Egg Recognition and Cowbird Egg Ejection Behavior in the American Robin (Turdus migratorius)

Abstract

American Robins (Turdus migratorius) typically eject parasitic cowbird eggs
from their nests. In order to successfully remove parasitic eggs, robins must first be able
to differentiate between their own and foreign eggs and then remove the unwanted
egg(s). This study addressed these behaviors. A robin's ability to recognize its own
eggs was tested by adding artificial cowbird eggs to robin nests at various parasite-tohost
egg ratios which created three treatments: (1) a majority of robin eggs in the nest,
(2) an equal number of robin and cowbird eggs, and (3) a majority of cowbird eggs. At
89% of nests (51 of 57), robins ejected all cowbird eggs, and latency to ejection did not
differ between these three treatments, indicating egg recognition. However, host-toparasite
egg ratio and date had a significant effect on the risk of ejection: a combination
of the likelihood of ejection and latency to ejection. Ejection was more likely to happen,
and happen faster, as the proportion of parasitic eggs in the nest increase and as the
nesting season progressed. Because the likelihood of birds undergoing their initial
nesting attempt is greater earlier in the nesting season, these results suggest that robins
may learn to recognize their own eggs during their initial nesting attempt.

Date of Award	2012
Original language	American English
Awarding Institution	Eastern Illinois University
Supervisor	Eric K. Bollinger (Supervisor)