ABS 2023
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Exploring the rules of egg rejection in the American robin through conjoint experimentation
Devin J. Goodson1, Jeffery P. Hoover2, Abbigail Turner3, Carena J. van Riper4, Mark E. Hauber1,2,3,5. 1Program in Ecology, Evolution, and Conservation Biology, University of Illinois at Urbana-Champaign, Urbana, IL, United States; 2Illinois Natural History Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Urbana, IL, United States; 3Department of Evolution, Ecology, and Behavior, School of Integrative Biology, University of Illinois at Urbana-Champaign, Urbana, IL, United States; 4Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, , Urbana, IL, United States; 5Advanced Science Research Center and Program in Psychology, Graduate Center of the City University of New York, New York, NY, United States

In this study we focus on the American robin (Turdus migratorius), a robust rejector of parasitic eggs laid in its nest by the brown-headed cowbird (Molothrus ater). Employing a randomized multiple simultaneous parasitism paradigm, we experimentally investigated how visual traits influence the rejection rates of eight distinct model eggs across 28 different pairings of two model eggs at a time. Utilizing a conjoint design analysis, we assessed egg-rejection decisions by evaluating how female robins made a choice to accept one egg while rejecting the other. Our findings reaffirmed the role of egg color, size, and maculation as key visual cues influencing model-egg rejection behavior in American robins. Specifically, we found that parasite-like white model eggs were 50% more likely to be rejected relative to robin-mimetic blue eggs, robin-sized model eggs were 39%��likely to be�accepted�compared to cowbird-sized eggs, and maculated eggs were 18% more likely to be rejected compared to immaculate eggs. These findings advance our knowledge of American robins' rejection behavior and showcase an innovative methodology for future research on bird decision-making.