The ability to fly gives birds the unique capacity to perform fast seasonal movements up to thousands of kilometres a year across heterogeneous landscapes and ecological barriers1. The way migrants undertake this complex journey often shows great flexibility in behaviour2. That flexibility is governed by an interplay between (1) internal drivers, such as motion capacity (dependent on, for example, wing morphology), orientation ability, and the individuals’ age, sex and/or reproductive state that shape the internal motivation to move3; and (2) environmental drivers such as weather conditions and landscape structure that influences connectivity and creates the so-called ecological barriers3,4,5. Variation in behaviour is also affected by differences between adults and juveniles and/or males and females through, for example, reproductive advantages associated with early arrivals of adult males to establish territories in pre-breeding migration6,7,8. Weather conditions of a landscape vary within the daily cycle and among seasons influencing migratory behaviour, e.g. flight-mode (soaring, flapping) or foraging patterns9,10,11. However, understanding the relative contributions of such environmental and internal drivers to variation in migratory behaviour is often hampered by the lack of high-resolution tracking data for representative samples of individuals7,11.
Studies that took account of the interplay of environmental and internal drivers in shaping migratory performance, which is most commonly measured via metrics such as the ground speed, distance travelled, duration of stopovers and route straightness12,13,14, present a bias towards large soaring birds, due to their fairly greater size and thus ability to carry large tracking devices. Studies on these species have demonstrated that variation in weather (e.g. winds, turbulence, and thermal or orographic updrafts) is often the prevailing factor explaining performance patterns, such as seasonal and regional differences in hourly and daily migration speeds10,12. For example, turkey vultures (Cathartes aura) achieved faster speeds and travelled more hours each day during the pre-breeding compared to the post-breeding migration, because thermal uplifts were stronger during the pre-breeding migration15. The Oriental honey buzzards (Pernis ptilorhynchus) traverse ecological barriers (East China Sea) during the post-breeding migration when supportive winds are available and detour them during the pre-breeding migration when weather conditions are less favourable for soaring flight16. Age and experience are important factors mediating the response to weather in several species which are more prone to soaring flight during migration (e.g. golden eagle, Aquila chrysaetos17; osprey, Pandion haliaetus18). Yet, sex differences tend to explain little variation in travel speeds of soaring migrants19,20.
We still know little about the relative importance of environmental, internal and other seasonal drivers in shaping the migratory performance of smaller, non-obligate soaring-gliding migrants21. This is an important bias in current migration research because smaller birds are more abundant and, unlike large soaring birds, can use different flight modes, including obligate-flapping passerines and waders as well as the so-called flight-generalists (swallows, bee-eaters, falcons, hawks and harriers), which are capable of covering long distances using both flapping and soaring flight22,23,24. Studies on small birds demonstrated seasonal differences in migratory strategies, such as a time-minimising behaviour during the pre-breeding compared to the post-breeding migration11. For example, a tracking radar study of long-distance migrant passerines found faster mean airspeeds (the bird’s flight speed relative to the surrounding air) during the pre-breeding compared to the post-breeding migration after accounting for seasonal differences in wind and body mass8. These findings suggested a greater seasonal motivation to reduce migration time during the pre-breeding migration due to an expected competition at the breeding areas for nests and mates8. The ability of flight-generalists to switch between flight-modes gives them more flexibility to cope with weather conditions and thus may make the role of internal and other seasonal drivers more predominant in shaping migratory decisions, compared to obligate-soaring and obligate-flapping birds21.
Flight-generalists migrants are capable of flapping flights that extend daily travel schedules (i.e. how migrants shape their daily travels24) into the night when thermal updrafts are not available25. They typically also achieve higher speeds during nocturnal than diurnal migration, enabling them to cross ecological barriers in non-stop flights (“sprints”)26. For example, the Amur falcon (Falco amurensis) undertakes the longest non-stop water crossing of any bird of prey studied so far, taking 3–4 days to cross the Indian Ocean, from India to East Africa (ca. 3000–4000 km) flying day and night22. On the other hand, birds that invest energy in flapping flight also have to recover that energy by foraging, which they may do prior to or after completing the migration, and often also during migration by (a) making stopovers4 or (b) intermittent diurnal fly-forage, i.e. combination of foraging and flying in the migratory direction27,28. Studies on African-Eurasian migrants like Eurasian hobbies (Falco subbuteo) and Eleonora’s falcons (Falco eleonorae) typically revealed significant seasonal variation in travel speed between regions, with fast and long flights over barriers and slower and shorter daily flights over non-barriers28. Studies on Eleonora’s falcon indicated that when age, landscape and wind conditions are simultaneously analysed, the type of landscape over which they migrate is the most influencing factor on migration speed, likely due to birds alternating between sprinting flights and slow fly-and-forage migration29. However, the interplay between such habitat-dependent time schedules with weather conditions and seasonal and sex-specific differences in shaping migratory flight performance in flight generalists is still being resolved.
