hypothesis holds that cowbirds may return to host nests to check on their eggs and offspring. If the host parents rejected the cowbird babies, the cowbirds retaliate, killing the hosts’ eggs and nestlings. That ensures that any host parent that comes up with some sort of innovative way to recognize the cowbird egg, to refuse the cowbirds’ young, or to get rid of them, will not have any surviving babies that could inherit this new behavior. It prevents the evolution of anti-cowbird defenses. You can read the original mafia paper here: https://doi.org/10.1073/pnas.0609710104.

Additional evidence in support of the Mafia Hypothesis is lacking, in part because testing it would ideally include genetic evidence linking individual cowbirds to the eggs rejected and movement data or video data showing that the parent cowbirds are the ones destroying the nest (an expensive and technically complicated project). However, scientists have seen tantalizing clues, including this video we got of a Grasshopper Sparrow nest in Northeast Kansas. In this video a cowbird kills a cowbird nestling and sparrow nestlings, revealing an unhatched cowbird egg in the nest. Was it her own egg and was she retaliating?

As if these behaviors weren't enough, cowbirds are expanding their breeding (parasitizing) range across the continent. They were historically a species of the open Great Plains of North America, but as settler colonists cut down the forests of the continent cowbirds have expanded into these newly-open habitats. This expanded range has put cowbirds in contact with vulnerable and even endangered species, like the Kirtland’s Warbler and the Black-capped Vireo. Both species are not well-equipped to deal with cowbird parasitism and their host babies often die when cowbirds are in the nest outcompeting them. Researchers working to save both species therefore use traps to kill adult cowbirds in the breeding range of the warblers and the vireos, a costly and labor-intensive solution to protect these species from extinction. More information: https://nationalzoo.si.edu/news/kirtlands-warbler-no-longer-needs-protection-brown-headed-cowbird-michigan.


So, are cowbirds villainous mobsters? I think you can argue that they are, and researchers have been compiling that evidence for decades. They poke holes in eggs and remove them to test the stage of the nest, reducing the number of babies that the host species can produce. They sometimes destroy whole nests in this farming behavior. They foist all that costly parental care on the host parents who have to raise their voracious babies. Young cowbirds may outcompete host offspring, even killing them from starvation. They stress and overwork host parents, even after they have fledged the nest. If the Mafia Hypothesis is supported, they may even kill host nestlings if the cowbirds are not well taken care of. And they are expanding their range, impacting endangered species.

Have I convinced you? Well, let’s pivot to why I give talks defending cowbirds as a falsely maligned native species and a good subject for biological research.

 

3.     What makes cowbirds a falsely maligned native species?

When I first started giving this talk, I asked folks on Twitter “are cowbirds good?” and surprisingly most people who had an opinion said yes, cowbirds are good. Of course, that could be a biased sample of the kind of people who follow a cowbird researcher online. But, the most common response I got was this: we can’t impose human moral standards on a non-human animal. Sure, cowbirds have evolved to do some behaviors that make us uncomfortable, and definitely wouldn’t fly in a human society, but that doesn’t make them evil or bad, it’s just nature.

Plus, cowbirds are not the only birds that hurt other birds. Here is a Cooper’s Hawk, half of a pair that built a nest right outside my office window. Sometimes they feed their babies dead birds, including dead nestlings that they pulled out of nearby nests and my beloved robins. Predators hurt other birds too, but we still enjoy them and most North American birders wouldn’t be advocating for destroying Cooper’s Hawk eggs. At least, I hope they’re not!

OSU campus Cooper's Hawk holding a dead American Robin
OSU campus Cooper's Hawk holding a dead American Robin

Cowbirds have expanded their range across North America in recent history because humans cut down forests, but that does not mean they are not native. Paleontologists have found fossil cowbirds on the East Coast, showing they once lived across much of the continent: https://doi.org/10.58782/flmnh.yklc1562.

Cowbirds are a native bird species, and like most native songbirds they are in decline. They are declining both in the regions where they recently expanded and in their historic range. [Source of graphs used in the video of cowbird declines in British Columbia: https://doi.org/10.1676/22-00074; and Missouri: https://doi.org/10.1371/journal.pone.0047591.]  

Commentators suggest that brood parasitism makes cowbirds “bad parents” but I prefer to see it as a shift in when they make their parental investment. Cowbird females spend the entire breeding season looking for safe nests in which to lay their eggs and they likely check back in on those eggs. They may have the physiological capacity to lay as many as 50 eggs in a breeding season, much more than the average songbird. This requires a large amount of nutritional resources to build this many eggs! In my mind they are good parents, just doing their parental care at a different stage of their offspring's lives.

