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1 October 2009 Daily Travel Schedules of Adult Eurasian Hobbies Falco subbuteo — Variability in Flight Hours and Migration Speed Along the Route
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Abstract

We analysed satellite tracking data to reveal the daily travel schedules of Hobbles Falco subbuteo migrating between northern Europe and southern Africa. By comparing movements during short-time intervals of 1–8 h at different times of the day between the three different regions Europe, Sahara Desert and tropical Africa, we found significant higher hourly speeds in the Sahara compared to the other two regions. The migration started early in the morning and continued throughout the day, resulting in daily travel time exceeding that of typical thermal soaring migrants. Due to fast hourly speeds in combination with the long travel days during Sahara crossings, the Hobbies exceeded daily travel distances recorded for soaring migrants. In Europe and tropical Africa, the speed was highly variable during the day, and lower flight speeds, especially during afternoons, suggest a fly-and-forage strategy. Our predictions about regular occurrence of nocturnal flight especially across geographical barriers as the desert were not fulfilled. In the Sahara, the Hobbies restricted their movement to diurnal migration. Nevertheless, 10% of the flight segments in tropical Africa were partly or completely during night hours and Hobbies often started their daily flight before or at sunrise. We attribute the special characteristics of the daily travel schedules of Hobbies primarily to their use of a fly-and-forage strategy, possibly in combination with preparedness for exploiting thermals during the daily thermal time window. Further studies are needed to understand why Hobbies do not increase daily travelling hours to include regular nocturnal flight in the Sahara Desert. We suggest that the daily travel schedules of the Hobby, which were distinctly different from those of other diurnal migrants like large raptors (which mainly exploit thermal soaring migration) or pipits, wagtails, finches and pigeons (which mainly travel during the morning hours) may be typical for bird species that primarily use a fly-and-forage migration strategy.

INTRODUCTION

Many bird species migrate during the night, while others travel mainly during daytime and still others are more flexible and perform both nocturnal and diurnal flights. The reasons for these differences in daily travel schedules are not fully understood, but several different factors that make either nocturnal or diurnal migration favourable have been suggested (reviewed by Kerlinger & Moore 1989).

A fundamental advantage of nocturnal migration is that the migratory flights do not occupy potential foraging and refuelling time during the day (for diurnal foragers), which will, other things being equal, make the resulting total speed of migration, including both flight transport and fuel accumulation, faster for nocturnal compared to diurnal migration (Alerstam 2009). Furthermore, the atmospheric structure is generally more suitable for migratory flights during the night than during daytime. This gives nocturnal migrants the benefits of reduced evaporative water loss, reduced exposure to turbulence and lower and less variable wind speeds (Kerlinger & Moore 1989). In addition, predation risk may be lower during nocturnal flights (Walter 1979).

There are two main factors that favour diurnal migration: exploitation of thermal air in soaring flight and the use of a fly-and-forage strategy (Alerstam 2009). Many birds like raptors, storks, pelicans and cranes reduce their energy costs of transport by thermal soaring migration, which takes place during daytime when thermal air develops over land (Kerlinger & Moore 1989, Kerlinger 1989). Birds that fly extensively during their foraging, e.g. when hunting on the wing for insect or bird prey, or making search flights to locate food on the ground or in water, may combine foraging with covering migration distance. The food intake will help to offset the net energy expenditure during travelling. Such a fly-and-forage migration strategy may be very favourable with respect to both energy and time minimization and may therefore be an important explanation for diurnal migration habits among many bird species such as many seabirds, harriers, sparrowhawks, falcons, terns, gulls, swifts, swallows and martins (e.g. Rudebeck 1950–51, Dorst 1962, Strandberg & Alerstam 2007).

Satellite tracking provides important possibilities to analyse the daily migration schedule of individual birds and its variability in different regions along the entire migration route. Such tracking results have confirmed that large raptors like Osprey Pandion haliaetus (Kjellén et al. 1997, 2001) and Honey Buzzard Pernis apivorus (Hake et al. 2003) migrate primarily between 8:00 am and 5:00 pm local time, which is typical for birds depending strongly on thermal soaring migration (Kerlinger 1989). Ospreys show a large degree of flexibility in flight time within the main daily thermal time window, depending on if they travel over habitats/landscapes that permit them to combine their thermal soaring flight with a fly-and-forage strategy or not (Klaassen et al. 2008).

