Movements Within The Nonbreeding Season

Whereas the individuals of some species migrate only between a single fixed breeding area and a single fixed wintering area, individuals of other species make substantial movements during the course of a non-breeding season, often extending ever further from their breeding areas as the season progresses. In some species such movements are regular, somewhat analogous to different stages in a single migration, but the birds stay in the same place for up to several weeks between each move. The separate moves occur at about the same dates each year, and each move is preceded by fat deposition. Many species that breed in Eurasia and winter in Africa perform such two-or-more-stage migrations, pausing for a time in the Sahel zone (where some of them moult, as described above), and then move on within the northern tropics or beyond the equator to the southern tropics (Jones 1995; Chapter 24). This latter movement brings the birds into the austral summer at a time when food is at its most plentiful.

Other species move between three areas each year that are not on the same direct route. As found by radio-tracking, Prairie Falcons Falco mexicanus leave their breeding areas in southwest Idaho between late June and mid-July (Steenhof et al. 2005). They migrate northeast across the continental divide to spend the rest of the summer in Montana, Alberta, Saskatchewan and the Dakotas. They stay there for 1-4 months, and then in October move southwards around 1000 km to the southern

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North America Map

Figure 16.3 Three-part migration of Prairie Falcons Falco mexicanus, as revealed by the satellite-tracking of radio-tagged birds nesting in southern Idaho. B, breeding area; PB, post-breeding area; W, wintering area. After breeding, the birds move northeast to spend the summer in cooler areas at higher latitude or elevation where ground squirrels remain available, and then after 1-4 months, they move southward to winter mainly in northern Texas, returning in spring directly to their breeding areas. Based on Steenhoff et al. (2005).

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Figure 16.3 Three-part migration of Prairie Falcons Falco mexicanus, as revealed by the satellite-tracking of radio-tagged birds nesting in southern Idaho. B, breeding area; PB, post-breeding area; W, wintering area. After breeding, the birds move northeast to spend the summer in cooler areas at higher latitude or elevation where ground squirrels remain available, and then after 1-4 months, they move southward to winter mainly in northern Texas, returning in spring directly to their breeding areas. Based on Steenhoff et al. (2005).

Great Plains, mainly in Texas. After five months there, they return on a direct course for their breeding areas (Figure 16.3). This three-step migration enables the falcons to exploit seasonally abundant food supplies in each region. Their absence from breeding areas at the hottest time of year coincides with the period when their main prey, ground squirrels, are underground and unavailable. But in the cooler areas occupied in late summer, ground squirrels remain active until winter. In their wintering areas, Prairie Falcons feed largely on migratory birds, especially wintering Horned Larks Eremophila alpestris. In addition, Ferruginous Hawks Buteo regalis, which also eat ground squirrels, perform a similar clockwise movement, first eastward and northward to higher latitude or higher altitude post-breeding areas, and then southward to wintering areas (Schueck et al. 1998).

Several species of hummingbirds also move through three or more different regions each year (Grinnel & Miller 1944, Stiles 1973). For example, Anna's Hummingbird Calypte anna breeds in spring in the coastal chaparral of southern California, summers in the high mountains of California, and winters in the deserts of Arizona and Mexico, thereby ensuring that its need for nectar is met year-round. Movements following flowering patterns have been described also in other nectar-eating birds, including the sunbirds of Africa and Asia and the honeyeaters of Australia.

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