Flight activity in pallid swiftsApus pallidusduring the non‐breeding period

Abstract

Flight activity recorders have recently confirmed that alpine and common swifts spend the majority of their non‐breeding period on the wing, which may last 6–10 months. Here we test the hypothesis that the closely related pallid swift, a species with a breeding distribution around the Mediterranean, lead a similar aerial life‐style during its migration and wintering periods. The pallid swift usually lays two clutches in one season and therefore spends more time in the breeding area than the common swift. We successfully tracked four pallid swifts with data loggers that record light for geolocation and acceleration every 5 min to monitor flight activity. The birds wintered south of the Sahel in west Africa from the Ivory Coast to Cameroon. The pallid swifts spent the majority of their non‐breeding time in flight, especially the first two months after leaving the breeding area in autumn, while a few landing events occurred during the winter. The total time grounded was < 1%, similar to that of the common and alpine swifts. The mass specific flight metabolic rate of swifts is similar to the average non‐breeding metabolic rate of a long distance terrestrial migrant, suggesting swifts are not more likely to procure oxidative damage as a consequence of continuous flight than other migrants. The open airspace used by swifts may provide a relatively safe habitat that explain the high survival rate found in swifts.

Highly mobile insectivorous swifts perform multiple intra‐tropical migrations to exploit an asynchronous African phenology

“…This temporal strategy may make them vulnerable to unusually dry years when late rains and insect concentrations do not occur (Boano and Bonardi 2005). Pallid swifts are mainly aerial outside the breeding season and given the large movements within the sub-Saharan non-breeding range documented here, they seem predisposed to opportunistically respond to variation in prey availability between and within years (Hedenström et al 2019). However, another common nonbreeding visitor south of the Sahara from the temperate zone, the common swift, winters further south in Africa in areas where the general timing of the birds in relation to the rainy season may differ from that in the Sahel (Åkesson et al 2012, 2016).…”

“…Similar to the closely related common swift A. apus (Pellegrino et al 2017), the pallid swift show general morphological modifications to an aerial life-style, such as high aspect ratio wings, a streamlined body and an energy efficient flight style (Hedenström et al 2016, Sachs 2017. It was recently also empirically shown that pallid swifts mainly spend the non-breeding season on the wings (Hedenström et al 2019), something earlier described in alpine swifts Tachymarptis melba (Liechti et al 2013) and common swifts (Hedenström et al 2016). Despite their energy-saving modifications, swifts are still likely facing higher daily energetic costs relative to other species with a more terrestrial life-style due to the elevated expenditure related to flapping flight (Norberg 1990).…”

“…The light-data were visually inspected for erroneous transitions, such as those caused by repeated shading events throughout the day. Pallid swifts are like common swifts predominantly aerial during the nonbreeding season (Hedenström et al 2019), and thus the continuous light exposure has resulted in very clean light data (Åkesson et al 2016). The exception was the breeding season when the birds repeatedly visited the nest-cavities.…”

“…Given their mobility and aerial life-style (Hedenström et al 2019) the pallid swifts have the prerequisites to prey upon air-borne insects while flying and would thus have the potential to continually 'surf ' the resource wave as it moves through the landscape. Apparently, the pallid swifts of this study did not exhibit such a movement strategy, but instead alternated long periods of residency with relatively rapid and directed movements similar to what have been described in other Palaearctic-African migrants (Thorup et al 2017).…”

“…Despite their energy-saving modifications, swifts are still likely facing higher daily energetic costs relative to other species with a more terrestrial life-style due to the elevated expenditure related to flapping flight (Norberg 1990). As swifts do not regularly land during the non-breeding season (Liechti et al 2013, Hedenström et al 2016, 2019, finding areas abundant with energy rich food resources is likely a critical factor to enable maintenance (i.e. feather moult, Boano et al 2015) and preparations for future energy expenditures (such as migratory phases when time is mainly spent on directed flight) while on their wings.…”