Satellite tracking has revealed that nocturnal flights are very uncommon among raptor species, and most often closely linked with long sea crossings (e.g. Cochran 1985, Fuller et al. 1998, Martell et al. 2001, Hake et al. 2003, Alerstam et al. 2006, Decandido et al. 2006, Strandberg et al. 2008). Radar observations at the Spanish south coast detected nocturnal sea crossings, in this case of small falcon species such as Common Kestrel Falco tinnunculus, Eurasian Hobby Falco subbuteo and Lesser Kestrel Falco naumanni (Meyer et al. 2003). Regular nocturnal migration over land has so far only been shown for one diurnal raptor, the Levant Sparrowhawk Accipiter brevipes (Stark & Liechti 1993, Spaar et al. 1998). It was estimated that about 5–10% of the population crossed the Negev Highlands in Israel during the night.

In this paper, we analyse the daily travel schedules of the Hobby in different regions along its long-distance migration route between northern Europe and southern Africa. This small raptor is not as strongly dependent on thermal soaring migration as larger raptor species, a fact that is well known from field observations (Kjellén 1997, Chapman 1999) and also expected for aerodynamic reasons because of its relatively small size (Pennycuick 1989, Hedenström 1993). We expect that the difference in the balance of gain from thermals and from fly-and-forage migration between the small Hobby and larger raptors will be associated with differences in daily travel schedule, and we aim to test the following predicted differences between the Hobby (with a comparatively low profitability associated with thermal migration but a potentially high profitability associated with fly-and-forage migration) and larger raptors represented mainly by the Osprey (Klaassen et al. 2008):

  • (1) We expect the Hobby to primarily be a diurnal migrant, exploiting the advantage of a fly-and-forage migration strategy in combination with opportunistic soaring in thermal air. Because of its habits of regularly hunting not only during the day but also during dawn and dusk (Chapman 1999, Bijlsma & van den Brink 2005), we predict that the daily travel period of the Hobby includes early morning and late evening hours in addition to the daily thermal time window.

  • (2) In the Sahara Desert, where we assume that foraging is not possible, we predict that Hobbies will travel during a large number of both day and night hours (Alerstam 2009), summing up to a longer migration period per 24 h than for the Osprey flying only in the daily thermal time window (Klaassen et al. 2008) as well as to a faster total speed per 24 h across the desert compared with the Osprey.

  • (3) We predict that the Hobby, as other smaller raptors, is generally more prone than larger thermal dependent raptors to performing nocturnal migratory flights over land when foraging possibilities are poor (Alerstam 2009). As suggested by Strandberg et al. (2009), the equatorial evergreen rain forest could, beside the Sahara Desert, be a potential barrier for migrating Hobbies on their routes to southern Africa.

METHODS

From 2005 to 2007 four adult Hobbies (three females and one male) were captured at their breeding sites in southern Sweden (55.7–55.9°N, 13.4–14.2°E) and equipped with Solar PTT-100 satellite transmitters (Microwave Telemetry Inc.), which were attached as backpacks. The transmitters weighed 9.5 g which comprised on average 4.1% of the Hobbies' body mass. The weight of the transmitters had no visible effects on flight behaviour and hunting performance (own observations) or timing of migration (e.g. Beaman & Galea 1974, Kjellén 1992).

The transmitters were programmed to operate on a transmission cycle of 10 h on and 24 h off and tracked by CLS/Service Argos in Toulouse, France. Depending on satellite orbits and local conditions we received 0–15 positions per 10-h period. Hence, the initiation of the 10-h transmission period shifted backwards by 4 h/day, which means that every day a slightly different part of the daily cycle was sampled. The evaluation of positions differing in accuracy was the same as described by Hake et al. (2001): we primarily used high quality positions (ARGOS-class 1–3, accuracy is most often within 1 km). Low quality positions (ARGOS-class 0, A, B, accuracy is unspecified) were only used when the locations seemed reasonable and were in agreement with typical travel rates and directions. For the autumn migration, the complete data set comprised 786 positions, of which 32% were high quality locations (for further information about ARGOS-classification see  http://www.argos-system.org).

We assumed that the Hobbies had initiated their autumn migration the day they moved >50 km away from their nesting area. When the Hobbies were stationary for more than one week in the southern Hemisphere we assumed that they had arrived at their wintering quarters.

