Data from: 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 control broods. This difference manifested during early
development and gradually disappeared as nestlings grew older. The thermal
environment in the nests differed between treatments during nestling
development, such that nest temperature in reduced broods was lower than
that in enlarged broods during all days and during nights at the end of
the experimental period. We suggest that the development of endothermy in
blue tit nestlings is not ontogenetically fixed, but instead may vary
according to differences in developmental, nutritional and thermal
conditions as determined by brood size.