Decoding the Dilemma: Exploring the Rules and Cues of Egg Rejection in the American Robin (Turdus migratorius) Through Conjoint Experimentation

Uncovering the cognitive bases of egg rejection behavior in avian hosts of brood parasites carries significant comparative implications not only for our understanding of host–parasite coevolution but also for cross-species research aimed at assessing decision-making. In this study, we focused on the American robin (Turdus migratorius), a species that lays large, elongated, and immaculate blue eggs and is well studied for its robust rejection of smaller, rounder, white, and maculated eggs laid by the brood parasitic brown-headed cowbird (Molothrus ater). Employing a randomized multiple simultaneous parasitism paradigm, we experimentally investigated how model egg color, dimensions, and maculation influenced the rejection rates of eight distinct egg types across 28 different pairings of two eggs at a time. First, we assessed whether egg rejection decisions depended on model egg features. Then, for a subset of trials (36%) where one egg was accepted and the other was rejected, we utilized a conjoint design analysis, a methodology borrowed from economics and marketing. Using the conjoint analysis, we showed that white model eggs were 50% more likely to be rejected relative to blue eggs, small-round eggs were 39% more likely to be rejected compared to large-elongated model eggs, and maculated eggs were 19% more likely to be rejected compared to immaculate eggs. These findings reaffirmed the roles of egg color, dimension, and maculation as key visual cues influencing egg rejection behavior in American robins. These findings also offer methodological advancements to study egg rejection behavior and lend themselves to future comparisons of human and nonhuman decision-making processes.