Relative breeding timing and reproductive success of a resident montane bird species

Wild populations appear to synchronize their reproductive phenology based
on numerous environmental and ecological factors, yet there is still
individual variation in the timing of reproduction within populations and
such variation may be associated with fitness consequences. For example,
many studies have documented a seasonal decline in reproductive fitness,
but breeding timing may have varying consequences across different
environments. Using 11 years of data, we investigated the relationship
between relative breeding timing and reproductive success in resident
mountain chickadees (Poecile gambeli) across two elevational bands in the
Sierra Nevada mountains, USA. Chickadees that synchronized breeding with
the majority of the population (“peak” of breeding) did not have the
highest breeding success; instead, birds that bred early performed best at
high elevation, while at low elevation early and peak nests performed
similarly. At both elevations, late nests consistently performed the
worst. Overall, breeding success decreased with increasing relative timing
at both high and low elevations, but the relationship between breeding
success and timing differed among years. Our results suggest that in
mountain chickadees, earlier breeding is associated with higher
reproductive success, especially at high elevations, while late breeding
is consistently associated with lower reproductive success at both
elevations.