Since the time of Darwin, subspecies have been recognized as an initial stage in the evolution of species. However, the impact of dispersal on subspecies richness in birds globally has received little attention, despite dispersal being a key factor in determining the rate of evolution and playing a significant role in evolutionary divergence. Therefore, it is important to conduct a comprehensive study to address this issue. In this study, we aimed to assess the association between subspecies richness (measured by the number of subspecies) and dispersal ability (measured by the hand-wing index) on a global scale, using a dataset of over 7000 bird species. Our results revealed a negative correlation between dispersal ability and the number of subspecies, without any quadratic correlation. The wing is a crucial phenotypic trait for birds, and the concept of subspecies is important in speciation theory and conservation biology. Therefore, our findings not only enhance our understanding of the association between avian morphological traits and evolutionary divergence but also have implications for the conservation of avian species diversity.

The brain is among the most energetically costly organs in vertebrates, and thus trade-offs have been hypothesized to exert constraints on brain size evolution. The energy trade-off hypothesis (ETH) predicts that reducing the energy consumption of reproduction or other costly tissues should compensate for the cost of a large brain. Egg production in birds requires a large proportion of the total energy budget, and a clutch mass in some bird species can outweigh the body mass of the female. To date, this hypothesis has mainly been tested in mammals and ectothermic animals such as anurans and fishes. We collated data on adult brain size, body mass and egg-production traits such as clutch size, egg mass and annual broods from published studies, and conducted a phylogenetic comparative test of the interplay between egg-production investment and brain size evolution across bird species. After controlling for phylogenetic relationships and body size, we find a negative correlation between brain size and clutch size across 1395 species, which favored ETH. However, when egg mass was integrated in models, positive associations were detected between brain size and mass of eggs (via egg mass, clutch mass and annual total egg mass). Our results suggest that brain size trades off against egg-production only via certain aspects (e.g., clutch size). By contrast, a positive relationship between brain size and total egg reproduction (e.g., clutch mass and annual total egg mass) implied increased total energy budget outweighing energy allocation across bird species. Our study shows that there is no general energy trade-off between brain size and egg-reproduction investment, and suggests that brain size evolution follows mixed strategies across bird species.