Does hyperthermia constrain flight duration in a short-distance migrant?

While some migratory birds perform non-stop flights of over 11 000 km, many species only spend around 15% of the day in flight during migration, posing a question as to why flight times for many species are so short. Here, we test the idea that hyperthermia might constrain flight duration (FD) in a short-distance migrant using remote biologging technology to measure heart rate, hydrostatic pressure and body temperature in 19 migrating eider ducks (Somateria mollissima), a short-distance migrant. Our results reveal a stop-and-go migration strategy where migratory flights were frequent (14 flights day(-1)) and short (15.7 min), together with the fact that body temperature increases by 1°C, on average, during such flights, which equates to a rate of heat storage index (HSI) of 4°C h(-1) Furthermore, we could not find any evidence that short flights were limited by heart rate, together with the fact that the numerous stops could not be explained by the need to feed, as the frequency of dives and the time spent feeding were comparatively small during the migratory period. We thus conclude that hyperthermia appears to be the predominant determinant of the observed migration strategy, and suggest that such a physiological limitation to FD may also occur in other species.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

On the importance of radiative heat exchange during nocturnal flight in birds.

Many migratory flights take place during cloudless nights, thus under conditions where the sky temperature can commonly be 20 degrees C below local air temperature. The sky then acts as a radiative sink, leading objects exposed to it to have a lower surface temperature than unexposed ones because less infrared energy is received from the sky than from the surfaces that are isothermic to air. To investigate the significance of this effect for heat dissipation during nocturnal flight in birds, we built a wind tunnel with the facility to control wall temperature (TASK) and air temperature (TAIR) independently at air speeds (UWIN) comparable to flying speeds. We used it to measure the influence of TASK, TAIR and UWIN on plumage and skin temperatures in pigeons having to dissipate a thermal load while constrained at rest in a flight posture. Our results show that the temperature of the flight and insulation plumages exposed to a radiative sink can be accurately described by multiple regression models (r2>0.96) based only on TAIR, TASK and UWIN. Predictions based on these models indicate that while convection dominates heat loss for a plumage exposed to air moving at flight speed in a thermally uniform environment, radiation may dominate in the presence of a radiative sink comparable to a clear sky. Our data also indicate that reducing TASK to a temperature 20 degrees C below TAIR can increase the temperature difference across the exposed plumage by at least 13% and thus facilitate heat flow through the main thermal resistance to the loss of internally produced heat in birds. While extrapolation from our experimentally constrained conditions to free flight in the atmosphere is difficult, our results suggest that the sky temperature has been a neglected factor in determining the range of TAIR over which prolonged flight is possible.

Intermittent hypoxia and plasticity of respiratory chemoreflexes in metamorphic bullfrog tadpoles.

We tested the hypothesis that intermittent hypoxia elicits plasticity in respiratory chemoreflexes in bullfrog tadpoles. Metamorphic tadpoles (Taylor-Kollros stages XVI-XX) were subjected to intermittent hypoxia (PW(O(2))=45 Torr; 12 h/day) or constant normoxia (PW(O(2))=156 Torr) for 2 weeks before ventilatory responses to hypoxia and hypercarbia were measured. Buccal pressure changes were used to quantify the frequency and amplitude of movements associated with gill and lung ventilation. Morphometric assessment showed that intermittent hypoxia delayed development in comparison with controls. Oxygen consumption was enhanced in tadpoles subjected to intermittent hypoxia; however, this increase was not sufficient to affect basal ventilatory activity or the hypoxic ventilatory response. During acute hypercarbic exposure, tadpoles subjected to intermittent hypoxia showed (1) a greater decrease in gill ventilation frequency and (2) a greater increase in lung ventilation frequency than tadpoles maintained under control conditions. We conclude that intermittent hypoxia augments the responsiveness to hypercarbia, thereby promoting lung ventilation when animals face this stimulus. This manifestation of respiratory plasticity may reflect uncoupling between physiological and morphological development in the bi-modally breathing bullfrog tadpole.