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 of 58). The frequency of ejection did not differ between the 3 treatments, indicating that robins typically recognized their eggs regardless of whether they were the majority egg type, providing strong support for the true egg recognition hypothesis. However, the risk of ejection for the artificial cowbird eggs was greater as the ratio of cowbird to host eggs increased, which suggests that robins respond adaptively to the increased fitness costs of multiple parasitism. Finally, the risk of ejection of our artificial cowbird eggs was greater later in the nesting season. Because many individuals were exposed to their eggs for the first time early in the season, this result suggests that robins learn to recognize their eggs during their initial nesting attempt.
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