Experimental evidence that hyperthermia limits offspring provisioning in a temperate-breeding bird

In many vertebrates, parental care can require long bouts of daily exercise, that can span several weeks. Exercise, especially in the heat, raises body temperature, and can lead to hyperthermia. Typical strategies for regulating body temperature during endurance exercise include modifying performance to avoid hyperthermia (anticipatory regulation hypothesis) and allowing body temperature to rise above normothermic levels for brief periods of time (facultative hyperthermia hypothesis). Facultative hyperthermia is commonly employed by desert birds to economize on water, but this strategy may also be important for chick-rearing birds to avoid reducing offspring provisioning when thermoregulatory demands are high. In this study, we tested how chick-rearing birds balance their own body temperature against the need to provision dependent offspring. We experimentally increased the heat dissipation capacity of breeding female tree swallows (Tachicyneta bicolor) by trimming their ventral feathers, and remotely monitored provisioning rates, body temperature, and the probability of hyperthermia. Birds with an experimentally increased capacity to dissipate heat (i.e., trimmed treatment) maintained higher feeding rates than controls at high ambient temperatures ( ≥ 25 ºC), while maintaining lower body temperatures. However, at the highest temperatures (≥25 ºC) trimmed individuals became hyperthermic. These results provide evidence that chick-rearing tree swallows use both anticipatory regulation and facultative hyperthermia during endurance performance. With rising global temperatures, individuals may need to increase their frequency of facultative hyperthermia to maintain nestling provisioning, and thereby maximize reproductive success.