Reproductive development according to elevation in a seasonally breeding male songbird

Abstract

Seasonal temperate zone breeders respond to increasing day length to anticipate the approach of spring breeding conditions. Other (supplementary) environmental cues, such as temperature and precipitation, were historically thought to play unimportant roles in reproductive timing. We demonstrate variation in reproductive timing across small geographic distances by examining the vernal testicular recrudescence of adult song sparrows (Melospiza melodia morphna) breeding in coastal (0–10 m elevation) and montane (280–1220 m elevation) habitats. Each year, these birds experienced the same photoperiod, but were exposed to different supplementary cues that varied with altitude. Coastal birds experienced warmer and more stable temperatures during late winter and early spring than did montane birds. We measured bud opening, emergence of new green shoots, and arthropod biomass to monitor the pace of spring’s approach. New spring shoots emerged 2 months earlier on the coast than in the mountains and buds on flowering trees and shrubs also tended to open earlier at the coast. Arthropod biomass was similar in both the mountains and the coast during early spring, and began to increase in early summer. Reproductive morphology (i.e. testis volume and cloacal protuberance length) developed up to 2 months earlier on the coast than in the mountains. Testicular recrudescence occurred earlier on the coast in most years and proceeded at a faster rate in 1 year. Circulating levels of luteinizing hormone, follicle stimulating hormone and prolactin increased through the season, but did not correlate with differences between sites. Both populations responded similarly when exposed to identical photoperiodic cues in the laboratory. Therefore, we suggest that an integrated response to cues characteristic of location and elevation account for differences in patterns measured in the field.