Huddled together in a nest for the first 3 weeks of their lives, tiny, defenceless blue tit chicks rely on their siblings and parents for warmth. The birds’ minute muscles are too small to generate heat through shivering, although by the time the chicks are ready to fledge, they are mature enough to generate their own warmth. Fredrik Andreasson, Andreas Nord and Jan-Åke Nilsson, from Lund University, Sweden, explain that several factors, such as the nestlings’ growth rate and development of plumage, affect when the youngsters develop the ability to maintain a stable body temperature. However, growing up in a large brood can reduce the amount of food available to a developing chick, despite reducing the urgency with which a chick must develop endothermy thanks to the insulation and warmth provided by family members, so Andreasson and his colleagues wondered how being reared in small and large families might affect the rate at which chicks develop the ability to regulate their own body temperature.
After reassigning blue tit chicks from the shores of Lake Krankesjön, Sweden, between broods to create large (15–16 chicks) and small (5–6 chicks) nests, the team then measured how the developing birds managed their body temperature when they placed individuals in a chilly cup for 5 min periods from the ages of 4–10 days. Even at the youngest age, the birds from the smaller nests coped better than the birds from the larger nests, which cooled 46% more (∼0.6°C g−0.67 min−1) than the birds from the smaller nests (∼0.4°C g−0.67 min−1). However, by day 10, chicks from both the large and small nests were all able to hold their temperature better and only cooled at ∼0.1°C g−0.67 min−1. And when the team checked the temperature in the chick's nests, the larger broods were warmer (37.9°C) than the smaller broods (37.1°C). So nestlings from smaller broods developed endothermy faster than chicks from larger, warmer families, and the team says, ‘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’.
