Bergmann's Rule notes a correlation between animal size reduction and geographical temperature increase in three dimensions. This study examines bird size change in the context of temperature change in a fourth dimension: time. The body size of six passerine bird species found year-round in Connecticut was measured using museum specimens collected between 1874 and 2009, during which time mean temperature in Connecticut increased by 0.94 °C (SD = 0.71). Mean wing length significantly decreased from a pre-1955 period (1874–1952) to a post-1955 period (1958–2010) for all species combined (P < 0.0025) and for three of the six species (P < 0.025), suggesting that some Connecticut passerines exhibit an evolved size decrease since 1874. This study joins a growing body of research suggesting a causal relationship between climate change and animal morphological change, and it demonstrates the importance of museum specimens in documenting such global trends.