Raptors have excellent vision, yet it is unclear how they use colour information. It has been suggested that raptors use ultraviolet (UV) reflections from vole urine to find good hunting grounds. In contrast, UV plumage colours in songbirds such as blue tits are assumed to be ‘hidden’ communication signals, inconspicuous to raptors. This ambiguity results from a lack of knowledge about raptor ocular media transmittance, which sets the limit for UV sensitivity. We measured ocular media transmittance in common buzzards (Buteo buteo), sparrowhawks (Accipiter nisus), red kites (Milvus milvus) and kestrels (Falco tinnunculus) so that, for the first time, raptor UV sensitivity can be fully described. With this information, and new measurements of vole urine reflectance, we show that (i) vole urine is unlikely to provide a reliable visual signal to hunting raptors and (ii) blue tit plumage colours are more contrasting to blue tits than to sparrowhawks because of UV reflectance. However, as the difference between blue tit and sparrowhawk vision is subtle, we suggest that behavioural data are needed to fully resolve this issue. UV cues are of little or no importance to raptors in both vole and songbird interactions and the role of colour vision in raptor foraging remains unclear.
All authors contributed equally to all parts of the article.
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/216/10/1819/DC1
No competing interests declared.
Funding for this research was provided by the Swedish Research Council [2009-5683 to A.K.], Human Frontier Science Program [to A.K.] and The Royal Swedish Academy of Sciences [to O.L.].
LIST OF SYMBOLS AND ABBREVIATIONS
- illuminating spectrum
- just noticeable difference
- long-wavelength-sensitive cone
- visual pigment of the LWS cone
- medium-wavelength-sensitive cone
- transmittance of ocular media
- transmittance of oil droplets
- quantum catch of photoreceptor
- spectral sensitivity of visual pigments
- spectral sensitivity of cone photoreceptors
- visual pigment of the MWS cone
- reflectance spectrum
- short-wavelength-sensitive cone
- visual pigment of the UVS/VS cone
- visual pigment of the SWS cone
- ultraviolet light
- ultraviolet- or violet-sensitive cone
- standard deviation of noise in an individual cone
- contrast in receptors
- number of cone types within a receptive field
- wavelength of 50% transmittance (of ocular media)
- cut-off wavelength (of oil droplet)
- peak wavelength (of visual pigments or cones)
- wavelength of 50% transmittance (of oil droplet)
- Weber fraction
- © 2013. Published by The Company of Biologists Ltd