Birds commonly adjust timing of breeding to spring temperatures. We examine 40 years of data from the Owyhee Mountains of southern Idaho to determine whether mean lay date of Mountain Bluebirds (Sialia currucoides) is associated with May temperatures and whether an increase in those temperatures has advanced the breeding phenology of this species. Average May temperatures increased by 2.20°C (CI = 0.58, 3.82) from 1980 to 2019, were negatively correlated with the mean lay date of Mountain Bluebirds, and thus were associated with an advancement in breeding phenology of 2.09 (CI = 1.16, 3.03) days per decade. Our study demonstrates the importance of long-term monitoring to detect effects of climate change on bird phenology.

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