Data from: Chronological and biological age predict seasonal reproductive timing: an investigation of clutch initiation and telomeres in birds of known age

Female vertebrates that breed earlier in the season generally have greater
reproductive success. However, evidence suggests that breeding early may
be costly, thus leading to the prediction that females with fewer future
reproductive events will breed earlier in the season. While chronological
age is a good indicator of remaining lifespan, telomere lengths may also
be good biomarkers of longevity as they potentially reflect lifetime
'wear and tear' (i.e. "biological age"). We examined
whether variation in the timing of the first seasonal clutch was related
to age and telomere length in female dark-eyed juncos (Junco hyemalis),
predicting that older females and those with shorter telomeres would breed
earlier. Both predictions held true and were independent of each other, as
telomere length did not significantly vary with age. These results suggest
that females may adjust their reproductive effort based on both
chronological and biological age.