Egg Size Variation in the Southern House Wren Troglodytes musculus bonariae

Egg size in birds is a crucial fitness trait with high individual consistency. While the causes of this consistency are not fully understood, different sources of egg size variation in birds have been proposed. This study investigated the repeatability of egg size in two south temperate populations of Southern House Wrens Troglodytes musculus bonariae and examined the influence of climatic factors and female traits on egg size. During the 2008–2011 seasons, we measured the length and width of eggs from 296 House Wren clutches and estimated their volume. Our analyses revealed high repeatability of egg size of southern House Wrens within and across breeding seasons, with no differences between study sites. This high repeatability suggests a substantial influence of consistent environmental factors as no evidence of significant heritability was found. However, environmental factors and female traits did contribute to a small portion of the egg size variation. These findings challenge the notion that high additive genetic variance drives the high repeatability of egg size in Southern House Wrens. Instead, our results support the hypothesis that individual female traits play a key role in determining egg size consistency in the species.