The roller coaster flight strategy of bar-headed geese conserves energy during Himalayan migrations.

The physiological and biomechanical requirements of flight at high altitude have been the subject of much interest. Here, we uncover a steep relation between heart rate and wingbeat frequency (raised to the exponent 3.5) and estimated metabolic power and wingbeat frequency (exponent 7) of migratory bar-headed geese. Flight costs increase more rapidly than anticipated as air density declines, which overturns prevailing expectations that this species should maintain high-altitude flight when traversing the Himalayas. Instead, a "roller coaster" strategy, of tracking the underlying terrain and discarding large altitude gains only to recoup them later in the flight with occasional benefits from orographic lift, is shown to be energetically advantageous for flights over the Himalayas.

The paradox of extreme high-altitude migration in bar-headed geese Anser indicus.

Bar-headed geese are renowned for migratory flights at extremely high altitudes over the world's tallest mountains, the Himalayas, where partial pressure of oxygen is dramatically reduced while flight costs, in terms of rate of oxygen consumption, are greatly increased. Such a mismatch is paradoxical, and it is not clear why geese might fly higher than is absolutely necessary. In addition, direct empirical measurements of high-altitude flight are lacking. We test whether migrating bar-headed geese actually minimize flight altitude and make use of favourable winds to reduce flight costs. By tracking 91 geese, we show that these birds typically travel through the valleys of the Himalayas and not over the summits. We report maximum flight altitudes of 7290 m and 6540 m for southbound and northbound geese, respectively, but with 95 per cent of locations received from less than 5489 m. Geese travelled along a route that was 112 km longer than the great circle (shortest distance) route, with transit ground speeds suggesting that they rarely profited from tailwinds. Bar-headed geese from these eastern populations generally travel only as high as the terrain beneath them dictates and rarely in profitable winds. Nevertheless, their migration represents an enormous challenge in conditions where humans and other mammals are only able to operate at levels well below their sea-level maxima.

Relating body condition to inorganic contaminant concentrations of diving ducks wintering in coastal California.

In wild waterfowl, poor winter body condition may negatively affect migration, survival, and reproduction. Environmental contaminants have been shown to adversely affect the body condition of captive birds, but few field studies have examined body condition and contaminants in wild birds during the winter. We assessed the body condition of carcasses from a collection of canvasbacks (Aythya valisineria) and lesser (A. affinis) and greater scaup (A. marila) wintering in coastal California. We used Akaike information criterion (AIC) to select the model with the best balance of parsimony and goodness of fit that related indices of body condition with concentrations of Cd, Cu, Hg, Se, and Zn. Total ash-free protein in canvasbacks decreased with increasing Se concentrations, and pancreas mass decreased with increasing Hg. We combined the closely related lesser and greater scaup in analyses and found that total carcass fat, pancreas mass, and carcass mass decreased with increasing Zn concentrations, and pancreas mass decreased with increasing Hg. Our AIC analysis indicated that some indices of body condition in diving ducks were inversely related to some environmental contaminants in this collection, but additional AIC analyses should be conducted across a wider range of contaminant concentrations to corroborate our findings.

Contaminant profiles of two species of shorebirds foraging together at two neighboring sites in South San Francisco Bay, California.

The San Francisco Bay estuary is used by over one million shorebirds during spring migration and is home to several hundred thousand during the winter. Most shorebird use occurs in the southern reach of the estuary (South Bay). The reduced water circulation and discharge from industrial sources in the South Bay are responsible for the highest levels of some trace elements in the estuary. Wintering shorebirds have been found to have strong site fidelity to areas as small as a few kilometers in the South Bay, which may increase their exposure to contaminants near local point sources. In addition, different shorebird species foraging at the same site have been shown to have different contaminant burdens. Thus, our objectives were to test whether contaminant burdens differed by species, or whether contaminant burdens differed in shorebirds collected at adjacent sites. We examined the contaminant profiles of two species of shorebirds, long-billed dowitchers (Limnodromus scolopaceus) and western sandpipers (Calidris mauri) that forage together at two sites, Hayward and Newark, separated by 8 km in the South Bay. We used multivariate analysis of variance tests to compare the composition of 14 elemental analytes in their liver tissues and estimated their molar ratios of Hg and Se. Composite samples were used for contaminant analyses because of the small body size of the shorebirds. Seven elemental analytes (Ag, Ba, Be, Cr, Ni, Pb, V) were below detection limits in a majority of the samples so statistical analyses were precluded. In the measurable analytes (As, Cd, Cu, Hg, Mn, Se, Zn), we found no significant intra-site differences of contaminant profiles for the two species. We pooled the samples to examine inter-site differences and found significant differences in contaminant profiles between shorebirds at the neighboring sites (P = 0.03). Shorebirds at Newark had higher (P < 0.05) concentrations of As, Cd, and Se than those at Hayward. Dowitchers at Newark had concentrations of Hg and Se which were highly correlated (P < 0.003) in a mean molar ratio of 1:19, similar to that reported in other birds. In the larger dowitcher species, we also examined exposure to 20 organochlorine compounds. Organic analyses showed that the dowitchers had been exposed to DDE, PCBs, dieldrin and trans-nonachlor, but with no significant differences in concentrations between Hayward and Newark (P > 0.05).