Contrasting sensitivity of nestling and fledgling Barn Swallow Hirundo rustica body mass to local weather conditions

Local weather can influence the growth and development of young birds,
either indirectly, by modifying prey availability, or directly, by
affecting energetic trade‐offs. Such effects can have lasting implications
for life history traits, but the nature of these effects may vary with the
developmental stage of the birds, and over timescales from days to weeks.
We examined the interactive effects of temperature, rainfall and wind
speed on the mass of nestling and fledgling Barn Swallows Hirundo
rustica, both on the day of capture and averaging weather across the time
since hatching. At the daily timescale, nestling mass was negatively
correlated with temperature, but the strength of this association depended
on the level of rainfall and wind speed; nestlings were typically heavier
on dry or windy days, and the negative effect of temperature was strongest
under calm or wet conditions. At the early lifetime timescale (i.e. from
hatching to post‐fledging), nestling mass was negatively correlated with
temperature at low wind speed. Fledgling body mass was less sensitive to
weather; the only weather effects evident were a negative correlation with
temperature at the daily scale under high rainfall that became slightly
positive under low rainfall. These changes are consistent with weather
effects on availability and distribution of insects within the landscape
(e.g. causing high concentrations of flying insects), and with the effects
of weather variation on nest microclimate. These results together
demonstrate the impacts of weather on chick growth, over immediate (daily)
and longer term (nestling/fledgling lifetime) timescales. This shows that
sensitivity to local weather conditions varies across the early lifetime
of young birds (nestling‐fledgling stages) and illustrates the mechanisms
by which larger scale (climate) variations influence the body condition of
individuals.