Anthropogenic climate change is predicted to be a major cause of extinctions in the future. But how exactly will this happen? Using two long-term datasets on common birds from the United Kingdom and the Netherlands, I show that warmer temperatures are resulting in earlier egg laying and lower body mass. Many studies assume that such climate sensitivities will have important population – and thus conservation – consequences, but this has rarely been explicitly tested. I use a hierarchical framework to explain and test when impacts of climate on phenology and physiology affect reproduction and in turn population dynamics. Although changes in laying dates with warmer temperatures were associated with improved reproductive success, this has no apparent effect on population trends. Decreased body mass from warmer temperatures were associated with strong positive and negative effects on reproduction in different populations, where reduced reproduction led to declining population sizes. However, there were no overall changes in population sizes for species due to high intraspecific variation essentially ‘buffering’ the overall impact. Consequently, although species show strong changes in phenology and physiology, this did not appear to influence the risk of extinction.