Warming temperatures drive at least half of the magnitude of long-term trait changes in European birds.

SignificanceClimate change is impacting wild populations, but its relative importance compared to other causes of change is still unclear. Many studies assume that changes in traits primarily reflect effects of climate change, but this assumption is rarely tested. We show that in European birds global warming was likely the single most important contributor to temporal trends in laying date, body condition, and offspring number. However, nontemperature factors were also important and acted in the same direction, implying that attributing temporal trends solely to rising temperatures overestimates the impact of climate warming. Differences among species in the amount of trait change were predominantly determined by these nontemperature effects, suggesting that species differences are not due to variation in sensitivity to temperature.