Cowbirds also need to have some pretty fascinating cognitive work-arounds to make sure that they know they are cowbirds. Most bird species learn what to look for in future mates by observing their own parents, but cowbirds cannot learn what a cowbird looks like from their host parents. Instead, they have evolved a “password,” a call given by adult cowbirds that, when heard by juvenile cowbirds, “unlocks” their cowbird identity in their brain. More information on the cowbird password hypothesis can be found here: https://www.audubon.org/news/scientists-pinpoint-secret-password-unlocks-cowbirds-self-identity.


At the end of the day, I think we are too harsh on cowbirds. We can’t impose human moral standards on a naturally evolved strategy, and after all cowbirds are not the only bird that hurts other birds. They are a native species, and they are in decline. We should be worrying for them, not worried about them.

 

4.     Should you remove cowbird eggs?

Even if I have not convinced you to think of cowbirds as a falsely maligned native species, I do want to emphasize that it is a terrible idea to remove cowbird eggs. They are (declining!) native species, protected by the Migratory Bird Treaty Act, so it is illegal to harm their eggs. Plus, doing so brings risk to the hosts themselves. Visiting bird nests (even for photos) can stress out the host parents, leading them to abandon the nest. Removing an egg may convince them that a predator has found their nest. Because some predators (like small snakes) will only remove one egg but return on subsequent days to remove the rest, birds may abandon the nest, assuming that their eggs are all doomed. Plus, if the Mafia Hypothesis is to be believed, cowbirds may return to the host nests and retaliate against them for removing the host egg. Even in areas where cowbirds are removed to protect endangered host species, researchers do not remove the eggs but remove the adults. For more information, see this Audubon article: https://www.audubon.org/news/is-it-okay-remove-cowbird-eggs-host-nests.

 

A Speckled Kingsnake about to eat Dickcissel and Brown-headed Cowbird eggs, NE Kansas
A Speckled Kingsnake about to eat Dickcissel and Brown-headed Cowbird eggs, NE Kansas

 5.     Who are the other obligate heterospecific brood parasites?


  • Black-headed Ducks are a species of South American duck that lays their eggs in the nests of other ducks but also gulls and coots. This is not a big problem for the host parents, as these ducklings are completely independent within 24 hours of hatching. More information: https://en.wikipedia.org/wiki/Black-headed_duck

 

  • Indigo birds, cuckoo-finches, and whydahs are altricial songbirds; their babies will depend on the host parents for care after they hatch. Hosts have evolved ways to avoid the parasitism of these finches so that they don’t waste energy raising the costly altricial babies that aren’t their own. For example, the host species known as the prinia and the cisticola have evolved variation in their egg coloration to help better distinguish their eggs from the parasite eggs, but the parasitic cuckoo finches have evolved egg coloration that matches the host eggs. They are locked in what we call an evolutionary arms race, the host innovating over generations to better distinguish their eggs from the parasite, and the parasite innovating over generations to trick the host into raising their babies.


    This evolved mimicry doesn’t stop with the eggs. The common waxbill host evolved fancy markings on the baby’s beaks, presumably to better distinguish the parasite babies so the host could preferentially care for their own offspring. However, the parasitic whydah, has evolved to match those too. Links to scientific papers reference in the talk: Spottiswoode & Stevens 2011 https://royalsocietypublishing.org/doi/full/10.1098/rspb.2011.0401; Schuetz 2005 https://www.sciencedirect.com/science/article/pii/S0003347205001995

 

  • Honeyguides have a neat relationship with local humans; over thousands of generations the birds have learned to guide humans to bee hives. When humans break open the hives to get the honey, the honeyguide gets to eat the bee larvae. More info: https://www.audubon.org/magazine/meet-greater-honeyguide-bird-understands-humans


    While honeyguides cooperate with humans, they do not cooperate with the host species that they parasitize. Baby honeyguides hatch with sharp hooks on the end of their beaks. Before they are even able to open their eyes, they use these sharp hooks to kill the host eggs and nestlings in the nest so only they receive the parental care. Link to scientific paper referenced in the talk: Spottiswoode and Stevens 2012, Biology Letters https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2011.0739

 

 

  • The New World (Western Hemisphere) cuckoos are not all obligate brood parasites, but there are some obligate brood parasites among the Central and South American species.


