To our big surprise, the Alpine swifts change their flight behaviour drastically between the breeding period, active migration and the non-breeding stage in Africa. The activity pattern in combination with the pitch values allows distinguishing between periods of flying and resting (Fig. 2). During daytime, all individuals showed an activity and pitch pattern, which excludes resting on the ground. At night, the low flapping activity during the wintering period is in accordance with nocturnal radar recordings of common swifts performing elongated gliding flights without wing flapping during their roosting flights9,10. Thus, nocturnal roosting on the wing is characterized by long gliding intervals (with almost zero activity) and stable pitch position. This combination can be observed mainly during the wintering period and to some extent during migration, but rarely during breeding (Fig. 2, blue points). In two birds there is some overlap between the reference kernels for resting with the nocturnal data during the wintering period. If this overlap would indicate resting on the ground during wintering, we would have to assume that these birds were always resting in exactly the same pitch position as during flight, because other pitch values are lacking (Fig. 2). As this is very unlikely, we conclude that all three individuals were most probably constantly on their wings during their stay in the sub-Saharan African winter quarters. We cannot rule out that Alpine swifts may interrupt their flight for a few minutes. Nevertheless, they must be able to accomplish all vital physiological functions in flight over a period of several months. Roosting common swifts hanging on trees have been observed in Europe, in relation to cold weather20. This gives a hint, that aerial roosting is driven by food availability, which might restrict aerial roosting of the large Alpine swifts to the equatorial region.

A dawn and dusk ascent described recently for common swifts21 let us assume that the observed increase in activity at dawn and dusk implies a corresponding ascent in the Alpine swifts. The function of such an ascent is not clear yet. Based on the radar data, which also recorded insect densities, it could be ruled out that the ascent was related to foraging21.

Birds are homoeotherms with the highest basal metabolic rate and with the highest metabolic rates attained during endurance exercise among vertebrates22. To date, long-lasting flights have been investigated under the aspect of fasting during flight22. Recent studies showed that immune responses seem to be affected by long-lasting flights23. Alpine swifts feed on the wing and might be able to compensate such negative effects on immune defence. In addition, such long-lasting flights raise the question of how or whether these birds sleep. For many decades sleep has been considered as an unavoidable necessity to restore at least some of the physiological functions of the brain24. In Alpine swifts there seems to be no necessity for physical inactivity to maintain any of the relevant physiological processes. However, our data indicate that there are distinct periods of increased and decreased activity, which could go along with some kind of sleep in flight. It has been argued that locomotive control of flight is possible in slow-wave sleep, either unihemispherically as observed in dolphins or even bihemispherically14. Various birds carrying out long-distance flights are able to reduce sleep without the need for compensatory sleep afterwards. However, they still do sleep for some time and some other birds do show some kind of sleep deprivation after long-distance flights14,15,22.

Although the sample size contains only three individuals, the consistent results confirm clearly that swifts do at least to some extent sleep while airborne and demonstrate that sleep deprivation does not necessarily have to occur after long-lasting flights. As a consequence, the whole organism must be maintained in flight, which has important implications for further models of sleep and other aspects of recovery, as well as for locomotive control of flight and the cost of flight in general. From the evolutionary aspect, we are interested to uncover the causes for this extraordinary behaviour.