Itinerant lifestyle and congregation of lesser kestrels in West Africa

Trans-Saharan migrants often spend a large proportion of their annual cycle wintering in the Sahel. Advances in fieldwork and tracking technology have greatly enhanced our ability to study their ecology in these areas. Using GPS-tracking we aimed to investigate the little-known non-breeding movements of the lesser kestrel Falco naumanni in sub-Saharan Africa. We segment non-breeding tracks (n = 79 tracks by 54 individuals) into staging events (131± 25 days per non-breeding cycle), itinerant movements between staging sites (11 ± 10 days), and non-directed exploratory movements (6 ± 5 days). We then describe timing and directionality of itinerant movements by male and female kestrels throughout the non-breeding season. Regardless of sex, lesser kestrels spent on average 89% of the non-breeding season staging at 2 (range = 1–4) sites in West Africa. At the end of September, kestrels arrived along a broad front throughout the northern Sahel. By December, however, they congregated into two distinct clusters in Senegal and along the Malian-Mauritanian border. The birds stayed for longer periods and showed greater daily activity in the latter areas, compared to their first and intermediate ones. Among 24 individuals tracked along multiple annual cycles, 20 individuals consistently used the Senegalese or Malian-Mauritanian cluster. The remaining four birds used these clusters only after 2-3 years of tracking or switched between clusters across years. The eastward and westward itinerant movements of lesser kestrels during the non-breeding season, coupled with their tendency to cluster geographically towards the end, differ from the southward movements of other insectivorous raptors in West Africa. While 31% of Spanish lesser kestrels converged in Senegal, where roosts of > 20,000 birds are known, 68% moved into the Malian-Mauritanian border region where more groundwork is needed.

Methods
Fieldwork was conducted in Spain during the breeding seasons of 2016–2020. A total of 216 adults were captured near the colony using balchatri or mist nets. They were also captured within the nest (such as nestboxes or other cavities) before egg laying, at the end of the incubation period or during the chick-rearing phase. We used two models of solar GPS-UHF biologgers from different manufacturers (GPSminiDatalogger, Microsensory LS, Córdoba, Spain; and NanoFix GEO+RF, Pathtrack Ltd., Leeds, UK.). The GPS-UHF loggers weighing 5.5 g (including harness, ~3.8 % of the mean weight at capture, males = 146.0 g ± 35 SD; females = 148.0 g ± 29) were attached as backpacks with a Teflon harness. Locations were stored on-board and downloaded via a UHF base station placed in the vicinity of the colony. Overall, we analysed 79 non-breeding tracks from the 54 adult birds (25 males and 29 females) from 20 breeding sites

All data analyses were conducted in R (V 4.2.3), and all figures were produced with ggplot2. The full data was resampled to a 1-h interval, allowing deviations of up to 20 min.  We calculated movement metrics using the R package ‘fossil’. All the mixed linear models were implemented using the ‘lme4’ package. We determined daily sunrise/sunset times using the "StreamMetabolism" package.

In addition, we used third-party public data from:

Country borders via: https://www.naturalearthdata.com/downloads/50m-cultural-vectors/

 Topography: alt_30s_bil via https://geodata.ucdavis.edu/climate/worldclim/1_4/grid/cur/

The main inland water (e.g. rivers and deltas) present in Senegal, Gambia, Mauritania and Mali via: https://www.diva-gis.org/