Daily travel schedules were analysed by calculating flight speeds for ‘short-interval segments’, which are segments with time intervals of 1–8 hours. By using a minimum of 1 hour the possible effect of low quality positions is reduced. Within each 10-h transmission period as many non-overlapping segments were defined as possible. Only data from travel days were used in our analyses, while stopover days (when the bird remained at the same site during one or more 24-h periods) were excluded. If there were several positions within one hour, we always used the highest quality position to define the segment. The speeds for the short interval segments were plotted in relation to the time of day for three main geographic regions along the migration route: Europe (north of the Mediterranean Sea); Sahara (36–30°N to 17–15°N, depending on longitude), and tropical Africa (south of Sahara).

Short-interval segments were subsequently classified in two groups, indicating active travel (segment length >5 km) or stationary behaviour (segment length <5 km; 1 h segments shorter than 5 km are best classified as stationary primarily due to variation in location accuracy). The mid-interval times for every segment were used to obtain the distribution of travelling and stationary periods throughout the 24 hours of the day, which reflects a simple daily time budget (cf. Klaassen et al. 2008).

The number of daily travelling hours in the different main geographic areas were estimated in three different ways: (1) by dividing the mean speed of migration on travelling days (km/24 h; based on locations between different transmission periods) by mean short-interval speeds (km/h; including only segments >5 km), (2) by dividing the mean speed of migration on travelling days (km/24 h; based on locations between different transmission periods) by median short-interval speeds (km/h; including only segments >5 km) and (3) by calculating the number of hours with active travel from the daily time budgets (i.e. the cumulative sum of hours with active travel from the hourly frequencies of travelling and stationary segments).

Times are given in local daily exact time (thus calculated by correcting location times in GMT for the longitudinal time difference). The exact local time of sunrise and sunset was obtained for every location from the website of the Astronomical Applications Department of the U.S. Naval Observatory ( http://aa.usno.navy.mil).

We compared the travel speed in the three different geographical regions in a GLM mixed model (SPSS 16.0) with individual included as random factor. Separate tests were conducted with hourly flight speed (as obtained from the short-interval segments) and daily migration speed as dependent variables, respectively. Post hoc tests were used to reveal differences in speed between the three regions.

RESULTS

We recorded autumn migrations of four adult Hobbies, one track for each bird, with all birds passing the equator, but only three reaching the wintering grounds in southern Africa (Table 1). The journey lasted on average 61 days of which 39 were travel days. The mean distance travelled was 9223 km (for a presentation and analysis of the routes of the falcons, see Strandberg et al. 2009).

Regional variation in travel hours

In Europe, the Hobbies were travelling from 4:00 am to 6:00 pm (Fig. 1, Fig. 2) and the average daily flight time on travel days was 7.9–10.5 hours (Table 2). Three of four morning tracks starting around or before sunrise were associated with water crossings (two over the Baltic Sea and one departing for the Mediterranean Sea).

Of the 22 segments recorded in the Sahara Desert, 16 were recorded during active flight. The estimated flight time on travelling days was 11.7–14.6 hours (Table 2) and flights were evenly spread out over the daylight hours (Fig. 2). Two flights were recorded before sunrise and two ended after sunset (Fig. 1).

The majority of positions were received from tropical Africa, for which we can thus construct the most detailed time budgets (Fig. 2). The average daily flight time on travel days was 10.1–13.0 hours (Table 2). The great majority of flights were recorded from 5:00 am to 6:00 pm. In total 18 active flight segments started before sunrise and seven ended after sunset, with additionally seven flight segments recorded during dark hours only (Fig. 1). The latter flights were made by three out of the four Hobbies north and south of the equatorial rain forest belt between 7.5–6.9°N (Nigeria) and 1.2–6.2°S (Congo).

Regional variation in travel speeds

The hourly speeds differed significantly between the three different regions (GLM mixed model, F2,163 = 6.79, P = 0.001). A post hoc test revealed that hourly speeds in Sahara (mean = 33.4 km/h, SD = 21.4, n = 16) were different (Tukey B test, P < 0.05) from speeds in tropical Africa (mean = 19.9 km/h, SD = 12.7, n = 123) and Europe (mean = 23.8 km/h, SD = 14.9, n = 27), whereas no significant differences (Tukey B test, P > 0.05) existed between Europe and tropical Africa.

Table 1.

