Beak height and nest structure shape avian bite force

Bite force in vertebrates has a role in various functions, including predation, defense, and competition. However, systematic comparative studies on avian bite force are lacking and there are multiple inconsistent morphological indicators associated with bite force. Additionally, several hypotheses (e.g., diet hypothesis, combat hypothesis and nest complex hypothesis) about potential evolutionary drivers of bite force need validation. Here, we measured the bite force and morphological indicators of 175 bird species, which were then subjected to correlation analysis. The results revealed that beak morphology (especially beak depth) correlated more strongly with bite force than the overall shape of head excluding the beak. In addition, we collected bite force data from existing literature for numerous bird species and finally collated the bite force measurements of 223 species. Utilizing the evolutionary drivers of bite force (e.g., body mass, diet, competition intensity, nest structure) proposed by different hypotheses, we constructed a phylogenetic generalized least squares model to verify the factors influencing the bite force of birds. The results indicated that, in addition to the scale effect represented by body mass, birds with more complex nest structures had a greater bite force. Consistent with the nest complex hypothesis, the results indicate that nest-building behavior is a significant driver in the evolution of bite force in birds. Our comparative study of numerous bird species offers novel insights into the morphological correlations and functional evolutionary drivers of bite force.