Can conditions experienced during migration limit the population levels of birds?

Abstract

Populations of migratory birds are usually considered to be limited by conditions in breeding or wintering areas, but some might be limited by conditions encountered on migration. This could occur at stopover sites where competition for restricted food supplies can reduce subsequent survival or breeding success, or during the flights themselves, when adverse weather can occasionally kill large numbers of individuals. Competition for food could act in a density-dependent manner and help to regulate populations, whereas weather effects are more likely to act in a density-independent manner. The evidence for these views is explored in this paper. When preparing for migration, birds must normally obtain more food per day than usual, in order to accumulate the body reserves that fuel their flights. Birds often concentrate in large numbers at particular stopover sites, where food can become scarce, thus affecting migratory performance. Rates of weight gain, departure weights, and stopover durations often correlate with food supplies at stopover sites, sometimes influencing the subsequent survival and reproductive success of individuals, which can in turn affect subsequent breeding numbers. Many studies have provided evidence for interference and depletion competition at stopover sites, relatively few for migration conditions influencing the subsequent breeding or survival of individuals, and even fewer for effects on subsequent breeding numbers. Migrants in flight occasionally suffer substantial mortality in storms, especially over water, sometimes involving many thousands of birds at a time. Other mass mortalities have resulted from atypical ‘winter-like’ weather, occurring soon after the arrival of summer migrants in their breeding areas or just before their departure in autumn. Again, many thousands of birds at a time have been killed in such incidents, causing reductions of 30–90% in local breeding densities. In some bird species, migration-related events can at times have substantial effects on the year-to-year changes in breeding population levels. Nonetheless, the difficulties involved in investigating migrating birds at different points on their migration routes have so far limited the number of studies on the influence of events during migration periods on population levels.

Appendix 1

In the ornithological literature, 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.

Ecological segregation

Nearly all bird species show some degree of ecological separation during migration stopovers, whether in habitat, foraging sites, foraging times or diet (Berthold 1988; Bairlein 1981; Streif 1991; Spina et al.1985; Farola and Fraticelli 1990; Moore et al. 1990). It has been argued that such segregation helps to reduce competition between species at stopover sites, as well as on 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 inter-specific competition for food is limiting for individual migrant performance, but cannot prove it. Nor can the idea be tested satisfactorily.

Temporal segregation of migration seasons

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 (for yellow wagtail Motacilla flava, see Curry-Lindahl 1958). 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. 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 would fall especially heavily on the later-migrating populations.