The main message to emerge from studies of the annual cycles of birds concerns the flexibility of such cycles and the enormous variation between species in the sequence of different events through the year, their duration and extent of overlap. This flexibility is manifest mostly by differences between species, but also between geographical populations of the same species, between sexes, and to
Table 11.2 Likely selection pressures and trade-offs influencing the timing of various events in the annual cycles of birds
Arrive early in breeding area
Arrive late in breeding area
Breed as soon as possible after arrival
Curtail period of parental care Finish breeding early in favourable season
Finish breeding late in favourable season.
Moult in breeding area
Moult in wintering area
Overlap between breeding and moult
Depart earlier in autumn
Depart later in autumn
Migrate in short flights with minimal fat deposition Migrate in long flights with high fat deposition
Obtain good nesting territory and have a longer breeding season
Decreased risk of adverse weather, and hence loss of body condition and starvation after arrival
Often better able to exploit the peak food supply. Fit in additional (or repeat) breeding attempts
May allow adults time for further breeding attempt or earlier moult or migration Moult at optimal time, with good quality feathers
More nesting attempts within season, or longer period of parental care.
Take advantage of summer food supply. Migrate with new feathers
Allows longer breeding season in summering area; more time available for moult than in breeding areas, so requires less extra food per day Saves time in the favourable season
Avoid unseasonably cold weather; reach stopover sites before food depleted, and obtain best territories in winter quarters
Make maximum use of potential breeding-moulting season
Saves time spent on fattening before starting migration, reduces predation risk, and saves fuel transportation costs Shortens journey time, and allows crossing of extensive unfavourable areas
Increased risk of adverse weather after arrival, loss of body condition or starvation
Relegation to poor territory, late start to breeding, and shorter breeding season
Increased risk of adverse weather during first nesting attempt, with higher costs to parents, especially laying females May jeopardise survival of young
Fewer nesting attempts within season, or shorter period of parental care
Moult overlapped with other activities or compressed to less suitable time, producing poor-quality feathers May occupy time that could be used for extended parental care, or for raising an extra brood, or delay departure from breeding areas
Adults must migrate with old worn feathers. Food may be scarcer than in summer, and winter movements could be hampered
Increases the daily food needs, and could reduce fitness and feather quality of parents and young
Fail to make maximum use of the potential breeding-moulting season
Risk of cold weather and reduced food supplies, which may prevent departure or cause starvation. May be relegated to poor territory in winter quarters Lengthens journey time. Suitable refuelling places may be far apart
Long periods of fattening, with high predation risk, and high fuel transportation costs
Partly after Mead (1983).
a lesser extent even between individuals in the same population. Interruptions can occur in moult or migration to permit another activity to occur, and a small number of species, with short breeding cycles, can make long movements between successive broods within a single breeding season (Chapter 16). In general, large bird species take longer to breed and moult than small ones, and show greater overlap between these activities. The timing of migration clearly cannot have evolved independently of breeding and moult, but only in concert with them. Conflicting pressures operate in the timing of any event (Table 11.2), giving different optimal solutions in different circumstances. If they had nothing else to do, the best time for most birds to migrate would be in early summer, when food reaches its peak abundance. The fact that adult birds do not migrate then, but breed instead, is evidence that breeding takes precedence.
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