Experimentally increased nest temperature affects body temperature, growth and apparent survival in blue tit nestlings

The thermal environment experienced by birds during early postembryonic development may be an important factor shaping growth and survival. However, few studies have directly manipulated nest temperature (Tn) during the nestling phase, and none have measured the consequences of experimental heat stress on nestlings’ body temperature (Tb). It is therefore not known to what extent any fitness consequences of development in a thermally challenging environment arise as a direct, or indirect, effect of heat stress. We, therefore, studied how experimentally increased Tn affected Tb in 8-12 days old blue tit (Cyanistes caeruleus) nestlings, to investigate if increased thermoregulatory demands to maintain normothermic Tb influenced nestling growth and apparent long-term survival. Nestlings in heated nest-boxes had significantly higher Tb compared to unheated nestlings during most of the experimental period. Yet, despite facing Tn > 50 °C (as measured in the bottom of the nest cup below the n...