Eye size and the time of arrival of birds at garden feeding stations in winter
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SHORT NOTE
Eye size and the time of arrival of birds at garden feeding stations in winter Nancy Ockendon Æ Sarah E. Davis Æ Mike P. Toms Æ Sarah Mukherjee
Received: 2 January 2009 / Revised: 27 February 2009 / Accepted: 17 April 2009 / Published online: 12 May 2009 Ó Dt. Ornithologen-Gesellschaft e.V. 2009
Abstract A nationwide volunteer survey was conducted to investigate the time at which common species of birds arrived at garden feeders in the morning during winter, and over 5,800 participants submitted observations. We examined the relationship between species’ eye size and their time of arrival at feeders, in order to investigate whether the time at which foraging was initiated was constrained by visual capability. There was a negative correlation between eye size and time of arrival at garden feeders across species, and this relationship remained significant when body mass was taken into account. This suggests that the time at which garden birds begin to forage on winter mornings may be limited by their visual capability at low light intensities. Keywords Feeding behaviour Eye size Visual sensitivity Garden birds
Introduction Different species of bird begin to forage at different times in the morning and one of the most likely explanations for this is that species vary in how well they can see under low light intensities. Birds might be expected to start foraging
Communicated by F. Bairlein. N. Ockendon (&) S. E. Davis M. P. Toms The British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK e-mail: [email protected] S. Mukherjee BBC Broadcasting House, Portland Place, London W1A 1AA, UK
as soon as they emerge from their roosts because of the loss of energetic reserves overnight, particularly for small bird species and during cold weather (e.g. Cresswell 1998; Lehikoinen 1987; Meijer et al. 1994). If the onset of activities is light-limited, and foraging is an important activity in mornings during winter, we predict that the time when a bird starts feeding will reflect its visual capability under low light levels. For example, European Robins Erithacus rubecula in artificially lit aviaries started singing and feeding significantly earlier than the same birds without artificial light, suggesting that the timing of onset of foraging is light-limited (Thomas et al. 2004). The efficiency of foraging in Great Tits Parus major has also been shown to be related to light levels, with searching efficiency limited by light intensity for up to 2 h after the time that birds become active in the morning (Kacelnik 1979). Reduced foraging efficiency in low light intensities could be caused by difficulties in detecting and handling prey items (Kacelnik 1979), or because prey are not yet active in the case of insectivorous species (Avery and Krebs 1984). Therefore, the diet of a species will influence the ease with which prey items can be found and handled under low light levels. As well as the constraints on foraging efficiency imposed by low light levels, other costs and benefits will affe
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