Another way in which birds can increase their intake is to concentrate on easily digestible, energy-rich food items. In both the Old and New Worlds, many passerines eat fruit at the time of migratory fattening. These include not only regular fruit-eaters, such as Sylvia warblers, but also others not normally considered as fruit-eaters, such as the Pied Flycatcher Ficedula hypoleuca and Yellow Wagtail Motacilla flava. This change of diet does not result simply because fruit is more available than insects in late summer, because captive birds, given a choice of insects or fruit, also select fruit then. In the Mediterranean region, at the time of autumn migration, frugivorous warblers increased in body mass about twice as rapidly as purely insectivorous ones, even though the fruit-eaters had to eat more than their own body mass in fruit per day (Ferns 1975, Thomas 1979, Izhaki & Safriel 1989). Likewise, pre-migratory weight gain in European Robins Erithacus rubecula showed a close relationship to fruit consumption, and was greatest from fruits relatively rich in lipids (Herrera 1981). Nonetheless, most birds take some insects at the same time as fruit, presumably to supply their protein needs, and captive Garden Warblers Sylvia borin, Red-eyed Vireos Vireo olivaceus, Hermit Thrushes Catharus guttatus and others gained weight more swiftly from a mixed diet than from fruit or insects alone (Bairlein 1998, Parrish 2000, Long & Stouffer 2003). For obvious reasons, most fruit-eating occurs in autumn, but many passerines also select fruit before the spring migration if it happens to be available. Some of the passerine species that spend the northern winter south of the Sahara eat many fruits in spring, especially those of Salvadora persica, on which they can also fatten more rapidly than on insects (Fry et al. 1970, Moreau 1972, Stoate & Moreby 1995).
Fruits are generally low in protein (most types 1-7% of dry mass), with a few exceptions such as Olive Oleo europaea (8%) and Elder Sambucus nigra (12-18%), both of which are favoured by birds (Jenni-Eiermann & Jenni 2003). However, most fruits contain high proportions of carbohydrate and unsaturated fatty acids that can be easily metabolised to produce body fat. Unsaturated fatty acids in fruit proved ideal for migratory fat formation in captive birds, while other individuals fed on diets containing only saturated fatty acids showed lower rates of fattening (Bairlein & Simons 1995).
The lipid content of fleshy fruits varies greatly between plant species. In temperate areas, the percentage of digestible lipid in most fruits is lower than in insects. In Mediterranean regions, where many warblers fatten in autumn before crossing the Sahara, fruits tend to be richer in lipids than those in the temperate zone; and in tropical regions, where fruits tend to be even richer in lipids, some bird species seem able to subsist on fruit alone for almost the whole year, some even raising their young on fruit (Snow & Snow 1988). Captive birds provided simultaneously with two diets identical in energy content but differing in lipid content, showed clear preferences for lipid-rich foods (Borowitz 1988, Bairlein 1990), confirming observations on wild birds (Borowitz 1988, Snow & Snow 1988). In contrast, American Robins Turdus migratorius preferred sugar-rich to lipid-rich fruits, and had higher absorption efficiency for sugars, although assimilation of lipids increased from summer to autumn (Lepczyk et al. 2000).
Another advantage of feeding on fruit is that it can be obtained more easily than insects; being concentrated, predictable and conspicuous, it requires the minimum expenditure of time and energy to obtain. Its low fibre content also makes fruit easy and quick to digest, and its high water content may reduce the need to drink. The main problem with some fruits is that they contain tannins or other toxic compounds which birds have to deal with in various ways.
Many passerines that remain insectivorous during migration prefer insects that are rich in fats (such as caterpillars) or carbohydrates (such as aphids), and can at times show fattening rates as high as those of fruit-eaters (for high rates of fuelling in Sedge Warblers Acrocephalus schoenobaenus eating aphids see Bibby & Green 1981). Other warblers on spring migration prefer nectar over insects, presumably for the same reason. Likewise, the Yellow-faced Honeyeater Lichenostomus chrysops in eastern Australia was found to increase the ratio of nectar to insects in its diet during both autumn and spring migrations (Munro 2003).
Most diet studies relevant to migration refer to small insectivorous-frugivorous passerines. It is unknown to what extent other birds change their diets at migration times, but fruit-eating is known then in such unlikely candidates as shore-birds, gulls and cranes (Glutz von Blotzheim et al. 1975, 1977). Also, Pink-footed Geese Anser brachyrhychus turned from grass to newly sown grain at migration time, more than doubling their daily energy intake (Madsen 1985), while Canada Geese Branta canadensis changed from a diet of corn to a mixture of corn and meadow grass in spring, the grain providing carbohydrate (which can be converted to fat) and the grass protein (McLandress & Raveling 1981). Many species of ducks turn from plant leaves and seeds to invertebrates at the time of spring migration, presumably to acquire more protein (Arzel et al. 2006). Raptors that eat migrant birds presumably have diets richer in lipids at migration times, when the fat contents of prey are higher than at other times.
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