High and low, fast or slow: the complementary contributions of altitude and latitude to understand life-history variation.

Dark-eyed juncos (Junco hyemalis) from two elevations in the Rocky Mountains of Canada display different life histories. Birds breeding at high elevation face a later and compressed reproductive season during which they raise on average half the number of broods (young) than their low-elevation conspecifics. Fledglings at high elevation are heavier, fatter, and have an increased chance of surviving to 25 days of age. Likewise, survival of adults (males) increases at higher elevation. Put into a broader perspective, high-elevation juncos show the life-history strategy characteristic for low-latitude birds. This raises questions about the mechanisms influencing life-history evolution.