Multiple Spatial and Climatic Conditions Affect Kingbird Flycatchers' Clutch and Egg Sizes

Variation in avian clutch and egg sizes across geographical gradients has long been debated, particularly regarding the mechanisms underlying latitudinal clines in reproductive investment. These patterns are primarily driven by climatic conditions, with latitude acting as a proxy for environmental gradients. Here, we assembled a robust dataset of eggs and clutches of eight species of kingbird flycatchers (Tyrannus) covering a broad geographic range to test the hypothesis that clutch and egg size vary systematically along environmental gradients, and predict that egg and clutch size would (a) increase with latitude, (b) be larger in more seasonal climate categories, (c) be larger at sites with cooler winters, and (d) be larger at sites with warmer and wetter long-term climatic conditions. We analysed clutch and egg data from 35 scientific egg collections representing the geographically widespread genus Tyrannus. After applying spatial, temporal and taxonomic data quality controls, we analysed the relationships between clutch and egg size and climatic variables. The analyses of 1332 clutches and 4680 eggs of eight species collected during 158 years (1858-2016) showed that clutch and egg sizes increased toward higher latitudes. Both traits varied according to major climate types, regional subclimates, and local temperature and precipitation conditions. More seasonal regions had the largest clutches, although sites with colder winters did not have the largest clutches. Egg size was larger at sites with less severe dry periods, and larger eggs were also associated with lower temperatures. Overall, residual variation in clutch and egg size is strongly associated with climatic conditions at both regional and local scales. These findings contribute to understanding how avian reproductive traits respond to climatic gradients and may help anticipate how species will respond to future climate scenarios.