American Society of Naturalists

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

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Carolyn M. Bauer, Jessica L. Graham, Mikus Abolins-Abols, Britt J. Heidinger, Ellen D. Ketterson, and Timothy J. Greives (June 2018)

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Older birds or those with shorter telomeres (closer to senescence) breed earlier (increase seasonal reproductive effort)

Birds use time and telomeres to determine when to breed

A banded female Dark-eyed Junco (<i>Junco hyemalis</i>) from Mountain Lake Biological Station.<br />(Photo by Jessica L. Graham)
A banded female Dark-eyed Junco (Junco hyemalis) from Mountain Lake Biological Station.
(Photo by Jessica L. Graham)

For seasonally breeding birds, individuals that breed earlier in the season tend to have higher reproductive success than birds that breed later. Why, then, do we see variation in the timing of when females lay their first clutch of eggs? Evidence from other studies suggests that it’s costly for females to breed early in the season, as early breeding females are less likely to survive to the next year. Lead author Carolyn Bauer (Adelphi University) wondered if the number of future breeding seasons a female is expected to have might influence timing of reproduction. The question is, do birds with fewer, future reproductive events increase their current reproductive effort by breeding earlier in the season?

A female Dark-eyed Junco (<i>Junco hyemalis</i>) incubating her eggs.<br />(Photo by Jessica L. Graham)
A female Dark-eyed Junco (Junco hyemalis) incubating her eggs.
(Photo by Jessica L. Graham)

To determine the number of “future reproductive events,” Bauer and colleagues (Jessica Graham, Britt Heidinger, and Timothy Greives from North Dakota State University, and Mikus Abolins-Abols and Ellen Ketterson from Indiana University), measured both chronological age and an estimate of biological age in a population of Dark-eyed Juncos (Junco hyemalis) at Mountain Lake Biological Station in western Virginia. Chronological age was determined from banding records. Biological age was estimated by measuring the length of telomeres, which are located at the ends of chromosomes and shorten during cell division and in response to stress exposure. Timing of breeding was determined via intensive searching for nests at the start of two breeding seasons.

Together, Bauer and colleagues found that both older females and those with shorter telomeres tended to breed earlier. Importantly, these effects were independent of each other, as telomere length did not differ between old and young birds. These findings suggest that females may increase their reproductive effort when they have fewer expected future breeding events. The next exciting question is which cues of remaining lifespan are birds potentially using to adjust reproductive timing?


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.