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Unformatted text preview: Received 21 September 2001 Accepted 3 December 2001 Published online 25 March 2002 Eye size in birds and the timing of song at dawn Robert J. Thomas 1 , Tama s Sze kely 2 * , Innes C. Cuthill 1 , David G. C. Harper 3 , Stuart E. Newson 4 , Tim D. Frayling 5 and Paul D. Wallis 6 1 Centre for Behavioural Biology, School of Biological Sciences, University of Bristol, Bristol BS8 1UG, UK 2 Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK 3 School of Biological Sciences, University of Sussex, Falmer, East Sussex BN1 9QG, UK 4 British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK 5 British Trust for Conservation Volunteers, Cayley Hall, Shepherd Road, Gloucester GL2 5DW, UK 6 24 Jireh Court, Perrymount Road, Haywards Heath, West Sussex RH16 3BH, UK Why do different species of birds start their dawn choruses at different times? We test the hypothesis that the times at which different species start singing at dawn are related to their visual capability at low light intensities. Birds with large eyes can achieve greater pupil diameters and hence, all other things being equal, greater visual sensitivity and resolution than birds with small eyes. We estimated the maximum pupil diameter of passerine birds by measuring the diameter of the exposed eye surface, and measured the times of the first songs at dawn of songbirds present in different bird communities, and the light intensities at these times. Using phylogenetic comparative analyses, we found that songbirds with large eyes started to sing at lower light intensities (and therefore earlier) than species with smaller eyes. These relationships were stronger when differences in body size were controlled for statistically, and were consist- ent between two phylogenies and when species were treated as independent data points. Our results therefore provide robust support for the hypothesis that visual capability at low light levels influences the times at which birds start to sing at dawn. Keywords: dawn chorus; eye size; song; light intensity; songbirds 1. INTRODUCTION One of the most striking features of the dawn chorus in any bird community is that different species start to sing at different times. For example, in a Welsh woodland in spring, common redstarts Phoenicurus phoenicurus begin to sing well before the first light of dawn is detectable to the human observer (this study). As dawn breaks, European robins Erithacus rubecula , common blackbirds Turdus merula , song thrushes Turdus philomelos and pied fly- catchers Ficedula hypoleuca join the chorus in turn, fol- lowed by a succession of other species. By the time birds such as chaffinches Fringilla coelebs and blue tits Parus caeruleus begin to sing, the earliest species are already beginning to fall silent. Species may differ by as much as 100 min in the timing of their first song (see 3). Whilst there are many theories for why the dawn chorus exists (reviewed by Mace 1987; Staicer...
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