Data from: Early to rise, early to breed: a role for daily rhythms in seasonal reproduction

Vertebrates use environmental cues to time reproduction to optimal
breeding conditions. Numerous laboratory studies have revealed that light
experienced during a critical window of the circadian (daily) rhythm can
influence reproductive physiology. However, whether these relationships
observed in captivity hold true under natural conditions and how they
relate to observed variation in timing of reproductive output remains
largely unexplored. Here we test the hypothesis that individual variation
in daily timing recorded in nature (i.e. chronotype) is linked with
variation in timing of breeding. To address this hypothesis and its
generality across species, we recorded incubation behavior data to
identify individual patterns in daily onset of activity for 2
temperate-breeding songbird species, the dark-eyed junco (Junco hyemalis
aikeni) and the great tit (Parus major). We found that females who first
departed from their nest earlier in the morning (earlier chronotype) also
initiated nests earlier in the year. Date of data collection and ambient
temperature had no effect, but stage of incubation influenced daily onset
of activity in great tits. Our findings suggest a role for daily rhythms
as one mechanism underlying the observed variation in seasonal timing of
breeding.