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One contribution of 17 to a theme issue 'Moving in a moving medium: new perspectives on flight'. 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 21 ) and short (15.7 min), together with the fact that body temperature increases by 18C, on average, during such flights, which equates to a rate of heat storage index (HSI) of 48C h 21. 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'.

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Dummy eggs were added to naturally incubated clutches of Greater Snow Geese (Chen caerulescens atlantica), an arctic-nesting species, in order to evaluate egg temperature during laying and incubation, and factors influencing egg cooling rate during female recesses. As laying progressed, both nest attentiveness by females and egg temperature progressively increased. Although the time spent at nest after laying the penultimate egg was relatively high (69% vs. 91% during incubation), mean egg temperature was still 5.7°C lower than during the early incubation period. This suggests that little embryonic development began before clutch completion. Thereafter, egg temperature averaged 37.1 ± 0.1°C during periods where females were present, a value that decreased only slightly when incubation recesses are included (36.8°C). This is a high temperature in comparison to other arctic-nesting geese. A modest increase (1.7°C) in mean egg temperature was observed as incubation progressed, but egg temperature was not influenced by clutch size or by the laying date of the first egg. During recesses lasting 24.7 ± 1.3 min on average, egg temperature dropped by 2.8 ± 0.3°C, at an instantaneous rate of 0.23 ± 0.02°C hr−1 °C−1. Cooling rates increased under windy conditions and decreased with high solar radiation, but were little affected by air temperature.