Brood size constrains the development of endothermy in blue tits

Altricial birds are unable to maintain body temperature when exposed to low ambient temperatures during the first days after hatching. Thermoregulatory capacity begins to form as postnatal development progresses, and eventually nestlings become homeothermic. Several factors may influence this development at both the level of the individual and the level of the whole brood, but to our knowledge no studies have focused on the effect of brood size per se on the development of endothermy in individual nestlings. We performed cooling experiments on blue tit (Cyanistes caeruleus) nestlings in the field, to study how different experimental brood sizes affected the development of endothermy in individual nestlings and the thermal environment experienced by the whole brood in the nest. Nestlings from all experimental brood sizes showed a decrease in cooling rate as they grew older, but birds from reduced broods showed an earlier onset of endothermy compared with nestlings from enlarged and contr...