Two other types of findings have been cited as evidence that food supplies at stopover sites can be limiting for migrants. Because these types of evidence are open to other interpretations, and are not amenable to experimental testing, they are discussed separately below.
Nearly all bird species show some degree of ecological separation during migration stopovers, whether in habitat, foraging sites, foraging times or diet (Bairlein 1981, Spina et al. 1985, Berthold 1988, Fasola & Fraticelli 1990, Moore et al. 1990, Streif 1991). It has been argued that such segregation helps to reduce competition between species at stopover sites, as well as in breeding or wintering areas.
While this may be true, the demonstration of ecological differences between species at a stopover site does not necessarily imply that food is limiting there. Such differences, which depend on the structure and behaviour of the species themselves, may result from food-based competition in the past, or in areas other than stopover sites, or they could result from causes other than food-based competition. Such ecological differences are thus consistent with the idea that interspecific competition for food is limiting for individual migrant performance, but cannot prove it - nor can the idea be tested satisfactorily.
Closely related species with similar ecology often pass through particular sites at somewhat different dates during the migration seasons (for shorebirds see Recher 1966, for warblers see Howlett et al. 2000). The same is also true for different populations of the same species. In general, different populations pass north in spring in the sequence in which their breeding areas become habitable, and south in autumn in the sequence in which their breeding areas become unsuitable (Chapter 12). Although such temporal segregation may reduce the opportunity for competition between the individuals in different populations, it might not have evolved for that reason, but have some quite different basis, related to the dates and periods that the nesting areas of different populations are suitable for occupation (Chapter 14). Also, the early migratory populations might, in some situations, deplete the food for later ones, in which case competition would not be eliminated, but its effects could fall especially heavily on the later migrating populations.
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