SPRING MIGRATION OF PACIFIC LOONS THROUGH THE SOUTHERN CALIFORNIA BIGHT - NEARSHORE FLIGHTS, SEASONAL TIMING AND DISTRIBUTION AT SEA

We studied the spring migration of Pacific Loons (Gavia pacifica) from a coastal promontory and during shipboard transects in the Southern C alifornia Bight. The coastline in the northern part of the bight acts as a leading line that diverts northbound migrants toward the west. We detected no effect of the onshore wind component on migration traffic rates along the coast and rejected the hypothesis that nearshore flig hts resulted from wind drift. Migration of Pacific Loons proceeded in headwinds up to 13 m/sec, but traffic rates decreased with increasing headwind strength, suggesting that loons avoided flying in the most en ergetically unfavorable conditions. The timing of peak migratory fligh ts varied from mid-April to early May in different years, perhaps refl ecting constraints imposed by the availability of food in wintering ar eas, at premigratory staging sites, and/or at en-route stopover sites. At sea, migrant loons were concentrated in cool waters in the norther n part of the bight, and observations of loons on the water were assoc iated with high densities of zooplankton at nearby sampling stations. We hypothesize that many northbound loons stop in the northern bight t o feed on macrofauna attracted by concentrations of zooplankton near t he frontal boundaries of upwelling plumes. Such stopovers should allow migrant loons to replenish energy stores efficiently and to resume no rthward migration rapidly.