Hyperphagia (eating more than needed to maintain a stable body weight) is evident especially in captive passerine migrants, which at appropriate seasons suddenly begin to eat around 25-30% more per day than usual (range 10-50%). This promotes mean weight gains of up to 10% per day (Figure 5.4). On a 20-30-g bird, with a fattening rate of 1.0-1.5 g per day, fattening may take 4-10 days. Higher rates of weight gain have been recorded at 20% per day in White-crowned Sparrows Zonotrichia leucophrys (King 1972); at 25-30% in Bobolinks Dolichonyx oryzivorus (Gifford & Odum 1965), and at 40% in Garden Warblers Sylvia borin (Bairlein 1990). In studies of captive shorebirds, which could feed for 23 of the 24 hours per day under artificial light, maximum daily energy intakes reached 300-500% above existence levels (Lindstrom & Kvist 1995, Kvist & Lindstrom 2003).
Before spring migration, White-crowned Sparrows Zonotrichia leucophrys showed two peaks in foraging, one in the morning and the other before sunset, as observed in both wild and captive birds (Morton 1967, Ramenofsky et al. 2003). Toward migration time this pattern changed, as birds began to feed throughout the daylight hours, gaining in body mass and fat content. Once migration activity started, some birds ceased feeding in the late afternoon, enabling them to empty their guts, reducing excess weight before the flight, which would normally begin after dark (Morton 1967, Brensing 1989). Shorebirds in West Africa increased their foraging periods from 6-10 hours per day in winter to 12 hours per day during spring fuelling. This change involved more feeding at night, but the total time spent foraging was still limited by the tidal regime (Zwarts et al. 1990a). Some birds may achieve hyperphagia at the expense of vigilance, spending less time scanning for predators, as noted in Ruddy Turnstones Arenaria interpres (Metcalfe & Furness 1984).
The most obvious way in which a diurnal bird could conserve feeding time is to migrate at night. While this may not increase feeding time over what is usually available, it at least prevents potential feeding time being reduced by flight time. Given this advantage, together with reduced predation risk, it is hard to
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