The correlates of intraspecific variation in nest height and nest building duration in the Eurasian blue tit Cyanistes caeruleus

Birds build nests primarily as a receptacle to lay their eggs in, but they
can also provide secondary benefits including structural support,
camouflage, and adjustment of the microclimate surrounding the eggs and
offspring. The factors underlying intraspecific variation in nest
characteristics are poorly understood. In this study, we aim to identify
the environmental factors that predict nest height variation and the
duration of nest building in blue tits (Cyanistes caeruleus), evaluating
latitude, elevation, temperature, and the timing of egg-laying as
predictors of nest height, while also taking into account female and male
parental identity. Using 713 nest height observations collected over a
period of five years along a 220km transect in Scotland, we found that if
the annual mean timing of egg-laying was earlier, nests were taller.
However, there was no correlation between nest height and elevation,
latitude, the minimum temperature in the 14 days pre-egg-laying or the
phenology of birds within a year. Female parental identity accounted for a
large amount of variation in nest height, suggesting that individual
behaviour has an influence on nest structure. We also found that nest
building duration was shorter in years when egg laying occurred earlier in
the year, and that across all observations taller nests took longer to
build. Overall, our results show that blue tits are able to alter their
nest characteristics based on conditions environmental gradients like
latitude (in the case of building duration) and the annual mean
phenological variation timing of egg laying, and that birds build
relatively taller nests faster.