Data from: Nest-box temperature affects clutch size, incubation initiation, and nestling health in great tits

Prenatal maternal effects can be a source of phenotypic plasticity and may
play a role in adaptation to climate change. However, we do not know how
far temperature could influence such effects, if at all. We studied the
influence of temperature during egg laying on maternal reproductive
investment and on the phenotype of adult females, adult males, and
nestlings. We expected temperature to have an effect, as it influences
maintenance costs for females, who can also use it as a cue of the
advancement of the breeding season. We experimentally increased night-time
nest-box temperatures by approximately 1 °C throughout the entire laying
period in great tits (Parus major). Clutch size was negatively correlated
with laying date in heated females. Heated females did not delay
incubation after clutch completion as frequently as control females did.
Finally, blood sedimentation rate, which is an indicator of acute
infections and inflammatory diseases, was positively correlated with
hatching date in control broods. This suggests that nestlings were of
lower quality in late-hatched broods than in early-hatched broods. This
seasonal effect was not detected in heated nests. Our results show that a
small increase in temperature during laying can influence breeding
strategy and nestling characteristics. These results suggest that birds
used temperature as a cue of seasonal advancement to adjust breeding
phenology, with beneficial effects on nestling health. To better
understand the consequences of maternal adjustments during egg laying, it
would be interesting to combine studies with heating treatment during
different periods of the breeding cycle.