The fall migration of Pacific Flyway Brent Branta bernicla in relation to climatic conditions

Abstract

Fall migrations of Brent from Izembek Lagoon, Alaska, to wintering areas in Baja California, Mexico were studied for 30 years from 1959 to 1988. Surface and upper air weather patterns were analyzed for departure, approximate mid-route and arrival locations. Radar was used to determine the average departure direction and altitude of 30 migrating flocks within 130 km of Izembek Lagoon. Based on radar observations, upper air wind directions at the 850 millibar level were used to estimate the most favorable migration route to wintering areas in Mexico. Estimated migration routes averaged 5301 km v a direct route distance of 4408 km. Estimated time in route averaged 54.3 hours and average ground speed was 99 kph based on probable routes over the 30 year period. During 1974, 1983 and 1984, observers in Alaska and Mexico documented Brent departures and arrivals, estimating times en route to be 60, 60 and 95 hours, respectively. The configuration of the departure weather system, and the direction and velocity of winds, are important factors causing variability in observed flight duration. The rapid fall migration of Pacific populations of Brent is energetically costly with males and females, respectively, losing an estimated 33% and 31% of their total body weights. Physiological demands of migration from Izembek Lagoon to Mexico may exceed the amounts of body reserves accumulated during the fall staging period.