We studied the migratory behaviour of a flight generalist, the lesser kestrel (Falco naumanni), a small-sized insectivorous trans-Saharan falcon with reversed size dimorphism. During foraging trips30 and occasionally on migration22, lesser kestrels are known to soar, especially during periods of intense solar radiation. Additionally, their wings have a relatively high aspect ratio, suitable for long bouts of high-speed flapping flight en route31, and thus are capable of extending their daily travel schedule into the night32,33. Migration of this species has been mostly studied using geolocators34,35, and satellite telemetry32,33, shedding light on departure and arrival dates, routes, and non-breeding areas. Sarà et al. (2019) revealed that European-breeding lesser kestrel migrates to African non-breeding areas on a broad front across ecological barriers, instead of concentrating at bottlenecks as many soaring migrants do. Moreover, it has been reported that lesser kestrels migrate faster during the post-breeding than the pre-breeding migration, although the mechanisms behind these performance patterns have yet to be studied in detail.
Here we computed travel parameters at coarse (trip), intermediate (daily), and fine (hourly) temporal scales. At the trip scale, we analysed seasonal and sex-specific patterns in migratory performance (i.e. mean trip duration, the proportion of travelling/non-travelling days, mean travelling speed and mean route straightness). At the daily scale, we evaluated patterns in performance (i.e. mean daily speed, mean daily distance and mean travelling time) between landscapes (i.e. ecological barriers vs. non-barriers). At the hourly scale, we further assessed spatiotemporal patterns in hourly performance (i.e. ground speeds) of males and females for both post- and pre-breeding migration, specifically comparing barriers vs. non-barriers and diurnal and nocturnal flight segments. We also analysed diurnal and nocturnal performance when flying over barriers and non-barriers in relation to weather conditions along the kestrels’ routes (i.e. tailwind strength, absolute crosswind strength and boundary layer height; BLH hereafter), accounting for sex and season-specific responses.
To assess which factors drive migratory variation in the lesser kestrel, we first described behavioural patterns by investigating seasonal and sex-specific differences in performance between landscapes, diurnal and nocturnal flights. Then we uncovered mechanisms by disentangling the effect of external (wind, landscape), internal (sex) and other seasonal drivers. It is well established that wind conditions have a significant influence in migration ground speeds1,22. We thereby expected a positive effect of tailwind strength will explain a large part of the variation in travel speed along the kestrels’ flight path. Accounting for these effects in our models should allow us to assess whether faster post-breeding migrations are a consequence of seasonal wind regimes, and whether such wind effects are masking sex-specific and other seasonal drivers, e.g. time-selected behaviour, particularly strong in males36, during the pre-breeding migration arising when there is a gain from arriving early at their destinations, which may be challenging to disentangle6,11,12.
Based on the expectations that: a) territory acquisition and competitions for mates increase selective pressure for early arrivals in the breeding relative to the non-breeding areas8,11; b) flight generalists species are less restricted by environmental (and especially weather) conditions compared to obligate soaring-gliding migrants, we hypothesised that models accounting for weather effects will show (1) significantly higher migratory performance (i.e. faster speed, longer daily distance and more travelling hours per day) during the pre-breeding than during the post-breeding migration; (2) significantly higher migratory performance for the smaller males than for the larger females, because flapping is theoretically less costly for the former37 and competition for securing a high-quality territory is weaker in the latter38. Furthermore, we hypothesised that (3) lesser kestrels migrate faster when flying over ecological barriers (Mediterranean Sea and Sahara Desert) than over non-barriers areas to reduce the time spent in harsh environments where there are few or no resting/drinking/feeding opportunities24,28,39. Accordingly, we expected that individuals will perform faster travel speed, larger daily beeline distance, and more extended daily travel schedules, including the use of nocturnal migration when flying over ecological barriers vs. non-barriers.