Body mass variation among latitudes and migration sectors
With the exception of the Desert sector during the spring migration, body mass did not show a significant association with latitude during the autumn and spring migration. This implies that birds do not show a substantial increase or decrease in energy reserve per latitude in a given sector. Most species showed an increase in body mass along the two African sectors but a decrease during their northward migration across the desert in the spring (Table 3). It has been reported that nearly all long-distance trans-Saharan migrant passerines landing in the Sahara Desert have both body masses well above the breeding level and sufficient energy reserves to cross the desert without refueling. Most of the birds checked at an oasis were not critically short of energy reserves, and only a few of them used the desert oases to regain fat (Bairlein et al. 1983; Bairlein 1985; Biebach 1985; Biebach et al. 1986; Biebach 1988; Bairlein 1992; Biebach 1995). This result suggests that energy reserves are either deposited before the birds start crossing the Sahara or at stopover sites in the desert.
After crossing the desert, most species showed a substantial decrease in body mass in NE Africa compared to values registered in Europe. The Sedge Warbler, Red-backed Shrike, Thrush Nightingale, Nightingale, Willow Warbler, Garden Warbler, Whitethroat and Barred Warbler showed a marked decrease in body mass in this sector (Table 2). Across the Eastern African flyway, several bird species perform step migration whereby they cross the Saharan or Arabian deserts without delay and arrive for extended stopover in NE Africa. Within this stopover period, many birds accomplish a partial or complete moult. Migratory passerines with such a stepwise autumn migration and moult include the Marsh Warbler, Whitethroat and Great Reed Warbler (Pearson 1973, 1975; Pearson and Backhurst 1976a; Curry-Lindahl 1981; Pearson 1982; Dowsett-Lemaire and Dowsett 1987; Pearson et al. 1988). However, individuals of other species may also perform moult in these areas. In such cases, a moult-related decline in body mass might be exhibited.
Along the NE Africa migration sector, slight gains in body mass were estimated for the Garden Warbler compared to those observed in Europe, the desert and Eastern Africa (Table 2). As indicated earlier by Ash (1980, 1994), some bird species arrive in NE Africa with a moderate body mass and show a substantial increase in body mass before departure to more southerly located wintering grounds.
In the spring, most of the study species gained considerable mass at the northeasterly sites in Africa (from which they take off for trans-desert migration to the northern latitude) followed by a substantial mass loss during migration across the deserts (Table 3). Birds caught in Eastern Ethiopia, Somalia and the Farasan Islands of Saudi Arabia revealed pronounced body mass increases relative to birds caught at study sites located in Eastern Africa. For example, on average, the Marsh Warblers in Jijiga, 09°22′N 42°51′E eastern Ethiopia, were 4.22 g heavier than birds in Kifufu (03°09′S 39°12′E), Tanzania. Marsh Warblers were not observed feeding at the stopover site in Jijiga (H. Biebach, personal communication and personal observation); therefore, the increase in body mass must have been achieved at feeding sites located south of this site.
Based on our analysis, the four strategies demonstrated earlier (e.g. by Schaub and Jenni 2000a) for crossing the desert by migratory birds of the Western African migration route (i.e. steady increase in body mass along the migration route, increase in body mass towards the south before reaching the Sahara or just before crossing the desert and regular stop-and-fuel strategies) are not directly applied by birds migrating along the Eastern African flyway/migration route (taken here as the route through Egypt, Sudan, Eritrea, Ethiopia, Somalia, Kenya, Uganda and Tanzania). We are aware that, unlike the data from Schaub and Jenni (2000a), our results are derived from a dataset (gathered from different sources) that do not account for fluctuations or unidirectional changes due to factors that affect body mass estimates. Hence, some of the estimates may have given a biased conclusion. Nevertheless, our results, which are based on a large number of birds and study sites, provide valuable information on the general trend of passerine migration strategies over a wider geographic range in both seasons.
Schaub and Jenni (2000a) note that they did not sample birds from the eastern population of Sedge Warbler, Whitethroats and Spotted Flycatcher. However, our results (which include data from the eastern population) indicate that while migrating along the Eastern African flyway, some species (e.g. Sedge and Garden Warbler) may have adopted a slightly different strategy than those reported earlier (e.g. Schaub and Jenni 2000a, b).
Similar to the results shown for change in body mass during the migration from Western Europe to Northern Africa, the Spotted Flycatcher did not show a significant change along the Eastern flyway. Elsewhere, observations and recoveries of Spotted Flycatchers have indicated that North Africa is used as a stopover site before crossing the Sahara (Moreau 1972; Bundy 1976; Sultana and Gauci 1982; Flint and Stewart 1983; Fransson 1986). In a study at an Egyptian oasis, only very few Spotted Flycatchers stayed several days and gained some mass before continuing migration to the south (Biebach 1985; Goodman and Meininger 1989), while all of the others had high masses and departed after a short stopover. Along the entire migration route considered in this study, the highest autumn mass for the Spotted flycatcher was reported from the stopover sites in the deserts. This implies that birds may intermittently stop and refuel while crossing the desert.