Timing, duration and distance of autumn migration of adult Eurasian Hobbies tracked during 2005–2007. Departure = departure date from the breeding area. Arrival = arrival date to the wintering area. Duration = total number of days from departure to arrival. Distance = the distance travelled. Travel = number of travel days (movement >50 km/day). Median values are given for departure and arrival dates for all birds. Mean values are given for duration, distance and travel days of all birds. The second female (data in italics) did not complete migration and is not included in the median arrival date and mean values. Note that the end position is not equivalent to wintering site, because the Hobbies are nomadic during this period.

t01_287.gif

In Europe, the three fastest flights (>45 km/h) were recorded on the first migration days, and in two cases this occurred during the crossing of the Baltic Sea (Fig. 1). All high-speed segments (>35 km/h) in the Sahara were directed to the southwest, which may be related to the dominating easterly winds in this zone (Erni et al. 2005).

Table 2.

Speed on travel days and number of daily travelling hours of four Eurasian Hobbies in different geographical regions during autumn migration. Segments are 1–8 h. Mean daily travel speed = mean values based on daily travel distance divided by number of travel days (mean number of travel days used for calculations: Europe = 7; Sahara = 4; tropical Africa = 20). The number of daily travelling hours (per 24 h) was estimated in three different ways; (1) by dividing daily travel speed with mean segment speed, (2) by dividing daily travel speed with median segment speed and (3) from the proportion of segments in Fig. 2 (cf. Methods).

t02_287.gif

Figure 1.

Daily distributions of speeds during short-time intervals (1–8 h) for adult Eurasian Hobbies during autumn migration in three geographical areas; Europe (top), Sahara (middle), tropical Africa (bottom). The dashed vertical lines show the sunrise and sunset periods, which differ slightly with latitude and season.

f01_287.eps

In the tropics, the highest speeds were predominantly achieved during morning hours (Fig. 1) and were made either over the equatorial rain forest of Cameroon and Congo or in the nearby areas, except for one segment in western Zambia, when the bird was approaching the wintering area.

The three regions also differed in the daily travel speed (GLM mixed model, F2,8 = 14.10, P = 0.002). A post-hoc test revealed that the highest speed achieved in the Sahara (mean = 391.3 km/day) was significantly different (Tukey B test, P < 0.05) from the speeds accomplished in Europe and tropical Africa (mean = 188.6 km/day and 200.2 km/day, respectively). Within tropical Africa the speed during the crossing of the rainforest was noticeably high (249.9 km/day), but not significantly different from the speed in the rest of tropical Africa, which was only 191.3 km/day (P = 0.056, t-test).

Figure 2.

Daily distributions of stationary (<5 km of movement) and active migration (>5 km of movement) segments, based on mid-interval times for adult Eurasian Hobbies during travel days in Europe (top), Sahara (middle), tropical Africa (bottom).

f02_287.eps

DISCUSSION

The adult Hobbies we studied adapted their flight speeds and flight hours to the local environmental conditions. Instead of avoiding geographical barriers along their routes to southern Africa they travelled with increased speed over adverse areas such as the Baltic Sea, Sahara Desert and equatorial rain forest (the latter also regarded as a barrier according to Strandberg et al. 2009).

While some of our predictions about travel schedules of the Hobbies were supported by the results, there were also important discrepancies between predictions and observations, especially concerning the birds' behaviour during the crossing of the Sahara Desert (prediction 2). As the sample size of our study is small our results should be interpreted with care. However, the four individuals (from three different years) behaved in a similar way with respect to speed and daily timing of migration, indicating that they showed the typical migratory behaviour for the species.

(1) Diurnal migration with many travel hours. The daily distributions of speed and activity (Figs 1 and 2) showed that the Hobbies were primarily diurnal migrants in Europe and in tropical Africa. They started migration earlier than raptors that are more specialized in thermal soaring flight (e.g. Osprey, cf. Klaassen et al. 2008). The number of daily travel hours were 8–11 in Europe and 10–13 in tropical Africa (Table 2), which exceeds the number of flight hours (about 5) in the daily time budget of migrating Ospreys in Europe (Klaassen et al. 2008). This is in accordance with our predictions. The Hobbies started their daily migration already at or before sunrise and reached the fastest travel speeds in the first half of the day. Still, the variation in short-interval speeds was very large, probably because of a large variation in the amount of foraging that is combined with their migratory flights.

The mean flight speed of a Hobby in active cruising flight is close to 40 km/h (similar means of 11.3 and 10.9 m/s reported by Bruderer & Boldt 2001 and Heiko Schmaljohann pers. comm., respectively). Faster speeds thus indicate that the Hobbies have travelled with tailwinds, while slower speeds indicate flight into headwinds or that the birds have made deviations or interruptions associated with a fly-and-forage strategy (Strandberg & Alerstam 2007). Also thermal soaring behaviour may have contributed to reduce the resulting speeds. Travel speeds decreased in the afternoon in Europe and tropical Africa, which was unexpected. This pattern may be due to increased foraging intensity towards the end of the day, higher flight cost for flapping flight in relation to an increase in air turbulence throughout the day and/or to preparations for the birds' settling for the coming night.

