Extreme Endurance Migration: What Is the Limit to Non-Stop Flight?

Migratory birds have a history of challenging conventional wisdom about the limits of their endurance. More than half a century ago, many ornithologists doubted that a non-stop flight of 860 km across the Gulf of Mexico was possible for migratory (humming) birds [1]. But circumstantial and more direct evidence gathered in the following decade revealed that the Gulf of Mexico is a mere ditch to migratory birds [2],[3], and that some are capable of non-stop flights of up to 5,000 km [4],[5]. And now migratory birds have given their observers reason to pause yet again. In the past year, Gill et al. [6] have provided direct evidence that a shorebird, the Alaskan bar-tailed godwit (Limosa lapponica baueri) (Figure 1), makes its eight-day, 11,000-km autumn migration from Alaska to New Zealand in one step, with no stopovers to rest or refuel. This roughly doubles the previous maximum direct flight distance in birds, challenging experts to square this remarkable marathon migration with our understanding of aerodynamic theory and endurance physiology. Figure 1 Bar-tailed godwits. Has this bird finally shattered the limits of long-distance, nonstop migratory flights, forcing researchers to rethink their theories and assumptions about flight and endurance? Or is it possible to show that such feats are possible given what we already understand about aerodynamic theory, metabolism, navigation, and evolution? Here I argue that we already have the tools in hand to understand how it can fly such a distance. What then are the limits to non-stop flight, and can we expect to see these records beaten in the future? Non-stop flights are common among migratory shorebirds (also known as waders) and often involve trans-oceanic crossings, with examples being American golden plovers (Pluvialis dominica) flying 4,000 km between Nova Scotia and South America [7], ruddy turnstones (Arenaria interpres) flying 4,000 km between the Pribilof Islands and Hawaii [8], and red knots (Calidris canutus) flying 4,800 km between the Wadden Sea and their breeding area on Taymyr [9]. The evidence that the flights are indeed non-stop has often been circumstantial up until now, being based on timing of departure and arrival at main staging sites and how long the journey might take given the amount of fuel accumulated before departure; it is only recently that satellite-based tags have become small enough to allow individual shorebirds to be directly tracked as they migrate [10]. This technique enabled Gill et al. [6] to make their recent observations, and there is more recent circumstantial evidence that sharp-tailed sandpipers (Calidris acuminata) make a similar non-stop flight [11] (Figure 2). Figure 2 Sharp-tailed sandpiper.