Variations In Annual Cycles

Most parts of the world are seasonal, in terms of daylength, warmth or precipitation, and hence also in terms of biological productivity. In particular regions, food supplies are therefore more plentiful at certain times of year than at others. At high latitudes the favourable season is relatively short, lasting only about one fourth of the whole year. Nutritionally, the most demanding event in the annual calendar of birds is reproduction, which normally overlaps the season of most abundant food supplies (Lack 1954). The other events fit around this. Migration is timed so that birds can be present on their breeding areas at least for long enough to breed successfully, taking advantage of the favourable season, but are absent for the unfavourable season when their survival chances would be much lower there. The first individuals normally arrive in their breeding areas as soon as conditions become suitable. In some species, especially at high latitudes, this is only a few days before nest-building and egg-laying begin, but in others up to several weeks might elapse between arrival and egg-laying (Chapter 14). Post-breeding migration is timed so that birds leave their breeding areas before their chances of survival there would become precarious.

Birds are further constrained in migration timing by the need for an integrated annual cycle, minimising the overlap with breeding and moult. All three processes need additional food intake, so for nutritional reasons these events are best separated as much as possible. In addition, while breeding, birds must remain in a fixed locality, at least until their young are well grown and mobile, which precludes simultaneous migration. Likewise, moulting birds might have missing and part-grown flight feathers, which could hamper migration (Hedenstrom 2004). Hence, for both extrinsic and intrinsic reasons, migration is normally confined to particular parts of the annual cycle, and in most species is separated from breeding and moult (for exceptions see below). Before they reach breeding age, immature birds of some species have more freedom in the times that they moult and migrate, and often do so at somewhat different times of year from adults, which are constrained by breeding.

The eight most common sequences of annual cycle events described among migratory birds are shown in Figure 11.1, and others could be added to this list. They reveal the various ways in which birds arrange the major events in their annual cycles to suit the circumstances in which they live and to take maximum advantage of breeding areas offering only a short favourable season. Comparing species, moult is much more variable in timing than is breeding or migration, probably because its

Sequence 2

Sequence 1

Pre-breeding migration

Breeding

Breeding

Pre-breeding migration

Breeding

Breeding

Post-breeding migration, phase 1

Pre-breeding migration

Moult

Pre-breeding migration

Post-breeding migration, phase 1

Post-breeding migration

Post-breeding migration

Moult

Post-breeding migration phase 2

Post-breeding migration phase 2

Moult

Sequence 4

Sequence 3

Breeding

Breeding

Post-breeding migration

Breeding

Breeding

Moult phase 1

Pre-breeding migration

Post-breeding migration

Pre-breeding migration

Moult phase 1

Pre-breeding migration

Pre-breeding migration

Moult

Moult

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