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 nestlings), the highest nestling Tb recorded was 43.8 °C with nestlings showing evidence of controlled facultative hyperthermia without any increased nestling mortality in heated nests. However, body mass gain was lower in these nestlings compared to nestlings from control nest-boxes. Contrary to our prediction, a larger proportion of nestlings from heated nest-boxes were recaptured during their first winter, or subsequently recruited into the breeding population as first- or second-year breeders. This result should, however, be treated with caution because of low recapture rates. This study highlights the importance of the thermal environment during nestling development, and its role in shaping both growth patterns and possibly also apparent survival.

Measurements of body- and nest temperatures, nestling biometrics and survival data of blue tit nestlingsBody-, ambient- and nest temperature together with nestling biometrics, survival data and direction of nest-boxes. The last sheet, named Metadata, contains descriptions of all variables.Andreassonetal2017.xlsx