Unpredictable and inclement weather is increasing in strength and frequency, challenging organisms to respond adaptively. One way in which animals respond to environmental challenges is through the secretion of glucocorticoid stress hormones. These hormones mobilize energy stores and suppress non-essential physiological and behavioral processes until the challenge passes. To investigate the effects of glucocorticoids on reproductive decisions, we experimentally increased corticosterone levels (the primary glucocorticoid in birds) in free-living female tree swallows, Tachycineta bicolor, during the chick-rearing stage. Due to an unprecedented cold and wet breeding season, 90 % of the nests in our study population failed, which created a unique opportunity to test how challenging environmental conditions interact with the physiological mechanisms underlying life-history trade-offs.
Our goal in this study was to investigate the effects of elevated cort levels on parental investment in breeding birds. We used exogenous cort pellets to experimentally increase cort levels in female tree swallows (Tachycineta bicolor) during chick-rearing. The study year (2013) was characterized by a period of exceptionally cold and rainy weather at our field site, which had a detrimental impact on the flying insects that tree swallows rely upon exclusively to feed their offspring . Repeated, sudden drops in temperature led to massive reproductive failure in the study population: 90 % of tree swallow nests failed, as opposed to an average of 53 % nest failure in this population over the previous 5 years [F. Bonier unpublished]. Although the immediate cause of this mortality remains unknown (adults may have deserted the brood, or the young could have died due to insufficient food supplies and/or hypothermia), harsh weather conditions throughout the season could have exerted strong selection on the combination of traits that enabled some individuals to cope with weather as nest desertion occurred throughout the season. Thus, our study represents a unique investigation of the combined effects of parental feeding rates prior to hormone manipulation and experimentally elevated cort levels on reproductive success under varying environmental conditions within a breeding season. We investigated how adverse weather conditions interacted with the experimental cort elevation to affect brood mortality. In addition, we investigated whether initial parental feeding rates were associated with brood survival and whether the hormonal treatment and weather conditions altered this effect.