Life-History and Ecological Correlates of Egg and Clutch Mass Variation in Sympatric Bird Species at High Altitude

SIMPLE SUMMARY: This study intended to understand the variation in egg and clutch mass among coexisting bird species at high altitudes. We investigated several life-history or ecological factors that could explain this variation. The results show that both egg and clutch mass were related to body mass across species. Contrary to the hypothesis of a trade-off between egg mass and clutch size, egg mass variation was not explained by clutch size when accounting for allometric effects. Clutch mass was found to be positively associated with parental care and negatively associated with predation rate. When considering clutch size and egg mass together, clutch size was significantly correlated with parental care, predation rate, and lifespan, while egg mass was only associated with development period. These results support the idea that reduced clutch size or mass is linked to a higher risk of predation, reduced parental care, and longer adult lifespan. Our findings suggest that clutch size has a greater impact on these factors compared to egg mass, possibly because smaller clutches represent a more significant reduction in energetic investment. This study has increased our understanding of how different factors influence the size of eggs and clutches in coexisting high-altitude bird species. ABSTRACT: The variation in egg and clutch mass in sympatric species at high altitudes is poorly understood, and the potential causes of variation are rarely investigated. This study aimed to describe the interspecific variation in avian egg and clutch mass among 22 sympatric bird species at an altitude of 3430 m. Our objective was to reduce potential confounding effects of biotic/abiotic factors and investigated hypotheses concerning allometry, clutch size, parental care, nest predation, and lifespan as possible correlates and explanations for the observed variation. Our findings indicated that both egg and clutch mass evolve with body mass across species. We found that egg mass variation was not explained by clutch size when controlling for allometric effects, which contrasts the “egg mass vs. clutch size trade-off” hypothesis. Additionally, we found that clutch mass was positively associated with parental care (reflected by development period) but negatively associated with predation rate. By substituting egg mass and clutch size into the models, we found that clutch size was significantly correlated with parental care, predation rate, and lifespan, while egg mass was only significantly associated with development period. Overall, these findings support life-history theories suggesting that reduced clutch size or mass is associated with a higher risk of predation, reduced parental care, but longer adult lifespan. Interestingly, our results indicate that clutch size has a greater influence on these factors compared to egg mass. This could be attributed to the fact that smaller clutch sizes result in a more notable decrease in energetic allocation, as they require a reduced effort in terms of offspring production, incubation, and feeding, as opposed to solely reducing egg size. These findings contribute to the growing evidence that life-history and ecological traits correlate with egg and clutch mass variation in sympatric species. However, further research is needed to explore the potential evolutionary causes underlying these patterns.