Social organisation is relevant to these issues because it provides the behavioural framework within which the regulation of density and distribution occurs. The spacing of birds in the non-breeding season can vary from solitary and territorial to gregarious and flocking, depending on the species, the habitat, the distribution of food supplies, predation pressures and other factors, as well as on the social status of the individuals concerned (Newton 1998b). In addition, while some migrant birds remain in the same place for the whole non-breeding period from arrival to departure, others occupy two or more sites in succession, at different points on the migration route, or they may remain continually on the move, again depending largely on the spatial and temporal occurrence of food. In the northern continents, such itinerancy is shown by boreal finches and others, while in the drier parts of the southern continents, some migrant species follow rain-belts for the food sources they promote. African examples include the Lesser Spotted Eagle Aquila pomarina and White Stork Ciconia ciconia (both studied by radio-tracking, Meyburg et al. 1995c, Berthold et al. 2002), while South American examples include the Swallow-tailed Kite Elanoides forficatus and Swainson's Hawk Buteo swainsoni (Brown & Amadon 1968, Bildstein 2004). These species illustrate a major difference between the breeding and non-breeding seasons: in the breeding season, mated pairs are based for up to several months in the localities in which they nest, but in the non-breeding season, individuals of at least some species are free to move around, concentrating wherever food is plentiful at the time. In some species, individuals can migrate at almost any date in the non-breeding season, if stimulated to do so by reduced food supplies, when they usually move further along the same migration axis, reaching the most distant parts of their wintering range only in extreme years (Chapter 18). Hence, if migrants are limited by conditions in wintering areas, for some species there may be more than one such area involved, and these areas may not necessarily be the same from year to year.
Individual birds may remain in the same localities throughout the non-breeding period if food supplies remain continually available there. Similarly, birds can normally return to the same sites each year if food supplies are consistent and predictable from year to year. This is true whether the birds live solitarily or gregariously. Solitary species that show both within- and between-year site-fidelity include the European Pied Flycatcher Ficedula hypo-leuca, Spotted Flycatcher Muscicapa striata, Melodious Warbler Hippolais polyglotta and Common Nightingale Luscinia megarhynchos wintering in Africa (Salewski et al. 2000), Pallas' Grasshopper Warbler Locustella certhiola and Great Reed Warbler Acrocephalus arundinaceus wintering in Malaysia (Nisbet 1967, Nisbet & Medway 1972), and various parulid warblers and others wintering in Central or South America (Rappole 1995). Flocking species that show within-year and between-year site-fidelity include some geese, cranes and shorebirds, among others (Chapter 17).
Some species seem to behave in much the same way in their breeding and wintering areas. For example, Northern Wheatears Oenanthe oenanthe are territorial in both, remaining at the same localities throughout their stay, and returning there year after year. But territories are held by pairs in breeding areas and by singles in non-breeding areas. Other species hold pair territories in breeding areas, remaining faithful to these sites throughout the breeding season and from year to year, but are much more flexible and mobile in winter. In West Africa, Eurasian Willow Warblers Phylloscopus trochilus often feed in single-species or mixed-species flocks, showing no obvious territorial behaviour. In one area, Willow Warblers were present from November to April, except for 3-4 weeks in February at the height of the dry season, yet at no time did individuals stay for more than a few days (Salewski et al. 2002).
Whether species feed in individual territories or gregariously seems to depend on the consistency of the food supply, and whether it is defendable (Newton 1998b). The same species can behave territorially on one type of food and gregariously on another. For example, Steppe Eagles Aquila nipalensis in Africa are mostly territorial when feeding on mammals, and gregarious when exploiting local abundances created by emerging termites or Quelea colonies. In addition, dominant individuals of a species may behave territorially, while subordinate ones, unable to get territories, feed solitarily or in flocks, sometimes on different foods (for White Wagtail Motacilla alba see Zahavi 1971, for Yellow Wagtail M. flava see Wood
1979, for Fieldfare Turdus pilaris see Tye 1986, for Sanderling Calidris alba see Myers et al. 1979, for Common Crane Grus grus see Alonso et al. 2004).
The occurrence of winter movements, the use of multiple sites within a winter, and often different sites from year to year, all combine to make studies of population limitation difficult in migratory bird populations. It is not surprising, therefore, that such studies have developed only relatively recently, and that the most detailed ones have concentrated on species in which individuals occupy the same breeding and wintering places year after year.
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