The common swift (Apus apus) is adapted to an aerial lifestyle, where food and nest material are captured in the air. Observations have prompted scientists to hypothesize that swifts stay airborne for their entire non-breeding period [], including migration into sub-Saharan Africa []. It is mainly juvenile common swifts that occasionally roost in trees or buildings before autumn migration when weather is bad []. In contrast, the North American chimney swift (Chaetura pelagica) and Vaux’s swift (C. vauxi) regularly settle to roost in places like chimneys and buildings during migration and winter []. Observations of common swifts during the winter months are scarce, and roost sites have never been found in sub-Saharan Africa. In the breeding season, non-breeding individuals usually spend the night airborne [], whereas adult nesting birds roost in the nest []. We equipped common swifts with a micro data logger with an accelerometer to record flight activity (years 1–2) and with a light-level sensor for geolocation (year 2). Our data show that swifts are airborne for >99% of the time during their 10-month non-breeding period; some individuals never settled, but occasional events of flight inactivity occurred in most individuals. Apparent flight activity was lower during the daytime than during the nighttime, most likely due to prolonged gliding episodes during the daytime when soaring in thermals. Our data also revealed that twilight ascents, previously observed during the summer [], occur throughout the year. The results have important implications for understanding physiological adaptations to endure prolonged periods of flight, including the need to sleep while airborne.

Results and Discussion

4 Åkesson S.

Klaassen R.

Holmgren J.

Fox J.W.

Hedenström A. Migration routes and strategies in a highly aerial migrant, the common swift Apus apus, revealed by light-level geolocators. 5 Åkesson S.

Bianco G.

Hedenström A. Negotiating an ecological barrier: crossing the Sahara in relation to winds by common swifts. Figure 1 Winter Locations for Common Swifts in Africa Show full caption Map of the part of Africa showing the mean (February) wintering positions for individual common swifts equipped with activity loggers that, in addition to flight activity, also recorded light levels for geolocation between 2014 and 2015. See also Figures S1 and S3 We equipped adult common swifts at two sites in southern Sweden with data loggers to record acceleration and monitor flight activity in 2013 and, in addition to acceleration, also light data for geolocation in 2014. The data loggers and sampling routine were tailored for economic data storage of both activity and light data (see the Supplemental Experimental Procedures ). We recaptured 11 birds in 2014 and eight birds in 2015. One data logger retrieved in 2015 was deployed in 2013 and contained activity recordings for 2 years. Of the retrieved loggers, two from 2014 showed technical problems and did not contain data. The light data showed that the swifts spent the winter in either West Africa (Liberia, Ivory Coast, and Ghana) or in Central Africa (Democratic Republic of Congo and Congo Brazzaville), with birds from both breeding sites wintering in the two main wintering areas ( Figure 1 ). This is in agreement with previous results on Swedish swifts [].

During migration and winter periods, there was almost a total lack of inactivity recordings, except for a few nights in February in 2014, when bird 1 settled in a vertical position during four whole nights (0.64% of the time from September–April) ( Figure 2 ). In the second winter period, there were no indications of whole-night inactivity, with only one recorded stop of 2 hr (0.03% of time) ( Figure 2 ), suggesting that this swift practically spent the entire non-breeding period airborne that year. In this case, the data suggest that the bird remained airborne for about 10 months (314 days). However, not all birds spent the entire non-breeding season airborne, as illustrated by bird 2, which showed signs of intermittent nocturnal flight inactivity from November–January in 2013–2014 ( Figure 3 A) and a similar pattern the following winter ( Figure 3 B). The pattern for this bird was very similar, but not identical, between the 2 years, although flight behavior was recorded with two different sampling routines (see the Supplemental Experimental Procedures ). This lends support for the notion that the measurements accurately reflect flight behavior. However, the duration of landings was short, and in the 2013–2014 season no inactivity period was longer than 2 hr, whereas in 2014–2015 the bird was recorded as totally inactive for 23 hr (0.4% of the time September–April). Notice that during short periods of nocturnal inactivity, the swifts are not necessarily roosting with the body axis vertically aligned, but they may do so also with the body aligned near horizontal ( Figures 2 and 3 ).

The remaining individuals show similar variation in flight behavior from being virtually completely airborne (birds 3, 7, 9, and 10; Figures S1 and S2 ) to clear patterns of periodic nocturnal inactivity similar to that of bird 2 (birds 4–6, 8, and 11–13; Figures S1–S3 ), although the amount of inactivity periods varies between individuals. Five birds were tracked during two consecutive non-breeding periods, and they show similar, but not identical, flight activity between the 2 years ( Figures 2 3 , and S1 ). For example, bird 3 was largely airborne and had no inactivity periods longer than 2 hr in the 2 years, whereas birds 4 and 5 did show periods of nocturnal flight inactivity ( Figure S1 ), although the accumulated duration of inactivity of 2 hr or more were only 9 and 11.5 hr, respectively. Nocturnal inactivity periods often seem to be of short durations, but whole-night inactivity was recorded in four birds (birds 1, 4, 10, and 13; Figures 2 S1 , and S3 ). The amount of accumulated inactivity duration, including periods of 2 hr or more, during the non-breeding period (September–April) varied between 0% (birds 3, 7, and 9) and a maximum of 0.64% (bird 13; Figures S1–S3 ).

