Literature Review: Cowbird Studies

by Carl Strang

Published scientific studies provide information that gives context and insight into our local natural history. I review these studies to some extent through the year, and make a big push toward the year’s end (yet to take place this year). Today I’ll share a couple studies of cowbirds published this year in the open-access on-line journal PLoS ONE (PLoS = Public Library of Science).

O’Loghlen AL, Rothstein SI (2012) When Less Is Best: Female Brown-Headed Cowbirds Prefer Less Intense Male Displays. PLoS ONE 7(5): e36130. doi:10.1371/journal.pone.0036130

Male cowbirds have two levels of intensity in their wing-spread song display, with the higher level directed at other males and the lower level toward females. Females preferred the lower level visual display (in the experiments, the auditory part was kept constant).

Here a male cowbird displays while a female critically observes.

I have mentioned cowbird displays before, for instance in my species dossier on them. The display consisted of fanning the wings out to the side, fanning the tail and lifting it above the back level, then bowing or leaning forward to the point of sometimes losing balance. While performing the display, the bird at least sometimes gives its high‑pitched call. I call this the falling-down-drunk display. I had noticed that males sometimes display in groups when no females were present, but had not noticed a difference in display intensity, something to watch for in the future.

Cox WA, Thompson FR III, Root B, Faaborg J (2012) Declining Brown-Headed Cowbird (Molothrus ater) Populations Are Associated with Landscape-Specific Reductions in Brood Parasitism and Increases in Songbird Productivity. PLoS ONE 7(10): e47591. doi:10.1371/journal.pone.0047591

Cowbird populations have been declining in recent decades, and this study demonstrated that nest-parasitized species have increased their productivity as a result. The opening of the landscape, providing access for cowbirds, is supported as a contributing factor in their impact on other species.

A female Baltimore oriole feeds a cowbird fledgling.

I was not aware that cowbird populations were decreasing. This is good news for all the songbird species that unwittingly raise cowbird young. Cowbirds lay their eggs in other birds’ nests, and the foster parents raise the cowbird babies often to the detriment of their own young. The study demonstrated that the other species are benefiting from the cowbird decline.

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