Many wild populations are showing changes in phenotypic traits. However, the common assumption that such changes are driven by climate change relies on three conditions: that local climate is changing over time, that trait(s) are sensitive to climate variability, and that other causal agents are not also changing. We used long-term datasets on 60 bird species to test these conditions and to quantify the contribution of warming temperatures to changes in three important life-history traits. Across species, approximately half of the magnitude of changes in traits could be attributed to a single temperature window, with averages of 50% for laying date, 40% for body condition and 56% for offspring numbers. Thus, although warming temperatures were a key driver of change, other unknown factors contributed substantially to temporal trends (typically reinforcing change). Further analyses showed that these non-temperature-driven contributions explained most of the inter-specific variation in trait changes.