Poor condition and mortality at stopover sites

Birds caught soon after their arrival at stopover sites often show great variation in body condition. At some sites, it is not uncommon to find a two-fold variation among the body weights of individuals of similar size (e.g. Moreau & Dolp 1970). Some individuals seem on the point of starvation, devoid of body reserves, while others have enough residual fuel for several hundred kilometres of further flight. Over three different years, some 39% of 1903 thrushes (Catharus, Hylocichla) caught on arrival in a Louisianan coastal woodland after crossing the Gulf of Mexico carried no obvious body fat. The body weights of 23% of these thrushes were at or below the estimated fat-free body mass; while only 5% had estimated

1For age differences see Veiga (1986), Serie & Sharp (1989), Ellegren (1991), Carpenter et al. (1993), Holmgren et al. (1993), Gorney & Yom-Tov (1994), Morris et al. (1996), Woodrey & Moore (1997), Lindstrom et al. (1990), Ellegren (1991), Yong et al. (1998), Woodrey (2000), Heise & Moore (2003); for sex differences see Morris et al. (1994), Otahal (1995).

fat stores that exceeded 20% of lean body mass (Yong & Moore 1997). Between 21 March and 9 May 1987, 13 individuals of five migrating species were found dead in wooded feeding habitat, apparently having starved, and another 66 corpses of 18 species were found on daily walks on a 2 km stretch of beach (Moore et al. 1990). Other instances of birds arriving in Panama in autumn in very poor condition (below fat-free mass) were given by Rogers & Odum (1966), and other severely emaciated birds were collected from oil drilling platforms in the Gulf of Mexico (Gauthreaux (1971, 1999), from ships at sea (Serle 1956, McClintock et al. 1978), and from an island of southern Italy (Baccetti et al. 1981, cited in Pilastro & Spina 1997).

Birds weakened by starvation are more easily caught by predators, and may account for some of the raptor kills found at stopover sites, among which young birds often predominate (Bijlsma 1990). Many other records of exhausted and starving migrants are mentioned incidentally in the ornithological literature, often in association with sea- or desert-crossings (for hirundines and others see Paynter 1953, Spendelow 1985; for other passerines see Pilastro & Spina 1997; for shore-birds see Dick & Pienkowski 1979, Bijlsma 1990; for raptors see Smith et al. 1986). They confirm that some birds die of starvation during the course of their journeys.

A striking example was provided by the 200-300 Chimney Swifts Chaetura pelagica which in October 1979 stopped and starved to death on the Caribbean Islas del Cisna (Spendelow 1985). All collected specimens were emaciated, with no visible body fat, and weighed around 40% less than healthy Chimney Swifts in spring and summer. Totalling less than 4 km2 in area, and lying about 180 km north of Honduras, these two islands were judged too small and isolated to provide sufficient food, and 21 migrants of various other species were also found dead there in the spring and autumn that year, including Blue-winged Teal Anas discors and Cattle Egret Bubulcus ibis. Such events may occur frequently on small islands and other potential stopping places offering insufficient food. Spendelow (1985) cites B. Robertson that larger birds, such as Cattle Egrets, may survive for weeks on the Dry Tortugas on inadequate food, with as many as 100 dying slowly through the summer on an island group of only 40 ha. Such instances provide examples of birds unable to continue migration, and eventually dying, because of inadequate food at stopover sites.

It is common for newly arrived birds, especially of long-distance migrants, to lose mass for another day or two before they begin to regain it (Rappole & Warner 1976, Moore & Kerlinger 1987). Explanations for this initial weight loss include: (a) effects of capture and handling (Nisbet & Medway 1972), (b) inefficient foraging because of unfamiliarity with the stopping site (Yong & Moore 1997), (c) rebuilding of the digestive system in birds that have undergone long non-stop flights (Chapter 5; Piersma & Gill 1998), and (d) competition among conspecifics (Rappole & Warner 1976, Moore & Yong 1991).

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