Effects of delays

A major advantage of intrinsic control of the annual cycle is that events normally follow one another in an appropriate sequence. Nevertheless, birds whose spring migration is delayed also delay the start of breeding, birds that are breeding late delay the start of moult, and birds that are moulting late normally delay the start of migration (Chapter 12). However, there is a limit to the delaying process. Birds that arrive too late on their breeding areas may abandon nesting altogether that year (as in geese, Newton 1977). Birds that breed late may start to moult while they are still feeding young (as in Bullfinch Pyrrhula pyrrhula, Newton 1966), or birds that moult late may start migrating before they finish feather growth (as in Greater Whitethroat Sylvia communis, Hall & Fransson 2001). Similarly, Common Swifts Apus apus normally moult in their winter quarters, but when held up in Finnish breeding areas by bad weather, they started moulting in October at the normal time (Kolunen & Peiponen 1991). This again implies the influence of an underlying endogenous rhythm, which allows some delay but limits each event to an appropriate season (its 'time window'). Moreover, as is evident from both wild and captive birds, it is not necessary for a bird to complete one event in the annual cycle before it can start the next. Birds that do not complete a breeding cycle may nevertheless moult and reach migration condition at an appropriate time (although somewhat earlier than usual). The relationships between successive events in the annual cycle are explored further in Chapter 12.

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