Spring and Autumn Migration in Arctic Shorebirds: Same Distance, Different Strategies

Abstract

The Arctic is an extremely inhospitable region for most of the year, but during the summer months it bursts with life. A major proportion of avian species nesting in the Arctic are shorebirds (order Charadriiformes; suborder Charadrii). They migrate thousands of kilometers from their wintering grounds to take advantage of abundant food resources each summer and display a variety of migratory strategies. In an attempt to classify this variation, not only between spring and autumn migration, but within a migration, we present four categories. These relate to the distance a species generally flies between stopovers: short distance bout, intermediate distance bout, long distance bout, and combinations. We then explore further differences between spring and autumn migration. Spring migrants experience poor weather and decreased food availability as they fly north. Many cope with huge flocks, which serve as protection from predators, but may also reduce foraging efficiency and increase aggression. In contrast, autumn migrants generally encounter favorable weather and ample food. Flock sizes are usually smaller, thus foraging efficiency is higher and aggression lower than during spring migration. Physiologically, spring migrants are preparing for breeding and reproductive hormones are secreted. In the Western Sandpiper ( Calidris mauri ), luteinizing hormone levels are higher for spring than autumn migrants. Late spring migrants have higher testosterone levels than either early spring migrants or autumn migrants. Corticosterone levels are also higher in spring vs . autumn migrants. Although spring and autumn migrants travel similar distances, their strategies differ behaviorally and physiologically.