10 Dokter A.M.

Åkesson S.

Beekhuis H.

Bouten W.

Buurma L.

van Gasteren H.

Holleman I. Twilight ascents by common swifts, Apus apus, at dawn and dusk: acquisition of orientation cues?. 11 Liechti F.

Witvliet W.

Weber R.

Bächler E. First evidence of a 200-day non-stop flight in a bird. 10 Dokter A.M.

Åkesson S.

Beekhuis H.

Bouten W.

Buurma L.

van Gasteren H.

Holleman I. Twilight ascents by common swifts, Apus apus, at dawn and dusk: acquisition of orientation cues?. Inspection of the activity diagrams reveal two periods daily, one around 7–8 a.m. (denoted A in Figure 2 ) and another at 6–7 p.m. (denoted B in Figure 2 ), discerned as vertical bands of an almost complete lack of flight inactivity indications. These bi-daily periods that last for about 1.5 hr suggest an elevated proportion of flapping flight, which is consistent with climbing flight. Common swifts have been shown to perform ascents to altitudes up to 2,500 m around dusk and dawn in the summer [], whereas our data suggest that common swifts make such ascents throughout the year. A similar pattern has been recorded in the Alpine swift (Tachymarptis melba) []. The reason for such ascending flights remains obscure, but it has been suggested they are involved in navigation rather than foraging []. Our results show that no matter what the reason for this behavior is, it occurs throughout the annual cycle.

11 Liechti F.

Witvliet W.

Weber R.

Bächler E. First evidence of a 200-day non-stop flight in a bird. Another diurnal rhythm in flight activity is a relatively higher proportion of gliding flight during the daytime compared to the nighttime, except for the occasions of nocturnal inactivity events ( Figures 2 3 , and S1–S3 ). Immediately after the period of high flight activity around dusk, there is a short period of reduced flapping flight activity (denoted B in Figure 2 ), which is concordant with a gliding descent after a flapping flight ascent. The diurnal pattern of relatively more inactivity indications during daytime cannot be due to actual landings, since they are never recorded as indications of complete quiescent behavior and are much too short. Instead, we interpret this daily rhythm of relative flapping flight activity as a result of longer glide phases due to increased thermal soaring in daytime. This is contrary to the pattern observed in the Alpine swift, which appears to show longer glide phases during the nighttime than during the daytime [].

Figure 4 Seasonal Flight Activity in Common Swifts Show full caption Daily flight activity pattern during four periods recorded in 2013–2014 for nine individuals providing complete data. The circles show hourly means of flight activity, where the proportion of flight activity is measured as the proportion of 5 min periods representing flight. The dashed lines denote the 50% and 100% activity levels, respectively. Shown are autumn migration (September; A), winter residency (February; B), spring migration (April; C), and breeding (June; D). See also Figure S4 The activity recorder used during the first year (2013–2014) allows us to illustrate seasonal differences in flight patterns. During autumn migration, the proportion of active flight is generally high, in particular during the nighttime, with the slightly lower activity values in the daytime probably reflecting prolonged gliding flight periods when soaring ( Figure 4 A). In mid-winter, there is some variation between individuals in flight activity during the nighttime ( Figure 4 B), reflecting individual variation in the frequency of flight inactivity (cf. Figure S1 ), whereas daytime flight activity is similar to that during migration ( Figures 4 A and 4B). During spring migration, flight activity is very similar to that of autumn migration ( Figures 4 A and 4C). The breeding period shows a dramatic change in flight activity compared with the non-breeding period, reflecting the nightly roosting inside the nest and frequent nest visits during the daytime ( Figure 4 D).

12 De Roo A. Age-characteristics in adult and subadult swifts, Apus a. apus (L.), based on interrupted and delayed wing-moult. 13 Lindström Å. The energetic cost of feather synthesis is proportional to basal metabolic rate. 14 Hedenström A.

Sunada S. On the aerodynamics of moult gaps in birds. Adult common swifts typically molt their flight feathers in the winter [], but sometimes they return to the breeding area with the outermost primary left unmolted. We recorded molt in 11 of the birds and divided these into one group that showed no or little flight inactivity during the winter (birds 1, 7, 9, and 10; Figures 1 S2 , and S3 ) and one group that showed a pattern of periodic flight inactivity (birds 2, 4, 5, 8, and 11–13; Figures 1 and S1–S3 ). Of the birds in the mainly airborne group, all four had completed wing and tail molt during the preceding winter, whereas in the group of periodic nocturnal flight inactivity, all but one out of seven birds retained unmolted outer primary. The difference between the groups is statistically significant (p = 0.0152, two-tailed Fisher’s exact test). As molt is an energetically and aerodynamically costly process [], especially in an aerial bird, this suggests that there could be a physiological correlate explaining the pattern of nocturnal flight inactivity in the winter.