Within the migration season, migratory flights may be influenced by prevailing weather and food supply (Chapters 4 and 27). The role of feeding conditions in affecting autumn fattening rates, body condition and departure dates has been established in the field for a range of species from Sedge Warbler Acrocephalus schoenobaenus (Bibby & Green 1981) to Greylag Goose Anser anser (van Eerden et al. 1991). In general, individuals that fatten most rapidly leave first, whether in autumn or spring, or from starting or stopover sites. Poor feeding conditions contribute to the variation between individuals within years, and can delay the progress of whole populations in some years (Chapter 27).
When they reach an appropriate weight, migrants normally leave immediately if weather permits. This would be expected because, once acquired, fat stores are dangerous and expensive to maintain, conferring increased vulnerability to predation (Chapter 5). Research has confirmed that the motivation to proceed with migration is related to fat stores, as is the strength of directional preference (Dolnik & Blyumental 1967, Bairlein 1885a, Yong & Moore 1993, Berthold 1996, Sandberg et al. 2002). In one study, four songbird species were caught on stopover at an oasis in Algeria, and kept in cages fitted with activity recorders (Bairlein 1985a, 1992). Generally, lean birds were active only during daylight, feeding as normal. In contrast, fat birds remained inactive by day without feeding, but became active at night when they would normally have migrated. Birds with moderate fat reserves fed during the day and were also active at night. Nocturnal migratory activity was thus clearly associated with fat levels. In another study, European Robins Erithacus rubecula were trapped in autumn an hour or two before sunset, placed in cages that provided a view of the sky, and their behaviour recorded (Bulyuk & Mukhin 1999). Nocturnal restlessness appeared in 18% of the birds tested, mainly in the fattest ones, which also became restless earlier with respect to sunset than leaner birds. In both autumn and spring, birds trapped and tested during waves of passage tended to start nocturnal restlessness in greater proportion, and earlier with respect to sunset, than conspecifics caught during migratory pauses.
At the start of migration, the precise relationship between extra fat deposition and migration varies between species, and between autumn and spring in the same species, in apparent adaptation to the journeys they have to make. Some populations accumulate large fat levels before they start migrating, others only at a later stage in the journey, when they have to make a long flight over sea or desert without feeding (Chapter 6). For example, captive Pied Flycatchers Ficedula hypoleuca in Germany began increasing body mass in autumn soon after showing migratory restlessness, whereas Collared Flycatchers F. albicollis began much later, long after the first appearance of restlessness, and then fattened rapidly. This difference between species was linked to their different migration patterns, as revealed by ring recoveries (Gwinner 1996b). Similarly, in captive Bobolinks Dolichonyx oryzivorus, migratory restlessness began soon after the start of fat deposition, but did not reach maximum intensity until much later, after the peak of fat deposition (Gifford & Odum 1965). This fits the fact that, like many other species, Bobolinks start their migration with short flights, which lengthen later in the journey.
During the course of a journey, birds alternate periods of migratory flight (when body reserves are expended) with periods of stopover (when reserves are replenished through feeding). In general, periods of flight and refuelling must be integrated in such a way that leanness leads to feeding and fatness to migration. At departure and stopover sites, migrants have often been caught and weighed, and whereas lean birds are often recaught on subsequent days, as they gain weight, heavy (fat) ones are seldom recaught (Bibby & Green 1981, Yong & Moore 1993). Such findings imply that the fat levels of a bird influence its tendency to resume migration (Chapter 27; see also Dolnik & Blyumental 1967, Berthold 1996, Sandberg et al. 2002).
Captive birds have provided additional information. Spotted Flycatchers Muscicapa striata and Garden Warblers Sylvia borin that were fed and then kept temporarily without food showed migratory restlessness (Biebach 1985, Gwinner et al. 1985a, Totzke et al. 2000). When they were again provided with ad lib food, their migratory restlessness initially ceased, but re-appeared later as their body mass and fat levels continued to rise. The implication was again that the fat content of a bird influenced its inclination to depart. In another type of experiment, Red-eyed Vireos Vireo olivaceus were trapped as they were about to cross the Gulf of Mexico in autumn. Each was weighed, and tested for directional preference. It was then fitted with a small chemiluminescent light stick, released on its own 1-2 hours after sunset, and followed with binoculars until lost from view (Sandberg & Moore 1996). Before their release, significantly more fat vireos (81%) than lean ones (61%) showed migratory activity in cage tests. After release, all the fat birds flew out of sight in the migration direction. Some 38% of the lean birds stayed at their current location, and most of the others took a direction diametrically opposed to the migration direction. Similar results on the relationship between fat levels, inclination to migrate and directional preferences were obtained in release experiments using Robins Erithaca rubecula and Pied Flycatchers Ficedula hypo-leuca (Sandberg et al. 1991), and in cage tests on several North American warblers (Able 1977), Chaffinches Fringilla coelebs (Backman et al. 1997), Snow Buntings Plectrophenax nivalis (Sandberg et al. 1998) and Swainson's Thrushes Catharus ustulatus (Sandberg et al. 2002). In general, the greater the level of stored fat, the more likely was the bird to show in test cages a preferred direction. Many lean birds preferred the opposite direction, but temporarily reversed migration is not uncommon in the wild when birds encounter a sea-coast or other barrier. It has been interpreted as an attempt to find more profitable feeding areas away from the crowded coast (Chapter 4; Akesson et al. 1996).
All these various findings confirm that the fat levels of a bird on migration influence both its tendency to depart on each leg of the journey, and the strength of its directional preference. However, most of the above studies were conducted at localities where the birds were about to cross the open sea or desert. The behavioural difference between lean and fat birds might be much less marked in birds migrating overland where feeding sites are frequent (for Red-eyed Vireo Vireo olivaceous see Sandberg & Moore 1996a). Birds might then simply vary the length of their flights, according to their available fuel levels.
If fat is extracted from plant or animal oils, or from the carcasses of other birds, and injected into the subcutaneous deposits of living birds, it is apparently utilised in the same way as the bird's own fat (Dolnik & Blyumental 1967). During the migration period, this procedure increased the activity of lean birds to resemble that of fat ones, but at other times of year and in sedentary species, it had no such effects. Again, the feeding and flight activity of migrants was related to the amount of fuel they contained. This conclusion was in line with findings from seven passerines species studied at Rybachi on the southern Baltic coast, where the intensity of migration on particular days (as shown by the numbers of birds trapped) was strongly correlated with the fat levels of the birds themselves. Nevertheless, the fattening patterns of birds do not entirely conform to the simple model: arrive lean, fatten rapidly to a threshold, and then depart as soon as possible. The proximate factors influencing fattening rates and stopover times in each species include date in season, available food supplies, weather, prior body condition, age, sex and social factors (Chapters 4 and 27).
Moreover, fattening and restlessness are not inseparably linked in birds, because captives of various passerine species kept lean by extreme food deprivation still showed nocturnal restlessness (King & Farner 1965, Lofts et al. 1963, Gwinner 1968, Berthold 1977). Moreover, restlessness is not an infallible reflection of migratory condition, for it sometimes occurs in other circumstances, and it is often a matter of interpretation what is considered true migratory restlessness (e.g. Berthold 1988; Box 12.1). Nevertheless, fattening at migration seasons is normally absent in birds from non-migratory populations in either natural or experimental conditions (e.g. Wolfson 1945), although some nocturnal restlessness has occasionally been recorded from non-migratory populations (Smith et al. 1969, Helm & Gwinner 2006).
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