A 47% decline in wind speed shapes phenology and selection on mismatching in Tree Swallows across a quarter century

Studies of the effects of climate change on birds typically focus on how temperature affects the average timing of breeding, often neglecting other climate variables such as wind or precipitation and their ultimate effects on reproductive success. Here, we conducted a comprehensive analysis of Tree Swallow (Tachycineta bicolor) breeding ecology in southeastern Wisconsin, USA, over a 26 year period (1997-2023). During this period there was no significant change in mean annual laying date or air temperature. Wind speed was the only weather variable that changed, slowing by 47% during the laying and hatching periods. This decline in wind speed increased food availability (Nematoceran flies), leading to larger clutches, heavier nestlings, and greater fledging success over time. It also allowed later-nesting birds to advance their laying dates. In contrast, earlier-laying birds did not advance their laying date. A changepoint analysis revealed consistently lower food availability and colder temperatures at the end of April than in early May when laying starts. This change in breeding conditions has not shifted during the study and likely acts as a constraint against earlier breeding. Over the years, the distribution of laying dates has been compressed against this early-season wall, which has resulted in a dramatic decrease in the variance of laying dates, but relatively little change in the population mean. Slowing wind speed has also relaxed selection against individuals that mistime egg laying relative to peak food abundance (i.e., phenological mismatching). Mismatching exerted relatively weak effects on reproductive success, which was driven mainly by food and weather conditions early in the season, prior to and during laying. This pattern contrasts with other Tree Swallow populations in which reproductive success is primarily influenced by temperature, particularly cold snaps, later in the season during the nestling period. Our study demonstrates that more comprehensive analyses of the effects of climate change are needed because temperature, food and wind speed can interact to produce different effects at the beginning and end of the breeding season, and this can reshape long-term trends in phenology and selection, even before eggs are laid.