Effect of parasite-to-host egg ratio on egg rejection by a Brown-headed Cowbird host

Abstract

ABSTRACT American Robins (Turdus migratorius) typically eject parasitic Brown-headed Cowbird (Molothrus ater) eggs from their nests. In order to successfully remove parasitic eggs, robins must first differentiate between their eggs and foreign eggs, and then remove the foreign egg(s). Our primary objectives were to determine (1) whether the robins reject cowbird eggs because they are in the minority (“discordancy hypothesis”) or because robins have learned the appearance of their own eggs regardless of whether they form the majority of eggs in the nest (“true egg recognition hypothesis”); and (2) whether the robin's ability to recognize its eggs and reject parasitic eggs was affected by the parasite-to-host egg ratio. We added artificial cowbird eggs to robin nests to create 3 treatments: (1) a majority of robin eggs, (2) an equal number of robin and cowbird eggs, and (3) a majority of cowbird eggs. Parasite-to-host egg ratios were between 1:3 and 3:1. Robins ejected all cowbird eggs at 88% of nests (51 o...