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Weir, J.T. & Wheatcroft, D. (2011), "A latitudinal gradient in rates of evolution of avian syllable diversity and song length", Proceedings of the Royal Society B: Biological Sciences, 278 (1712): 1713–1720, doi:10.1098/rspb.2010.2037, PMC3081773, PMID21068034
Weir, J.T. & Wheatcroft, D. (2011), "A latitudinal gradient in rates of evolution of avian syllable diversity and song length", Proceedings of the Royal Society B: Biological Sciences, 278 (1712): 1713–1720, doi:10.1098/rspb.2010.2037, PMC3081773, PMID21068034