<b>Corticosterone shortens foreign egg ejection distance but not latency in American Robins (</b><b><i>Turdus migratorius)</i></b>

Obligate brood parasitic birds often face the removal of their eggs by rejector hosts; for example, in North America, American robins (<i>Turdus migratorius)</i> are robust egg rejectors of non-mimetic brown-headed cowbird (<i>Molothrus ater)</i> eggs. Recent studies have investigated the role of various hormones during the anti-parasitic egg-rejection process. Corticosterone, a steroid hormone often released in response to environmental stressors, has previously been found to increase the propensity for female American robins to reject non-mimetic model eggs. To better understand how corticosterone affects other behaviors during the egg-ejection process (i.e., distance the egg is taken from the nest, latency of rejection decision), we combined previously used techniques for non-invasive corticosterone delivery and model egg spatio-temporal tracking in American robins (<i>sensu</i> Turner et al. 2020, Turner et al. 2023). All female subjects rejected the non-mimetic eggs, and through a repeated measures design, we found that the same female robin reduced her egg ejection distance in the corticosterone treatment relative to the control. In turn, we did not find this effect for ejection latency, perhaps because all but one female already rejected the non-mimetic egg rapidly (within one hour) irrespective of the hormone treatment. Future work should examine if the other known endocrine mediators of egg rejection, including prolactin, similarly affect aspects of egg rejection behavior by this and other hosts of obligate avian brood parasites.专性育巢寄生鸟类（obligate brood parasitic birds）常会遭到拒卵宿主（rejector hosts）移除其卵；例如在北美地区，美洲旅鸫（Turdus migratorius）会强力拒除非拟态的棕色头牛鹂（Molothrus ater）卵。近期已有研究探讨了多种激素在抗寄生卵移除过程中的作用。皮质酮（corticosterone）作为一种常因环境应激释放的类固醇激素（steroid hormone），此前被发现可提升雌性美洲旅鸫拒除非拟态模型卵的倾向。为进一步明晰皮质酮如何影响卵移除过程中的其他行为（即卵被带离巢窝的距离、拒卵决策潜伏期），我们结合了此前用于美洲旅鸫的非侵入性皮质酮递送（non-invasive corticosterone delivery）技术与模拟卵时空追踪（model egg spatio-temporal tracking）方法（遵循Turner等人2020年、Turner等人2023年的研究范式）。所有受试雌性个体均拒除了非拟态卵；通过重复测量设计（repeated measures design），我们发现相较于对照组，经皮质酮处理的同一雌性旅鸫的卵移除距离有所缩短。反之，我们未在拒卵潜伏期观测到该效应，这可能是因为除1只个体外，其余所有雌性均会快速（1小时内）拒除非拟态卵，且不受激素处理的影响。未来研究可探讨其他已知的卵移除行为内分泌调控因子，如催乳素（prolactin），是否同样会影响该类及其他专性鸟类育巢寄生宿主的卵移除行为特征。