(2) Travel time during the Sahara passage. The Hobbies had the longest travel days (with 12–15 h of travel time, Table 2) during the crossing of the Sahara Desert, as predicted. However, they still seemed to travel primarily during daytime and not very much during the night, a result that was in disagreement with the predicted behaviour of a distinct increase in nocturnal travel time during the desert crossing. There were only six short-term intervals indicating stationary periods in Sahara and all of these took place during the night between 6:00 pm and 4:00 am. This indicates that Hobbies tended to interrupt the desert crossing mainly at night, but there were also four short-time intervals to show that they sometimes continued to fly during the dark hours. It should be kept in mind that data are limited with only 22 short-time intervals available from the Hobbies' crossing of Sahara, and further data are necessary to evaluate the relative occurrence of diurnal and nocturnal flight more accurately.

As a result of the fast short-interval travel speeds in combination with long travel days, the total average daily travel distance reached close to 400 km for the Hobbies crossing Sahara. This exceeds daily average travel distances by typical thermal soaring migrants like Osprey and Honey Buzzard during their crossing of the Sahara Desert (with average speeds on travel days of 200–300 km/day; Hake et al. 2003, Alerstam 2003, Klaassen et al. 2008).

It is difficult to understand why the Hobbies during the desert crossing did not increase the number of daily travelling hours even further to include nocturnal flight to a larger degree (cf. Alerstam 2009). They appeared to travel both before and after the thermal time window (9:00 am – 5:00 pm) used by the Osprey (Klaassen et al. 2008) so resort to thermal soaring migration cannot provide the full explanation. Travel speeds were fast during the desert crossing, possibly indicating that the Hobbies were flying more consistently in their migratory direction without much deviations and interruptions for foraging. Further studies are needed to investigate if Hobbies can benefit in a sufficient way from thermal soaring and from foraging during their flights across Sahara to explain why they seem to travel mainly during the daytime in this region. The behaviour of the Hobbies stands in stark contrast to the welldefined nocturnal travel schedules used by many songbirds during their Sahara crossings (Biebach et al. 1986, Bairlein 1988, Biebach 1990, Biebach et al. 2000, Schmaljohann et al. 2007).

(3) Possible nocturnal flights when foraging conditions are poor. The most extensive data set was obtained from the Hobbies' travel schedules in tropical Africa south of Sahara. A striking feature was the early start of migration, well before sunrise. A small proportion of observations (seven out of 69 short-time intervals between 7:00 pm and 4:00 am) showed that also true nocturnal flights may occur occasionally. This pattern was in reasonable accordance with predictions, even if the decreasing activity and speeds during the afternoon and the low frequency of nocturnal flights were unexpected.

The nocturnal flights recorded in the tropics were mainly undertaken in association with degraded forest areas north and south of the rain forest belt. If these areas represents bad foraging habitats for the falcons we do not know, but we can suspect that the intact forests do. The daily rains associated with this habitat may also limit the foraging hours for the falcons and that might explain why they increase speed over and close to the forest.

CONCLUSIONS

The daily migration schedules of the Hobby differed from those of thermal soaring migrants like the Osprey by extending flights outside the thermal time window (especially by the early morning start of migration) and by reaching a morning peak in speed. The Hobby schedules also differed from those of diurnal migrants not using thermals, like pigeons, pipits, wagtails and finches (cf. Alerstam 1990) by the extended duration of travel throughout the afternoon. We attribute the special characteristics of the daily travel schedules of Hobbies primarily to their use of a fly-and-forage strategy, possibly in combination with preparedness for exploiting thermals during the daily thermal time window.

Further studies are needed to show if the Hobby type of daily travel schedule is typical among birds using a fly-and-forage migration strategy. For such migrants one should expect that the daily travel schedule would change when migrating through regions where foraging is no longer possible, e.g. during the crossing of the Sahara Desert (Alerstam 2009). From the limited data available it seems like the Hobbies keep to a dominantly diurnal travel schedule during this crossing, in stark contrast to the vast numbers of songbirds that avoid evaporative water losses by a nocturnal schedule for their flights across Sahara (Schmaljohann et al. 2007). Perhaps there are after all enough possibilities for hunting during daytime in the desert (e.g. pursuing the songbirds that land in the desert during the day) to make diurnal migration favourable for the Hobbies?

