Variation in bird clutch and egg sizes across geographical gradients are issues often debated among ecologists, where latitudinal cline is a central question in several discussions. It is understood that these patterns are primarily driven by climatic conditions, where latitude acts as a proxy. Here, we achieve a robust dataset that covers a large territorial extent to test the hypothesis clutch and egg size will show measurable variation based on environmental gradients. We predict that these traits will: a) increase with increasing latitude, b) be larger in more seasonal climates (Köppen-Geiger), c) increase in sites experiencing cooler winters, and d) increase in sites with warmer and wetter long-term climatic conditions. We considered the geographically diverse Tyrannus genus (kingbirds) and collected breeding data from 35 scientific egg collections. After several data control processes, including spatial, temporal and taxonomic checking, we analysed the relationship between kingbird’s clutch and egg sizes with different climatic conditions. The analyses of 1358 clutches and 4750 eggs collected during 158 years (1858-2016) confirmed that Kingbirds’ clutch and egg sizes increase towards the poles. Both breeding traits varied according to main climates, regional sub-climates, and local temperature and precipitation conditions. More seasonal regions had the largest clutches, but sites with colder winters did not have the largest clutches. Tyrannus egg size increased in environments with less extreme dry periods. The significant relationship between larger eggs with sites with lower temperatures provides insights about the increase of egg size with latitude. Our findings suggest a robust correlation of residual variation in breeding traits with climatic conditions at both regional and local levels. Highly locally adapted species using climatic conditions as cues should also respond to interannual weather variations. The insights provided in this work can assist in understanding how species will cope with future climate scenarios.