SPRING AND AUTUMN MIGRATION IN ARCTIC SHOREBIRDS - SAME DISTANCE, DIFFERENT STRATEGIES

The Arctic is an extremely inhospitable region for most of the year, b ut during the summer months it bursts with life. A major proportion of avian species nesting in the Arctic are shorebirds (order Charadriifo rmes; suborder Charadrii). They migrate thousands of kilometers from t heir wintering grounds to take advantage of abundant food resources ea ch summer and display a variety of migratory strategies. In an attempt to classify this variation, not only between spring and autumn migrat ion, but within a migration, we present four categories. These relate to the distance a species generally flies between stopovers: short dis tance bout, intermediate distance bout, long distance bout, and combin ations. 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 serv e as protection from predators, but may also reduce foraging efficienc y and increase aggression. In contrast, autumn migrants generally enco unter favorable weather and ample food. Flock sizes are usually smalle r, 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 Sandpi per (Calidris mauri), luteinizing hormone levels are higher for spring than autumn migrants. Late spring migrants have higher testosterone l evels than either early spring migrants or autumn migrants. Corticoste rone levels are also higher in spring vs. autumn migrants. Although sp ring and autumn migrants travel similar distances, their strategies di ffer behaviorally and physiologically.