Climate change and constraints on breeding

Although climate change apparently affects the breeding patterns of many animals1,2,3, the wider implications for breeding success are unclear. Here we describe an energy trade-off between reproduction and maintenance that occurs during cold weather in great tits (Parus major L.), pointing to a thermal constraint on the timing of egg laying. Our observations indicate that the fine-scale pattern of climate change could be critical to the reproduction of some species and underlies previously unexplained variation in the breeding success of other temperate birds.