Autumnal Migration Departure Schedules in Mountain White-Crowned Sparrows

Abstract

During seven autumn seasons, Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) departed on migration from a subalpine meadow in September and October. On average, juveniles migrated 3.2 days earlier than adults and there was no difference in schedules of males and females within these two classes. Data from frequently trapped individuals showed that they departed as soon as they became fat. Peaks in departure dates did occur but only once did one occur just before a storm. Mean departure dates among years varied by 14 days in juveniles and eight days in adults. This interannual variation was related to reproductive schedule; departure of the population being delayed by about one day for every two days of delay in nesting (the latter being caused by late-lying snows). Reasons for a decrease in the fledging-migration interlude in juveniles in heavy snow years could have involved an overlapping of postjuvenal molt with premigratory fattening and/or telescoping of molt. Late-born young did not migrate later than those born earlier nor were they leaner at the time of departure. Temporal compensation in adults during heavy snow years was accomplished primarily by overlapping postnuptial molt with parental care and sometimes with premigratory fattening because molt duration did not decrease with season. Premigratory fattening occurred in about nine days in all birds with 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 not 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 usually served as the proximate stimulus for migration.