Auk’s Top Cited, #8: Yellow Warblers ID Impostor Eggs by Size

FIGURE 1. (A) Real and model eggs used in the experiment. (B) Real Brown-headed Cowbird eggs from the field site.

The next featured paper in our series on our most cited papers from 2014 and 2015 is from The AukRejection of parasitic eggs in passerine hosts: Size matters more for a non-ejecter by M.F. Guigueno, S.G. Sealy, and A.M. Westphal.

When a nesting bird realizes that a nest parasite such as a cowbird has added an egg to its nest, it has several options – it can eject the egg (kick it out), or it can bury it under nest material or desert the nest entirely. Yellow Warblers opt for burying or desert over ejecting, and Guigueno, Sealy, and Westphal hypothesized that they would use tactile clues like size to identify alien eggs, unlike birds who eject parasite eggs and rely on color to identify them.

The researchers used model eggs that differed from real Yellow Warbler eggs in size, color, or both, placing them in Yellow Warbler nests to test the birds’ reactions. As model eggs diverged in size and color from the Yellow Warblers’ own eggs, rejection frequency increased, with the data indicating that size was indeed more important than color in eliciting rejection in warblers. A literature review demonstrated that, as the authors predicted, the opposite is true for ejecter host – they use color more than size to identify impostor eggs.

This study provides insight into the evolution of egg rejection. The authors propose that (1) rejection frequencies based on egg size increase as the difference in size between host and parasite eggs and the reliance on a tactile cue increases; (2) rejection based on color is constant across a gradient of sizes; and (3) as the differences in size diminish, due to coevolutionary convergence by parasites, for example, size as a cue becomes less reliable.

Read the full paper at