Split migrations

Although autumn migration is usually considered as a single event, consisting of alternating periods of flight and fattening, some species break their autumn journeys for periods of several weeks, much longer than is needed for refuelling. Some species moult during this break in migration. This behaviour is shown by many Eurasian migrants to East Africa, such as the Marsh Warbler Acrocephalus palustris and Garden Warbler Sylvia borin, which remain in the northern tropics for several weeks, and only later move on to the southern tropics (Jones 1995). In this way, they get the best from both regions, remaining in the northern tropics until conditions deteriorate, and arriving in the southern tropics at the optimal time, after fresh rains have promoted vegetation growth. In captivity, as mentioned already, such species show two separated peaks of migratory restlessness (Berthold 1993). Equivalent behaviour is shown by some small insectivores that migrate between North and South America (Terrill & Ohmart 1984). Interrupted migrations are also evident in various irruptive and other facultative migrants that break their journeys to exploit food supplies they encounter en route, and travel much further from their breeding areas in some years than in others (Svardson 1957, Newton 1972; Chapter 18). One consequence of split migration is that the outward journey (including the break) takes more than four months in some species, whereas the return journey in spring can take only 1-2 months.

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