Similarly, the western population of Sedge Warbler already starts to accumulate a larger fuel store in Central Europe, long before arriving at the northern edge of the Sahara (Bibby and Green 1981; Schaub and Jenni 2000a). Along the eastern route, this species does accumulate its energy store in Europe, but it still maintains a steady body mass towards the south before crossing the desert. Similarly, Whitethroats showed no significant increase in body mass in the east, while in the west, it accumulates the larger energy reserves in North Africa before its flight across the Sahara.
These differences could mainly be accounted for by the fact that the eastern populations have to cover a longer distance of inhospitable area (including the Arabian Desert and the Red Sea followed by the Sahara Desert). As such, the different landscape pattern may place constraints that require a different type of migration strategy. Thus, it could be speculated that different populations might have evolved different migration strategies of energy storage during migration. Using body mass measurement of Garden Warblers caught on southbound migration across SW Europe, both Bairlein (1991) and Schaub and Jenni (2000a) independently found a similar pattern of a relatively larger increase in body mass in eastern birds than in western birds (comparing 10° east and west) during the autumn migration from the breeding ground in Europe to North Africa. These researchers argue that the western population of Garden Warblers migrates through the Iberian peninsula and crosses a shorter stretch of the Mediterranean Sea, while the Eastern population is required to cross the eastern section of the Mediterranean Sea and then crosses the Sahara Desert (Biebach 1996); this latter route provides limited opportunity for refueling and, hence, larger energy reserves needs to be stored.
Several factors may affect the body mass measured at various study sites. These include body size, time and intensity of moult, time of day and year as well as inter-year variation, method and effort of capture, geographical location, food availability in the study site, errors and accuracy of measurements by various investigators, among many others (Clark 1979; Murphy 1996; Schaub and Jenni 2000a). In addition, different populations may be associated at different study sites and in different seasons. Similarly, other factors, such as sampling bias towards lower body mass values, the arrival and departure of the birds caught at each site, the stopover duration, fuel deposition rate of the birds at each site, variation between different years and weather conditions, contribute to the variation in body mass.
For some of the species, body mass values differed between geographic sectors, indicating that birds possibly modulate their energy budget depending on their current location (geographic sector) en-route. This is in agreement with a recent appraisal and study of spatio–temporal precision in bird migration. For instance, in an experimental study, Fransson et al. (2001) and Kullberg et al. (2003, 2007) illustrated that birds may use geomagnetic cues as external indicators for successful migration, and they explain that these cues (simultaneously operating with bird’s orientation behavior and endogenous rhythm) provide information about the geographical position when such information is required, for instance, for a fueling decision. Given this, depending on the species and the route taken, a certain relationship would be expected between body mass and latitude of a given geographic area.
Palearctic birds that migrate through eastern and northeastern Africa are likely to belong to populations from different breeding grounds. Some passerines cross the Sahara to reach breeding grounds in the eastern Mediterranean while others presumably migrate over a broader front to move into Southwest Asia through the Arabian regions (Moreau 1972). These two subpopulations may differ in terms of the timing of migration and fuel deposition (and hence extent of fattening), which in turn may be related to the physical and geographic differences between the migration routes.
Some birds bound for the breeding areas in the north–northeast may start to increase body mass as far south as the equator. For example, Pearson (1971) has shown that in the spring, the heaviest Sedge and Marsh Warblers encountered in Uganda, Lake Victoria had sufficient fat reserves for migrating directly to the Middle East. However, a considerable number of birds caught in late spring in Uganda had moderate body masses. These birds may keep their reserves by frequently feeding, while moving either to more northerly sites or to further staging areas close to the desert. The extent and rate of fattening may also vary between sites (Pearson 1971, 1978).
Many locations in Eastern and NE Africa serve as important fattening areas for Palearctic migratory birds in both south- and northbound migration seasons. In the autumn, birds show an increase in body mass in NE Africa before migrating to more southerly wintering grounds. In the spring, birds gain considerable mass in NE Africa and immediately to the south of the Arabian/Sahara Desert before takeoff for trans-desert migration. Thus, the quality of these stopover sites may very well exert a significant influence over the success of the journey as well as the reproductive fitness of the birds in their breeding grounds.
Our results are based on body mass estimates of 12 Palearctic migratory passerines monitored at successive sites in Europe, Eastern Africa, the Middle East and Arabia. We show that, in general, passerine migrants adopt different patterns of accumulating energy reserves for crossing ecological barriers and migration. These patterns vary depending on the geographic sector and seasons (autumn and spring). Moreover, different populations or subpopulations may follow specific strategies that particularly fulfill the eco-physiological demands of each geographic sectors en-route.