Evolution of a multifunctional trait: shared effects of foraging ecology and thermoregulation on beak morphology, with consequences for song evolution

While morphological traits are often associated with multiple functions, it remains unclear how evolution balances the selective effects of different functions. Birds' beaks function in foraging, but also in thermoregulating and singing, among other behaviours. Studies of beak evolution abound, however most focus on a single function. Thus, we quantified relative contributions of different functions over an evolutionary time scale. We measured beak shape using geometric morphometrics and compared this trait to foraging behaviour, climatic variables, and song characteristics in a phylogenetic comparative study of an Australasian radiation of songbirds (Meliphagidae). We found that both climate and foraging behaviour were significantly correlated with beak shape and size. However, foraging ecology had a greater effect on shape, and climate had a greater effect on size. We also found that evolutionary changes in beak morphology had significant consequences for vocal performance: species with elongate-shaped beaks sang at higher frequencies, while species with large beaks sang at a slower pace. The evolution of the avian beak exemplifies how morphological traits can represent an evolutionary compromise among functions, and suggest that specialization along any functional axis may increase ecological divergence or reproductive isolation along others.