While morphological traits are often associated with multiple functions, it remains unclear how evolution balances the selective effects of different functions. Birds' beaks function not only in foraging but also in thermoregulating and singing, among other behaviours. Studies of beak evolution abound, however, most focus on a single function. Hence, we quantified relative contributions of different functions over an evolutionary timescale. We measured beak shape using geometric morphometrics and compared this trait with 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 the beak shape and size. However, foraging ecology had a greater effect on shape, and climate had a nearly equal 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 be an evolutionary compromise among functions, and suggests that specialization along any functional axis may increase ecological divergence or reproductive isolation along others.

Biodiversity and Biocomplexity Unit Okinawa Institute of Science and Technology Graduate University Onna son Okinawa Japan

Biodiversity and Biocomplexity Unit Okinawa Institute of Science and Technology Graduate University Onna son Okinawa Japan Department of Zoology and Laboratory of Ornithology Faculty of Science Palacký University Olomouc Czech Republic

Biodiversity and Biocomplexity Unit Okinawa Institute of Science and Technology Graduate University Onna son Okinawa Japan Graduate School of Information Science and Technology Hokkaido University Sapporo Hokkaido Japan

Cornell Laboratory of Ornithology Cornell University Ithaca NY USA

Department of Zoology and Laboratory of Ornithology Faculty of Science Palacký University Olomouc Czech Republic Department of Ecology Faculty of Science Charles University Prague Czech Republic

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