It is a challenging task to evaluate the primary selection factors determining daily travel schedules among migrating birds. Temporal analyses based on detailed tracking of individual birds over a larger range of species could provide crucial information about the daily travel schedules among different ecological categories of migrants and about the variability of schedules in different regions. For example, it would be very interesting to make a detailed comparison between Hobbies, which do not travel so much during the night, and Levant Sparrowhawks, which regularly travel during night (Stark & Liechti 1993, Spaar et al. 1998). Are Hobbies better in exploiting thermals during migration than the sparrowhawks, or is fly-and-forage migration more important for Hobbies than for Levant Sparrowhawks? Such detailed information and knowledge will be necessary for an understanding of the adaptive values of these important temporal aspects of migration ecology.

ACKNOWLEDGEMENTS

We are grateful to Mikael Hake for supervising and participating in the capturing of birds, to Mikael Svensson for his climbing efforts during field season 2005 and to Anthony Chapman and Mikael Hake for encouraging discussions about Hobby ecology. We also thank Heiko Schmaljohann and an anonymous reviewer for constructive comments on the manuscript. We are grateful for support and excellent equipment from Microwave Telemetry Inc. The study was approved by the Ethical Committees in Uppsala (C92/6) and Malmö/Lund (204-06). Permission to capture the birds was obtained from the Swedish Environmental Protection Agency (412-429-05 Nf). This research was financed through grants from the Swedish Research Council (to T.A. and to R.K.) and from the Niels Stensen Foundation (to R.K.).

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Appendices

SAMENVATTING

In dit artikel wordt met behulp van satelliettelemetrie nagegaan hoe Boomvalken Falco subbuteo tussen Europa en zuidelijk Afrika trekken. Door de afstand tussen opeenvolgende posities gedurende de dag (met tijdsintervallen van 1 tot 8 uur) te bepalen, is de treksnelheid gemeten. Snelheden waren boven de Sahara significant hoger dan boven Europa en tropisch Afrika. Boven de Sahara vlogen de valken van 's morgens vroeg tot 's avonds laat. Het aantal vlieguren dat ze dagelijks maakten, was daardoor groter dan bij roofvogels die grotendeels afhankelijk zijn van thermiek. Door de hoge vliegsnelheden en ‘lange werkdagen’ legden de valken per dag veel meer kilometers af dan zwevende roofvogels. Boven Europa en tropisch Afrika varieerde de vliegsnelheid sterk gedurende de dag. Vooral in de middag was de snelheid daar vaak laag. De lage snelheden suggereren dat de valken trekbewegingen afwisselen met voedsel zoeken (vlieg-en-foerageerstrategie). Onze voorspelling dat de valken geregeld gedurende de nacht zouden trekken, in het bijzonder boven ecologische barrières zoals woestijnen, werd niet door de gegevens ondersteund. Boven de Sahara vlogen de valken vrijwel uitsluitend gedurende de dag. In tropisch Afrika viel echter 10% van alle bewegingen gedurende (een deel van) de nacht. De valken begonnen daar vaak al voor of gedurende zonsopgang te trekken. De speciale manier waarop Boomvalken over de verschillende regio's trekken, wordt vooral toegeschreven aan de vlieg-en-foerageerstrategie en de neiging om in de loop van de dag van thermiek gebruik te maken. De vraag waarom Boomvalken 's nachts niet trekken als ze de Sahara oversteken, vraagt om verder onderzoek. We veronderstellen dat de trekstrategie van Boomvalken een typisch voorbeeld is van een trekvogel die trek afwisselt met voedsel zoeken. Deze strategie verschilt sterk van dat van andere dagtrekkers zoals grotere roofvogelsoorten (die sterk afhankelijk zijn van thermiek) en piepers, kwikstaarten, duiven en vinken (die vrijwel uitsluitend gedurende de ochtend vliegen).

Roine Strandberg, Raymond H.G. Klaassen, Patrik Olofsson, and Thomas Alerstam "Daily Travel Schedules of Adult Eurasian Hobbies Falco subbuteo — Variability in Flight Hours and Migration Speed Along the Route," Ardea 97(3), 287-295, (1 October 2009). https://doi.org/10.5253/078.097.0304
Received: 23 September 2008; Accepted: 1 May 2009; Published: 1 October 2009
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