Extreme endurance flights by landbirds crossing the Pacific Ocean: ecological corridor rather than barrier?

Abstract

Mountain ranges, deserts, ice fields and oceans generally act as barriers to the movement of land-dependent animals, often profoundly shaping migration routes. We used satellite telemetry to track the southward flights of bar-tailed godwits (Limosa lapponica baueri), shorebirds whose breeding and non-breeding areas are separated by the vast central Pacific Ocean. Seven females with surgically implanted transmitters flew non-stop 8,117-11,680 km (10153+/-1043 s.d.) directly across the Pacific Ocean; two males with external transmitters flew non-stop along the same corridor for 7,008-7,390 km. Flight duration ranged from 6.0 to 9.4 days (7.8+/-1.3 s.d.) for birds with implants and 5.0 to 6.6 days for birds with externally attached transmitters. These extraordinary non-stop flights establish new extremes for avian flight performance, have profound implications for understanding the physiological capabilities of vertebrates and how birds navigate, and challenge current physiological paradigms on topics such as sleep, dehydration and phenotypic flexibility. Predicted changes in climatic systems may affect survival rates if weather conditions at their departure hub or along the migration corridor should change. We propose that this transoceanic route may function as an ecological corridor rather than a barrier, providing a wind-assisted passage relatively free of pathogens and predators.

Figure 1

Southward flight tracks of nine bar-tailed godwits fitted with satellite transmitters (PTTs) during 2006 and 2007. Circles denote Argos locations collected during 6–8-hour intervals, and solid lines show interpolated 24–36-hour tracks between the PTT-reporting periods (see §2). Dotted lines are extensions of tracks between the last report of a PTT from a bird in flight and a confirmed sighting elsewhere of that bird. The dashed line represents the portion of flight following a confirmed stopover by a bird. Tracks are plotted on a Blue Marble image, geographic (Plate Carrée) projection (Stöckli et al. 2005). Inset shows individual track directions of nine PTT-tagged godwits departing on southward migration from Alaska (light blue circles) relative to directions towards which wind was blowing at 850 mb geopotential height (approx. 1500 m) during departures (orange circles). Arrows show mean direction of departing godwits (193°, light blue) and associated winds (174°, orange); length of arrows indicates strength of directionality (r=0.95, godwits; r=0.90, wind).

Figure 2

Average track speed (m s⁻¹) of bar-tailed godwits during transmitter-reporting duty cycles (1.5 to 9.2-hour duration) relative to the distance tracked from Alaska (km). Track speed varied as a quadratic function of distance from departure site (p=0.04, r²=0.17) and reflected latitudinal differences in wind speed. Solid curve shows best-fit regression and dotted curves show 90% prediction intervals. Godwit H4 (diamond) encountered extremely strong headwinds north of Hawaii; godwit Z7 (square) was assisted by moderate tailwinds near Fiji. Track speeds of all godwits during other duty cycles (black circles) fell within 90% prediction intervals.

Figure 3

Mean SLP (mb) across the North Pacific Ocean during staging and departure of bar-tailed godwits. (a) Averaged for 8 days when godwits departed Alaska. (b) Averaged for 55 days during intervening periods when no tagged birds departed. Areas of low pressure (blue) are associated with storm centres and strong cyclonic (anticlockwise) winds in the region. Filled circle (a) denotes the site from which all godwits departed.