Timing of breeding range occupancy among high-latitude passerine migrants

The brief subarctic summer limits the time available for birds to complete their reproductive activities, yet the temporal requirements of high-latitu de passerine migrants are not well understood. Our analyses examined the ti ming of spring and autumn migration among 18 passerine species to obtain in direct estimates of the time they occupy their breeding ranges in northwest ern North America. From 1992 to 1998, the Alaska Bird Observatory (64 degre es 50 'N, 147 degrees 50 'W) banded 31,698 individuals during the most inte nsive standardized mist-netting study ever conducted in subarctic North Ame rica. Among the migrants examined, the estimated number of clays that speci es were present in interior Alaska ranged from 48 days for adult Alder Flyc atchers (Empidonax alnorum) to 129 days for American Robins (Turdus migrato rius). Adults departed significantly later in autumn than immatures in 10 o f 18 species we examined and significantly earlier than immatures in only o ne species, Alder Flycatcher. Breeding range occupancy of Nearctic-Neotropi c migrants occurs in this region within the range of average frost-free tem peratures in Fairbanks, Alaska, and is significantly shorter in duration th an among Nearctic-Nearctic ("short-distance") migrants at this latitude.