Data from: Breeding season length and nest mortality drive cryptic life history variation in Dark-eyed Juncos (Junco hyemalis) breeding across a montane elevation gradient

The manner in which individual life history traits respond to the
environment and to each other, and how these traits combine to form
overall patterns of life history variation, remains poorly characterized
in wild populations. We monitored breeding Dark-eyed Juncos (Junco
hyemalis) across a 700-m elevational range. We compared breeding season
length, temporal patterns of breeding activity, adult body size, clutch
size, brood size, nestling quality, and nest mortality among elevations.
We also compared environmental measures across the studied elevations to
determine whether abiotic factors explained life history trait variation.
We used 12 microsatellite loci to test for genetic differentiation in
populations at different elevations. Finally, we constructed a computer
simulation to evaluate the combined effects of observed variation in life
history traits. We found differences among elevations in breeding season
length and in patterns of reproductive timing, which did not match each
other and which were not explained solely by abiotic factors. We found no
differences among elevations in adult body size, clutch size, brood size,
or nestling quality. Nest mortality increased significantly with
elevation. Genetic differentiation was too low to define distinct
subpopulations. The simulation suggested that differences in mortality, in
combination with differences in breeding season length, contributed to
substantial differences in reproductive success among elevations. Thus,
although individual life history traits showed little evidence of
variation in response to the environment or to each other and little
genetic differentiation, variation in breeding season length and in nest
mortality were potential drivers of substantial elevational variation in
overall life history in this system. These results demonstrate that
individual life history traits may vary substantially in their patterns of
variation, and that some life history traits may have disproportionate
effects on overall life history.