The study analyzed the breeding distributions of British birds over a 20-year period, finding that the northern margins of many species have moved an average of 18.9 km further north. This northward shift occurred during a period of climatic warming, suggesting a potential causal link. The analysis controlled for overall population expansions and retractions, ensuring that the results reflect genuine range shifts rather than population changes. For southerly species, the northern margins shifted northward, while for northerly species, the southern margins shifted southward. The most parsimonious explanation for these shifts is climate change, supported by evidence that bird reproduction is influenced by warmer spring conditions, and that bird diversity is correlated with temperature. The study parallels findings in European butterflies, where northern margins have expanded more than southern margins have retracted.The study analyzed the breeding distributions of British birds over a 20-year period, finding that the northern margins of many species have moved an average of 18.9 km further north. This northward shift occurred during a period of climatic warming, suggesting a potential causal link. The analysis controlled for overall population expansions and retractions, ensuring that the results reflect genuine range shifts rather than population changes. For southerly species, the northern margins shifted northward, while for northerly species, the southern margins shifted southward. The most parsimonious explanation for these shifts is climate change, supported by evidence that bird reproduction is influenced by warmer spring conditions, and that bird diversity is correlated with temperature. The study parallels findings in European butterflies, where northern margins have expanded more than southern margins have retracted.