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.
However, global warming actually often has a bigger impact on the autumn and winter months. Where this leads to milder winters, species with high numbers of resident birds will benefit as they can begin breeding very early and on a large scale before they start encountering competition from the migratory birds that will eventually arrive.
High rainfall and freak weather impact almost all bird species by drastically reducing their breeding success. It’s not possible to explain all these phenomena in detail here, but hopefully this information goes some way to showing just how complex the effects of changing weather conditions can be for birds as they breed. In other climate zones (the subtropics, the tropics, the arctic, etc.) and other habitats (semi-desert, deserts, permafrost regions, etc.), the conditions are entirely different again, and species must adapt to a totally different environment if they want to survive.
We still don’t know how capable birds will be of adapting to the changes in the long term, and their success will depend heavily on how exactly climate change ends up affecting vegetation and water. What is certain is that different species will react to the changes in very different ways. While some are sure to go into rapid decline as they find that their traditional territories are no longer suitable, others will benefit a great deal from the new conditions. It is argued that specialist species and those that cannot tolerate much variation in environmental factors will struggle with climate change, while the generalists will benefit. But even that is very likely to vary from one large region to the next.
This question was answered by Dr Hans-Günther Bauer of the Max Planck Institute for Ornithology in Radolfzell-Möggingen.