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Brood parasitism of an experimental nest - Brown-headed Cowbirds lay in unattended nests.

photos: experimental nest, cowbird egg

Katherine Thorington, an intern in Archbold’s Avian Ecology Lab, has been conducting artificial nest experiments to examine how nest placement within habitat islands in a suburban matrix influences rates of nest predation. During this portion of the experiment, Katherine placed 120 artificial nests, wicker canary nests available from pet supply stores, in a variety of locations. Each nest contained a coturnix quail egg and a plasticine sham egg. Nests were left exposed for 18 days and checked for evidence of predation every 6 days. On April 26, 2001, Katherine found an egg added to one of her experimental nests, presumably by a brood parasite. The nest had been exposed for 12 days. Brown-headed Cowbirds have been increasing in abundance in this area and Shiny Cowbirds have been observed, although they are rare relative to Brown-headed Cowbirds. However, brood parasitism of natural nests by Brown-headed Cowbirds has yet to be recorded for this county and yet to be confirmed by Shiny Cowbirds for the entire state of Florida. The egg seems most like a cowbird egg, but differs in several respects. The spotting seems dense, the base color is dark, and the egg is relatively long and narrow (21.8 x 14.6 mm). However, cowbird eggs are highly variable in color, pattern, and size. The egg somewhat resembles those of other species that nest in this area, but for many (i.e. Northern Cardinal), the average size is quite different from this egg. Consensus seems to be building for Shiny Cowbird, but we hope to submit the egg for molecular analyses to unambiguously determine its origin.

Update: Using DNA isolated from this egg and archived genetic resources from GenBank, Robert Fleischer at the Smithsonian Institution found a nearly perfect sequence match with Brown-headed Cowbird. The only other possibility was Shiny Cowbird but Dr. Fleischer found 7 sequence differences in the 300 bp of Cytochrome b between the egg and Shiny Cowbird and none with Brown-headed Cowbird. This is a rare record of a cowbird laying in an unattended nest and the first breeding record of Brown-headed Cowbird in Highlands County, FL.

Upper photo: Experimental nest with (largest to smallest) coturnix egg, sham egg, and Brown-headed Cowbird egg. Lower photo: The cowbird egg. Digital photos by Reed Bowman.

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