ABS 2022
Elevated corticosterone levels increase rejection of foreign eggs in the American robin 
Abbigail M. Turner1, Alexander J. Di Giovanni1, Nicholas D. Antonson1, Hannah M. Scharf1, Mikus Abolins-Abols2, Mark E. Hauber1. 1University of Illinois at Urbana-Champaign , Urbana-Champaign, Illinois, United States; 2University of Louisville, Louisville, Kentucky, United States

Egg rejection is one of the most effective and frequently studied defenses by avian hosts in response to obligate brood parasitism. For example, American robins (Turdus migratorius)  successfully recognize and eject Brown-headed Cowbird (Molothrus ater) eggs from their nests. Little is known about the physiological mechanisms involved in antiparasitic defense behaviors.  Corticosterone is a steroid hormone often produced in response to environmental stressors and has been shown to be elevated in hosts that are parasitized by non-mimetic eggs. We experimentally and non-invasively elevated corticosterone levels in incubating female robins using corticosterone-infused gel applied atop of natural robin eggs. Two hours after the initial application, we parasitized females with a deep-blue, cowbird-sized model egg. We found that females with elevated corticosterone levels were more likely to reject this type of foreign egg than control females, when accounting for the known effects of variable clutch size on egg rejection. Future studies are needed to assess how hormones, that are essential for avian parental care, are involved in hosts’ anti-parasitic defense behaviors.