AUTUMNAL MIGRATION DEPARTURE SCHEDULES IN MOUNTAIN WHITE-CROWNED SPARROWS

During seven autumn seasons, Mountain White-crowned Sparrows (Zonotric hia leucophrys oriantha) departed on migration from a subalpine meadow in September and October. On average, juveniles migrated 3.2 days ear lier than adults and there was no difference in schedules of males and females within these two classes. Data from frequently trapped indivi duals showed that they departed as soon as they became fat. Peaks in d eparture dates did occur but only once did one occur just before a sto rm. Mean departure dates among years varied by 14 days in juveniles an d eight days in adults. This interannual variation was related to repr oductive schedule; departure of the population being delayed by about one day for every two days of delay in nesting (the latter being cause d by late-lying snows). Reasons for a decrease in the fledging-migrati on interlude in juveniles in heavy snow years could have involved an o verlapping of postjuvenal molt with premigratory fattening and/or tele scoping of molt. Late-born young did not migrate later than those born earlier nor were they leaner at the time of departure. Temporal compe nsation in adults during heavy snow years was accomplished primarily b y overlapping postnuptial molt with parental care and sometimes with p remigratory fattening because molt duration did not decrease with seas on. Premigratory fattening occurred in about nine days in all birds wi th the average gain per day being 2.3 to 2.6% of initial body mass for the four sex-age groups. Propensity to return to the study area was n ot related to the body mass at departure in the previous autumn or to its date. It was hypothesized that the attainment of peak fat stores u sually served as the proximate stimulus for migration.