  • Cowbirds are a group of Western Hemisphere blackbirds that have evolved obligate heterospecific brood parasitism, including our North American Brown-headed Cowbird. Some cowbird species are host-specific, like the Screaming Cowbird, while others are generalists that parasitize hundreds of hosts, like the Brown-headed and Shiny Cowbirds. Hosts may even be parasitized by multiple species of cowbirds. More information on cowbirds: https://en.wikipedia.org/wiki/Cowbird


    For information on Baywings parasitized by both Screaming and Shiny Cowbirds, see De Mársico et al. 2012 Proc B: https://royalsocietypublishing.org/doi/full/10.1098/rspb.2012.0612


6.     Cowbird research referenced in the video

In the seminar, I share highlights from a few recent research projects. You can find the links to those projects below, in order of appearance:

 

  • Research on the distribution of Grasshopper Sparrow territories:

Winnicki, S.K., S.M. Munguía, E.J. Williams, and W.A. Boyle. 2020. Social interactions do not drive territory aggregation in a grassland songbird. Ecology 101: e02927. Link: https://doi.org/10.1002/ecy.2927.

  • Research on grassland bird (including cowbird) growth:

Winnicki, S.K. 2019. Growing up prairie: ecological drivers of grassland songbird nestling development. Thesis from Kansas State University: https://krex.k-state.edu/server/api/core/bitstreams/f49fc186-3657-4c86-afba-2d1c58b82670/content.

  •  Exposure to cowbird egg models increases robin rejection of blue eggs:

Xu, K., M.R. Servedio, S.K. Winnicki, C. Moskat, J.P. Hoover, A.M. Turner, and M.E. Hauber. 2023. Host learning selects for the coevolution of greater egg mimicry and narrower anti-parasitic egg-rejection thresholds. Evolution Letters 7: 413-421. Link: https://doi.org/10.1093/evlett/qrad041.

  • Robin rejection of eggs of different shapes:

Hauber, M.E., S.K. Winnicki, J.P. Hoover, D. Hanley, and I.R. Hays. 2021. The limits of egg recognition: testing acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest. Royal Society Open Science 8: 201615. Link: https://doi.org/10.1098/rsos.201615.

  • Review of robin egg-rejection studies:

Turner, A.M. and M.E. Hauber. 2021. The American robin (Turdus migratorius): a focal species for anti-parasitic egg rejection studies among hosts of the brown-headed cowbird (Molothrus ater). Ethology 127: 490-503. Link: https://doi.org/10.1111/eth.13158.

  • Effect of stress-associated hormones on robin egg rejection:

Turner, A.M., A.J. Di Giovanni, N.D. Antonson, H.M. Scharf, M. Abolins-Abols, and M.E. Hauber. 2022. Non-invasive elevation of circulating corticosterone increases the rejection of foreign eggs in female American robins (Turdus migratorius). Hormones and Behavior 146: 105278. Link: https://doi.org/10.1016/j.yhbeh.2022.105278.

  • Yellow warblers giving calls to warn about cowbird presence:

Hobson, K.A., and S.G. Sealy. 1989. Responses of yellow warblers to the threat of cowbird parasitism. Animal Behavior 38: 510-519. Link: https://doi.org/10.1016/S0003-3472(89)80044-2.

Lawson, S.L., J.K. Enos, C.S. Wolf, K. Stenstrom, S.K. Winnicki, T.J. Benson, M.E. Hauber, and S.A. Gill. 2021. Referential alarm calling elicits further vigilance in a host of an avian brood parasite. Biology Letters 17: 20210377. Link: https://doi.org/10.1098/rsbl.2021.0377.

  • Red-winged Blackbirds know what the Yellow Warbler cowbird alarm call means:

Lawson, S.L., J.K. Enos, N.C. Mendes, S.A. Gill, and M.E. Hauber. 2020. Heterospecific eavesdropping on an anti-parasitic referential alarm call. Communications Biology 3:143. Link: https://doi.org/10.1038/s42003-020-0875-7.

  • Cowbird growth in Song Sparrow and Red-winged Blackbird nests:

Winnicki, S.K. B.M. Strausberger, N.D. Antonson, D.E. Burhans, J. Lock, A.M. Kilpatrick, and M.E. Hauber. 2021. Developmental asynchrony and host species identity predict variability in nestling growth of an obligate brood parasite: a test of the “growth-tuning” hypothesis. Canadian Journal of Zoology 99: 213-220. Link: https://doi.org/10.1139/cjz-2020-0147. Also see the “Research” tab on this website for PDF access.

  •  Cowbirds grow best in Prothonotary Warbler nests with two host young and manipulate nests to suit them:

Antonson, N.D., W.M. Schelsky, D. Tolman, R.M. Kilner, and M.E. Hauber. 2022. Niche construction through a Goldilocks principle maximizes fitness for a nest-sharing brood parasite. Proceedings of the Royal Society B 289: 20221223. Link: https://doi.org/10.1098/rspb.2022.1223.



 
 
 

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