By Ben Young
And the winner for the “Most Despised Native Bird of North America” award is . . . the Brown-headed Cowbird (Molothrus ater).
“An open solicitation for cowbird recipes.” Such titles as this in a prominent North American birding journal in 1994 illustrate the collective disdain for this native songbird among bird-watchers and scientists alike. But why is there such widespread hysteria and negative attitude toward our smallest blackbird species? Why are some states spending over one million dollars annually to control Brown-headed Cowbird populations?
As the world’s most extensively-studied brood parasite, the Brown-headed Cowbird is best known for its exploitative reproductive strategy, in which it lays eggs in the nests of other host species, allowing the surrogate host to raise its young. Brilliant, right?
Well, yes, but how can you generate positive PR with such shady practices? Sure, it might win some over with its native nature: how can you not like a bird whose behavior is closely associated with the American Bison, with a historic range thought to be confined to the grasslands of the Great Plains? But imagine the labels it gets when we anthropomorphize their behavior: a bird that “abandons its own babies;” a bird that “kills the babies of others.”
While a close inspection of the methods of the cowbird’s techniques may not persuade you to start buying cowbird-imprinted stationery or 1000-piece puzzles, it may leave you marveling at the complexities of this intriguing bird.
Once migrant cowbirds arrive in Montana in mid-April to early May, multiple males will frequently chase single females. The courting males can be identified by their inferior position on a branch below a perched female, where they may perform a variety of courtship displays.
Following fertilization, cowbird females will use perches in forested areas or edge habitat to monitor with stealth the nests of other species, synchronizing the laying of its own eggs with that of the other species. The large hippocampus of the brain in female cowbirds is thought to aid in remembering the location of host nests. Preferring to parasitize open-cup nests of species with an insect-rich diet for hatchlings, Brown-headed Cowbirds are host generalists, their surrogate host species numbering 247 (second only to the Shiny Cowbird: 266 species). Throughout North America, the top two cowbird hosts are the Yellow Warbler and the Song Sparrow. In western Montana riparian habitat, the top four hosts are the Dusky Flycatcher, Warbling Vireo, Yellow Warbler, and American Redstart.
Once a viable host is identified, the females wait until the pre-dawn hours to quickly lay their eggs, attempting to avoid detection. In the process of laying her own eggs, the female may remove the host’s eggs with her beak, increasing the survival rates of her own chicks due to higher incubation efficiency and enhanced nutrition of chicks with less competition.
With an unusually long reproductive period, each female will have an average production of 40 eggs throughout the breeding season. Unlike normal passerines, cowbird ovaries and oviducts don’t regress between clutches. These birds are truly the chickens of the songbirds! The eggs, typically thicker-shelled and able to endure the puncture attempts by the host, typically hatch more quickly than those from the host (11-13 days). While only 3% of the cowbird eggs laid in host nests are successful, a parasitized nest may force the host to abandon it and renest, or bury the cowbird’s eggs with a new lining and start over, creating additional opportunities for cowbird parasitism.
Yet, if you’re a host, there’s a price to be paid for messing with cowbird eggs that were laid in your nest. Experimental data from Hoover and Robinson (2002) suggest that cowbirds may respond to ejection of their eggs from a host nest with retaliatory “mafia” behavior. Their results showed that in 56% of nests where warbler hosts ejected cowbird eggs, cowbirds returned to damage, destroy, or remove warbler eggs.
When these hosts renested, 85% of the new nests were parasitized. However, in nests where cowbird eggs were not ejected by the host, only 6% experienced depredation by returning cowbirds. In fact, hosts who accepted cowbird eggs had 60% more offspring than those who ejected cowbird eggs.
With such a potential impact on host productivity, it’s no surprise to see cowbird control measures being enacted in regions with federally endangered host species that are susceptible to cowbird parasitism. Such species include the Kirtland’s Warbler (Michigan), Black-capped Vireo (Texas), Southwestern Willow Flycatcher (desert SW), and the Least Bell’s Vireo (California). Although the potentially serious threat of cowbirds to the populations of these four species is undeniable, they may be a scapegoat for population declines among other native passerines which are more likely the result of habitat loss and fragmentation.
REFERENCES: ¨ Banks, A.J. and T.E. Martin. 2000. Host activity and the risk of nest parasitism by brown-headed cowbirds. Behavioral Ecology 12: 31-40. ¨
Hoover, J.P. and S.K. Robinson. 2007. Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs. Proc Natl Acad Sci 104: 4479-4483. ¨
Lowther, P.E. 1993. Brown-headed Cowbird (Molothrus ater), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: https://birdsna.org/Species-Account/bna/species/bnhcow/introduction ¨
Marks, J., Casey, D., and P. Hendricks. 2012. Birds of Montana: Brown-headed Cowbird. Retrieved from Montana Audubon page (no longer available), 10 August 2014. ¨
Peer, B.D., Rivers, J.W., and S.I. Rothstein. 2013.The Brown-headed Cowbird: North American’s avian brood parasite. Chinese Bird 4(1): 93-98. ¨
Rothstein, S.I. 2001. Relic behaviors, coevolution, and the retention versus loss of host defenses after episodes of avian brood parasitism. Animal Behaviour 61: 95-107. ¨ Rothstein, S.I. 2004. Brown-headed Cowbird: Villain or Scapegoat? Birding 36: 374-384. ¨
Schram, B.A. 1994. An open solicitation for cowbird recipes. Birding 26:254–257. ¨
Scott, D.M. and C.D. Ankney. 1983. The laying cycle of Brown-headed Cowbirds: Passerine chickens? The Auk 100: 583-592.