Environmentally relevant bouts of cooling stimulate corticosterone secretion in free-living eastern bluebird (Sialia sialis) nestlings: Potential links between maternal behavior and corticosterone exposure in offspring

• We studied the effects of cooling on corticosterone secretion in bluebird chicks.
• A < 10 °C reduction of chick surface temperature elevated corticosterone secretion.
• Thermal and hormonal responses to cooling increased in an age-dependent manner.
• Some broods naturally experienced such bouts of cooling in the wild.
• Variation in maternal brooding may affect glucocorticoid exposure early in life.

In vertebrates, exposure to stressful stimuli or to elevated glucocorticoids early in development can contribute to phenotypic variation that may have significant fitness consequences. In species with altricial young, offspring may be partially buffered from elevations in glucocorticoids by a period of low glucocorticoid responsiveness to stressors coupled with high levels of parental care. Because altricial young depend heavily on their parents for warmth, parental brooding behavior could buffer offspring from glucocorticoid exposure associated with cooling. We studied eastern bluebirds (Sialia sialis) with two goals: (1) to determine whether an experimental drop in body temperature such as that which might occur when a brooding female is off the nest was sufficient to stimulate glucocorticoid secretion in young chicks, and (2) to examine the extent to which chicks experienced such bouts of cooling in the field. We subjected chicks to treatments simulating nest temperatures while females were brooding or absent from the nest. We also recorded chick surface temperatures and ambient temperatures at nests during the first week of the brood period. Reductions of surface temperature of less than 10 °C significantly elevated corticosterone secretion in chicks as young as 5 days old, and thermal and hormonal responses of chicks to cooling increased in an age-dependent manner. One quarter of broods experienced repeated, natural bouts of cooling of this magnitude or greater in the nest. Our data suggest that natural variations in maternal brooding patterns can result in differential exposure of offspring to glucocorticoids, and this may have important phenotypic consequences later in life.