Climate change influences animal population dynamics via effects on survival or reproduction. However, attributing changes in mortality to specific climate variables is challenging as it is often not known exactly when individuals died within a year. Here, we investigated climate effects on adult mortality in Australian superb fairy-wrens (Malurus cyaneus). Over a 27-year period, mortality outside the breeding season nearly doubled. This non-breeding season mortality increased with both lower minimum and higher maximum temperatures in winter, and with higher heatwave intensity in the previous summer. Fine-scale analysis showed that higher mortality in a given week was associated with higher maxima two weeks prior, as well as with lower minima in the current fortnight. Increases in summer heatwaves and in winter maximum temperatures collectively explained 62.6% of the increase in mortality over time. Warming climate in both summer and winter can thus adversely affect survival, with potentially substantial population consequences.