Data from: 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 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.