The effects of temperature on nestling growth in a songbird depend on developmental constraints.

Climate change can adversely impact animals, especially those that cannot independently thermoregulate or avoid exposure to temperature variation. Altricial bird nestlings may be particularly vulnerable to temperature variation, with cold, hot, and variable temperatures leading to reduced nestling growth. At a broad scale, temperature effects on nestling growth vary across climatic zones, but how temperature effects vary with early-life developmental constraints imposed by the timing of thermoregulatory development, competition with siblings, and the amount of parental care has received less attention. We investigated whether the effects of temperature on the body mass of wild barn swallow (Hirundo rustica erythrogaster) nestlings (n = 113 nestlings, 31 nests) in Boulder County, CO depends on timing of exposure during development, relative size within the brood, or level of parental feeding. Lower minimum temperatures were associated with lower nestling mass in early but not late development (before but not after the putative development of thermoregulatory independence). Additionally, we found marginal evidence that the smallest nestling in the brood was more vulnerable to extreme and variable temperatures than other brood mates. Similarly, we found evidence that cold temperatures had stronger negative effects on nestlings in nests receiving low versus high levels of parent feeding. These findings indicate the existence of fine-scale heterogeneity in which the effects of temperature on nestling development are sensitive to metabolic constraints and early-life social environment, providing insight into the factors that may provide resilience to or exacerbate temperature effects on individual birds.