Because migrants occupy different parts of the world at different seasons, their numbers and geographical ranges at one season may be limited by the size of area available to them at the other season. In other words, a species with a limited wintering area, and no scope for expansion, may be unable to achieve the numbers that would enable it to occupy its full potential breeding area, or vice versa. A small wintering range could thereby limit both the breeding population and the breeding range. For example, the Bristle-thighed Curlew Numenius tahitiensis breeds in Alaska, but winters on Hawaii and other Pacific Islands. As the wintering habitat on the islands is restricted, the species could never have been numerous there, limiting it to small numbers in its Alaskan breeding range. Moreover, the situation may have worsened in recent centuries because the species becomes flightless during moult, and many of the islands colonised by humans and associated predators have now become unsuitable for it (Marks et al. 1990). Another species that winters on islands, and has a small breeding population and range on a continent, is Kirtland's Warbler Dendroica kirtlandii (winters Bahamas, breeds Michigan). It is now one of the rarest breeding birds in North America, but even at its peak, it may never have been numerous if it was always restricted to its present wintering range.
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