Using metabolic data to investigate the role of brood size in the development of endothermy

Altricial songbirds transform themselves from naked poikilotherms to fully
feathered endothermic homeotherms over a matter of days from hatching to
fledging. The ontogeny of endothermy is a developmental milestone for
birds that not only face warmer average temperatures, but also
increasingly frequent cold snaps and extreme weather. The timing of
development of endothermy has been studied in altricial birds for over
half a century. However, the determinants and constraints 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. The thermogenic response to a cooling challenge (15°C) increased
with age overall. We found that 8-day-old nestlings from reduced broods
had a slightly increased capacity for endothermic heat production compared
to enlarged broods. This difference cannot be explained by body mass
because this trait did not differ between brood size categories. Although
a metabolic response was present in most nestlings by day 6, it was brief,
not lasting more than a few minutes, and not sufficient to maintain a
stable body temperature in any age group. Our study shows that incipient
endothermy is present at an early age in nestling blue tits and may
advance faster in reduced broods, but that individual nestlings lack
sufficient insulation and thermogenic performance to maintain homeothermy
independently during the first week of life.