Costs of egg punctures and host defenses in the system Grayish Baywing-Screaming Cowbird.

PERUSIN, NATALIA G.; MÉNDEZ, ADRIÁN; SABIO, MARÍA DE LAS NIEVES; MIRAVALLES STASTA, CAMILA; REBOREDA, JUAN C.; DE MÁRSICO, MARÍA C.

Gramado

Congreso; II Ornithological Congress of the Americas; 2023

Association of Field Ornithologists (AFO)

The Screaming Cowbird [Molothrus rufoaxillaris] is a specialist brood parasite that has the Grayish Baywing [Agelaioides badius] as its primary host. A. badius often suffers egg losses due to punctures caused by M. rufoaxillaris, which can favor the evolution of antiparasitic defenses. Here, we addressed the question of the possible role of nest defense in reducing host egg losses. Under the hypothesis that the fitness costs of egg punctures select for host defenses, we tested whether: 1) egg losses to punctures increases the risk of nest abandonment and 2) host aggression against adult parasites can reduce egg puncturing. During the breeding season 2022-2023 (Nov-Feb) we recorded the number of host and parasite eggs laid, the occurrence of egg punctures during the laying stage, and whether the nest reached the incubation stage or was abandoned. Additionally, we quantified host defenses through the presentation of stuffed models of M. rufoaxillaris and a control species. All nests were parasitized (mean intensity of parasitism =3.3±1.5 eggs/nest), and 33 of them (53%) had eggs punctured. The likelihood of nest abandonment was positively related to the occurrence of punctures and incubated clutches had a higher proportion of surviving host eggs than abandoned clutches. A. badius was more aggressive towards the parasite models than the control, but the data did not show a relationship between host defenses thus measured and egg puncturing. In the future, it would be important to study host-parasite interactions during nest visits to evaluate if A. badius has specific defenses against egg punctures.