Lombardo et al Tree Swallow first egg dates.xlsx

Many bird species have advanced their first egg dates in response to recent milder winters and increases in spring temperatures. Tree Swallows begin egg laying earlier in warmer springs at most, but not all, study sites. We examined whether Tree Swallows that nested in boxes in west Michigan responded to an increase in spring temperatures from 1993-2018 by advancing their first egg dates. May, but not April, mean daily temperatures got significantly warmer from 1993-2018. We found that the first egg dates, but not mean first egg dates, of both second year (SY) and after second year (ASY) females got earlier between 1993-2018. The first egg dates of SY females did not get earlier in response to warmer April or May temperatures but got earlier when there were more days in May with measurable rain. The first egg dates of ASY females got earlier with warmer April and May temperatures. Overall, the relationships between weather conditions and first egg dates of both SY and ASY females were complex and varied yearly. The climatic conditions that influence Tree Swallow first egg dates are complex because stochastic rain and wind conditions interact with air temperatures to affect the availability of the aerial insect prey of swallows thereby influencing the ability of females to begin egg laying.