Data from: Variation in selective regimes drives intraspecific variation in life-history traits and migratory behaviour along an elevational gradient

Comparative studies, across and within taxa, have made important
contributions to our understanding of the evolutionary processes that
promote phenotypic diversity. Trait variation along geographic gradients
provides a convenient heuristic for understanding what drives and
maintains diversity. Intraspecific trait variation along latitudinal
gradients is well-known, but elevational variation in the same traits is
rarely documented. Trait variation along continuous elevational gradients,
however, provides compelling evidence that individuals within a breeding
population may experience different selective pressures. Our objectives
were to quantify variation in a suite of traits along a continuous
elevational gradient, evaluate whether individuals in the population
experience different selective pressures along that gradient, and quantify
variation in migratory tendency along that gradient. We examined variation
in a suite of 14 life history, morphological, and behavioral traits,
including migratory tendency, of yellow-eyed juncos along a continuous
1000-m elevational gradient in the Santa Catalina Mountains of Arizona.
Many traits we examined varied along the elevational gradient. Nest
survival and nestling growth rates increased, while breeding season
length, renesting propensity, and adult survival declined, with increasing
elevation. We documented the migratory phenotype of juncos (partial
altitudinal migrants) and show that individual migratory tendency is
higher among females than males and increases with breeding elevation. Our
data support the paradigm that variation in breeding season length is a
major selective pressure driving life history variation along elevational
gradients and that individuals breeding at high elevation pursue
strategies that favor offspring quality over offspring quantity.
Furthermore, a negative association between adult survival and breeding
elevation and a positive association between nest survival and breeding
elevation helps explain both the downslope and reciprocal upslope seasonal
migratory movements that characterize altitudinal migration in many birds.
Our results demonstrate how detailed studies of intraspecific variation in
suites of traits along environmental gradients can lend new insights into
the evolutionary processes that promote diversification and speciation,
the causes of migratory behavior, and how animal populations will likely
respond to climate change.