PERFITO, N; TRAMONTIN, A.D.; MEDDLE, S.L.; WINGFIELD, J.C.: Environmental and endocrine regulation of reproduction in song sparrows along an altitudinal gradient.

While the direct effects of daylength on seasonal reproduction are well understood, less is known about how supplementary cues (e.g. temperature, rainfall) influence this process. We compared seasonal gonadal growth among non-migratory populations of song sparrows Melospiza melodia morphna breeding along an elevational gradient from the coast of western Washington (3 m) to the Cascade Mountain Range (1600 m). The study sites along this gradient were located on the same latitude, and therefore all sites experienced identical daylength. Vernal gonadal growth was generally delayed in birds breeding in the mountains relative to those breeding on the coast. However, birds from both populations exhibited similar rates of reproductive growth when held in the laboratory under identical photoperiodic conditions. This suggests that individuals from different populations respond similarly to changing photoperiod, and that supplementary cues characteristic of location and elevation account for different patterns of gonadal growth observed in the field. Two supplementary cues that were strongly correlated with gonadal volume in the field, temperature and water availability, were tested separately in the laboratory and both influenced photoperiodically induced gonadal growth. The hormonal correlates of these adjustments in timing of gonadal growth will be discussed. These data suggest that supplementary cues play an important role in reproductive timing in the field for these sedentary populations of songbirds.