1. The optimum body mass of passerine birds typically represents a trade-off between starvation risk, which promotes fat gain, and predation pressure, which promotes fat loss to maintain manoeuvrability. Changes in ecological factors that affect either of these variables will therefore change the optimum body masses of populations of passerine birds. 2. This study sought to identify and quantify the effects of changing temperatures and predation pressures on the body masses and wing lengths of populations of passerine birds throughout Britain and Ireland over the last 50 years. 3. We analysed over 900,000 individual measurements of body mass and wing length of blue tits Cyanistes caeruleus, coal tits Periparus ater, and great tits Parus major collected by licenced bird ringers throughout Britain and Ireland from 1965 to 2017, and correlated these with publicly available temperature data and published, UK-wide data on the abundance of a key predator, the sparrowhawk Accipiter nisus. 4. We found highly significant, long-term, UK-wide decreases in winter body masses of adults and juveniles of all three species. We also found highly significant negative correlations between winter body mass and winter temperature, and between winter body mass and sparrowhawk abundance. Independent of these effects, body mass further correlated negatively with calendar year, suggesting that less well understood dynamic factors, such as supplementary feeding levels, may play a major role in determining population optimum body masses. Wing lengths of these birds also decreased, suggesting a hitherto unobserved large-scale evolutionary adjustment of wing loading to the lower body mass. 5. These findings provide crucial evidence of the ways in which species are adapting to climate change and other anthropogenic factors throughout Britain and Ireland. Such processes are likely to have widespread implications as the equilibria controlling evolutionary optima in species worldwide are upset by rapid, anthropogenic ecological changes.