Social interactions

Dominance relationships influence the outcome of competitive interactions at stopover areas, just as in breeding or wintering areas, even though the individuals concerned may be present for only short periods (Pienkowski & Evans 1985). In some species, dominant individuals hold short-term territories at stopover sites, and subordinate individuals unable to obtain territories must feed in less good places or leave (Rappole & Warner 1976, Kodric-Brown & Brown 1978, Bibby & Green 1980, Sutherland et al. 1982, Hixon et al. 1983, Veiga 1986, Sealy 1989, Carpenter et al. 1993). Territorial Northern Waterthrushes Seiurus noveboracensis were found to accumulate body reserves at a rapid rate, whereas non-territorial ones gained no weight until after they had acquired a feeding territory (Rappole & Warner 1976). The same was found among European Robins Erithaca rubecula (Mehlum 1983). Similarly, in non-territorial species, dominants had higher feeding and fuel deposition rates than subordinate conspecifics (for Bluethroat Luscinia svecica see Lindstrom et al. 1990, Ellegren 1991, for Brent Goose Branta bernicla see Prop & Deerenberg 1991). At similar environmental food levels, average fattening rates were lower at high than low densities of individuals (Moore & Yong 1991, Kelly et al. 2002). There is thus no doubt that birds compete for food at stopover sites, and that the fuelling rates of some individuals are thereby lowered. Competition may also help to regulate local densities in relation to resource levels, and result in some birds making longer stopovers, or leaving with lower fat reserves than others, reducing the distance they could travel before the next stop.

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