THE ALTITUDE OF BIRD MIGRATION IN EAST-CENTRAL ALASKA - A RADAR AND VISUAL STUDY

Flight altitudes of birds were monitored in east-central Alaska during spring and fall migration periods with visual (1987-1989) and radar ( 1988-1989) methods. Visual observations indicated that diurnal flights occurred primarily below 300 m above ground level (agl). Radar observ ations indicated that nocturnal flights generally occurred below 500 m agl. Flight altitudes were significantly higher in fall than in sprin g, and there were interannual differences in both seasons. There was h igh night-to-night variability in nocturnal flight altitudes during bo th seasons. This night-to-night variability probably was not related t o daily changes in the magnitude of migration: there was a very low co rrelation between mean nightly flight altitudes and mean nightly migra tion rates. Nocturnal flight altitudes were higher from April to early May than from mid- to late May and higher from late August to early S eptember than from mid-September to mid-October. These seasonal differ ences in flight altitudes largely were due to changing species composi tion within a season. When spring radar data were partitioned among li ght conditions, flight altitudes were found to be similar between dayl ight and crepuscular periods and between daylight and nocturnal period s, but birds flew significantly higher in crepuscular than nocturnal p eriods. In fall, nocturnal altitudes were significantly higher than cr epuscular and daylight altitudes, which were similar. Within a night, there was no near-midnight peak in flight altitudes in either spring o r fall.