Energy Reserves

The energy cost of soaring flight has been calculated at only 1.5-2.0 X BMR (basal metabolic rate), a cost spent mainly in maintaining the wings in gliding position. Per unit time, this is only about 5-25% of the energy consumed in flapping flight, making gliding by far the cheapest mode of flight (Chapter 3). One would expect, therefore, that soaring birds would lay down less migratory fuel as other same-size birds making similar overland journeys, or that they would travel further on a given amount. Comparing different species of raptors and owls, larger species tend to have significantly higher fat levels (and females higher than males), findings that hold summer and winter, and possibly also at migration times (Overskaug et al. 1997).

One detailed study concerns Steppe Buzzards Buteo buteo vulpinus caught on spring migration through Israel, just after crossing the Sahara (Gorney & Yom-Tov 1994). Fat levels were generally low at this stage in the journey, but significantly higher in adults (mean 4.5% of body mass) than in immatures (3.8%); neither group could have completed the rest of their migration to northern Europe (1400-6000 km) without eating, and the fact that the birds were caught in baited traps showed that they were prepared to feed. The mean weights of the birds caught in Eilat were around 25% lower than those of birds caught in the Cape Province of South Africa before spring departure (adults 579 g vs. 762 g, immatures 529 g vs. 709 g). Even if the entire weight difference was due to fat, at expected rates of usage, the birds could not have completed the 7800 km journey from the Cape to Israel without feeding. These data indicated that, although substantial body reserves were apparently accumulated by Steppe Buzzards at the start of their spring migration, representing more than 30% increase in body weight, further feeding en route was necessary to reach Israel, and still further feeding to reach the northern Eurasian breeding areas. This species performs one of the longest migrations of any bird of prey, at 9200-14 200 km each way.

Of 37 Broad-winged Hawks Buteo platypterus caught on their winter territories in Panama in March-April before setting off on migration, 62% were 'moderately to very fat' (Smith et al. 1986). This compared with 35% of 46 birds caught in mid-migration in autumn. Fasting for at least part of the journey was suggested by the absence of food in the guts of Broad-winged Hawks found dead in Panama and by the absence of pellets or faeces at large roosts of migrants (Smith 1980, Smith et al. 1986). If these birds had fed in the mornings, however, such food could have passed through by the evening, so it remains uncertain whether these soaring raptors accumulate large fat reserves before departure, and perhaps at certain sites en route, but fast during much of the journey. Turning to other species, Western Honey Buzzards Pernis apivorus and Oriental Honey Buzzards P. ptilorhynchus make no obvious attempt to feed over much of their route, although they do come down to drink. Museum labels on skins of Swainson's Hawks Buteo swainsoni and Mississippi Kites Ictinia mississippiensis collected at migration times often carried the note 'very fat' (W. S. Clark), and Bald Eagles Haliaetus leucocepha-lus have been tracked on migration for up to 12 days and were not seen to feed (Harmata 2002). Long-distance fasting has also been proposed for other soaring raptors, and recent satellite tracking of various species shows many examples of long, uninterrupted travel steps by soaring migrants (Berthold et al. 1992b, Meyburg et al. 1995a, Kjellen et al. 1997, Hake et al. 2003; Chapter 8). They could only do so on the basis of stored fuel reserves. Even though they make much of their flight on energy-saving gliding, therefore, it is clear that some raptors accumulate migratory fat like other birds.

In theory, such migrants could forage in the early mornings when conditions are unsuitable for soaring, and suffer no reduction in overall migration speed, but whether they do so is another question. Many species migrate partly through desert or other terrain offering little food and often birds travel in such large numbers over such narrow routes that most would have little chance of a meal. Of the soaring species that pass through Israel, only three make regular feeding stops in autumn (White Stork Ciconia ciconia, Great White Pelican Pelecanus onocrotalus, Booted Eagle Hieraaetus pennatus), but others can be caught in baited traps in spring or are occasionally seen with swollen crops. Other raptors that eat regularly on migration include the bird-eating falcons and accipiters which migrate at the same time as their prey, and also those insectivores which can take advantage of migrating insects (for Plumbeous Kites Ictinia plumbea eating drag-onflies, see Smith 1980).

Some smaller raptor species are said to accumulate large fat reserves, notably the Amur Falcons Falco amurensis that cross the sea between India and Africa (Moreau 1972) and the Mississippi Kites Ictinia mississippiensis that migrate between North and South America (Bent 1938). On the other hand, American Kestrels Falco sparverius were found with relatively low fat reserves in early autumn, females slightly but significantly more than males (7.0% vs. 5.3% of body weight), resulting from a doubling of usual fat levels immediately before migration (Gessaman 1979). Fat levels of 3-12% were reported for Cooper's Hawks Accipiter cooperi and Sharp-shinned Hawks A. striatus in New Mexico in autumn and spring, with females containing significantly more fat than males, and adults more than juveniles (DeLong & Hoffman 2004). Other observers have noted large variations in the visible fat reserves of raptors caught on migration (Geller & Temple 1983, Clark 1985), and many immature Red-tailed Hawks Buteo jamai-censis from northern populations were thin during their autumn passage through Wisconsin (Geller & Temple 1983). Other emaciated raptors have been reported by ringers, notably among Northern Goshawks Accipiter gentilis caught during irruptions (Mueller et al. 1977).

In many studies of migrating raptors, the birds were caught in baited traps. As lightweight birds are perhaps more likely to enter such traps than heavy ones, the weights of trapped birds may not be typical of the population as a whole. In two species studied at Eilat in spring, Gorney et al. (1999) compared body weights between individuals caught in baited traps and those caught without bait in mist nets. In the Levant Sparrowhawk Accipiter brevipes the trapped birds were significantly lighter than the netted ones, whereas in the Eurasian Sparrowhawk Accipiter nisus no difference was found between the two groups. In at least one of these species, therefore, individuals caught in baited traps were not representative. However, the fact that both species entered baited traps showed that they were prepared to feed, at least during this part of their journey.

Turning to non-raptorial species, White Storks Ciconia ciconia seem to feed every morning when passing through suitable areas, but would not be expected to do so in desert. Some other species clearly accumulate substantial reserves. For example, Sandhill Cranes Grus canadensis increased in body weight by about 34% (males) and 30% (females) in spring while at the Platte River in Nebraska and Last Mountain Lake in Saskatchewan before migrating to their northern breeding areas (Krapu et al. 1985). Carcass analyses revealed that most of this weight increase was due to fat deposition. This was not surprising considering that cranes use soaring-gliding flight much less than the other species discussed in this chapter, especially at the high latitudes where most of them breed. They frequently travel by flapping flight, or by a mixture of flapping and gliding, and often migrate at night as well as by day.

Pelicans killed in Israel at migration times were found to contain only 3.4% fat, compared with 5.4% in wintering birds, which does not suggest extensive pre-migratory fattening. On the basis of a soaring flight cost of 1.5 X BMR, Great White Pelicans Pelecanus onocrotalus were calculated to need only about 480 g of fat (about 5.5% of body mass) to migrate the 2500 km between Israel and their wintering areas in the Sudd swamps of Sudan, with a travel time of 7-8 days, but as they probably travelled south along the Nile Valley, they may have been able to feed en route (although no regular stopping sites are known) (Schmueli et al. 2000). Clearly, more information is needed on the migratory fuelling of soaring migrants.

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