Using metabolic data to uncover the ontogeny of endothermy in an altricial bird

Altricial songbirds transform themselves from naked poikilotherms to fully-feathered endothermic homeotherms over a matter of weeks from hatching to fledging. The timing of development of endothermy has been studied in altricial birds for over half a century. However, the potential determinants, constraints, and flexibility of the onset of endothermy are not yet fully understood. We experimentally investigated whether brood size influences the ontogeny of endothermic heat production in 4-8 day-old nestling blue tits ( Cyanistes caeruleus ) in southern Sweden. We found that both nestlings’ capacity to produce heat and the proportion of nestlings exhibiting a metabolic response to a cooling challenge (15 °C for 15 min) increased with age. However, we did not find an effect of brood size on the ontogeny of endothermy and this may be explained by the lack of differences in body mass of nestlings in enlarged and reduced broods in our study. The metabolic response in all age groups was brief and not sufficient to sustain a stable body temperature. Our study shows that the development and use of endothermy is present at an early age in nestling blue tits, but further study is needed to disentangle the factors contributing to individual variation in the timing of these developments. This will serve to improve our understanding of a developmental milestone of cold tolerance in birds in the temperate zone that not only face warmer average temperatures, but also increasingly frequent cold snaps and extreme weather. Summary Statement This study serves to improve our understanding of the ontogeny of endothermy in altricial birds while examining the potential role of brood size in the timing thereof. ### Competing Interest Statement The authors have declared no competing interest.