Meeting Abstract
Birds with biparental care can maximize their fitness by producing many offspring, but producing too many offspring in a single reproductive event can be costly. Parents with large broods may feed nestlings more frequently to provide sufficient resource, but nestlings in large broods still commonly experience more competition with siblings for parental resources than nestlings raised in smaller broods. Nestlings in larger broods may experience higher levels of food stress and develop more slowly than nestlings in smaller broods. Several studies have increased or reduced brood sizes to examine effects on parents. Relatively fewer studies have considered effects on offspring and the results have been inconsistent. We conducted a brood size manipulation using eastern bluebirds (Sialia sialis) in Payne County, Oklahoma, USA. When nestlings were 2 days old, we moved 1-3 nestlings between nests with the same hatch date to create reduced and enlarged broods. Some broods were unmanipulated. To quantify parental care, we analyzed nest attendance and number of nest visits from video recordings of the adults. We weighed nestlings on days 2, 5, 11, and 15 post-hatching to measure growth rates. On day 15, we also took a blood sample to measure corticosterone (CORT) concentrations as an indicator of stress exposure. Neither nestling CORT nor body mass differed across any of the brood size groups. Adult bluebirds raising enlarged broods may have been able to compensate by increasing feeding and nest attendance. In resource abundant years, increased parental care may be less costly than in years when resources are scarce.
