The benefits of early arrival in breeding and wintering areas could lead to the whole breeding cycle becoming earlier and earlier, if it were not environmentally constrained in some way. The brake on this process occurs in spring, when birds cannot occupy breeding areas and start nesting until conditions become suitable. In addition, multi-brooded species gain from remaining in breeding areas later than otherwise, if they could thereby raise another brood, even though this may delay to some extent their departure to wintering areas. Again the birds face a trade-off between the advantages of extra reproduction and the disadvantages of a late migration, when food supplies are likely to be depleted, and when better habitat in wintering areas is likely to be already occupied. The advantages of early arrival in wintering areas are perhaps most obvious in populations in which individuals occupy the same areas throughout a winter. Yet many migratory bird species occupy different areas in different winters, or move around within a winter, and it is not yet clear whether in these circumstances early arrival could carry any advantage.
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