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This question needs a very careful answer, as the impact of global warming is not the same the world over. In fact, climate change has very different effects on different regions. At our latitude, and in temperate zones, increases in temperature are very noticeable. Towards the North Pole (and, it seems, in just some areas of the South Pole), the impact is even stronger. However, these increases are neither exclusively nor even predominantly concentrated on the breeding season. Nonetheless, in places where the warming (also) happens in spring, we can expect early plant growth and a before-schedule arrival of the insects and other invertebrates that almost all species of birds rely on as food for their chicks. When this happens, birds can begin breeding sooner and – further north and at high altitudes – extend what is usually a very short breeding season. This increases the narrow window normally available for courtship displays, incubation, fledging, moulting, dispersal, migration, etc. It might even give the birds time to have a second brood or lay replacement eggs, or allow them to extend their breeding grounds to areas that weren’t suitable before. The changes might also affect the size of their clutch, depending on whether the birds can quickly synchronise their breeding habits with the new conditions.
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