Ultraviolet sensitivity and colour vision in raptor foraging

Abstract

SUMMARY 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.

Assessing Sexual Dicromatism: The Importance of Proper Parameterization in Tetrachromatic Visual Models

“…In our study, differences in light environment had relatively small but non-negligible effects on the mean dichromatism scores and ranks in Galliformes. The JNDs of some of the patches changed by more than 25%, differences considerably larger than those modelled by Lind et al [22]. Our largest observed differences included the daylight 65 (D65) illuminant for both UVS and VS eye types (Table 4).…”

“…The importance of this parameter was further demonstrated in behavioral trials which showed that the absence of UV wavelengths reduced the foraging efficiency of three-spined sticklebacks ( Gasterosteus aculeatus Linnaeus; [43]). In contrast, the light environment has almost no effect on the discriminability of vole urine against the vegetation background (~4% JNDs) suggesting little influence on the behavior of foraging raptors [22]. The results from these studies, and others (e.g., [41, 42, 81]), indicate that the importance of the light environment in visual models is context dependent.…”

“…Recent work has demonstrated that phylogeny and eye type can be used to estimate the approximate high-pass cut-off values of the ocular media in birds [22], but that variability within UVS and VS eye types, and in certain groups, is very high (e.g., waterbirds [21]). The absorption curves of ocular media in birds are all very similar and can be well approximated (function from [24] reproduced in S2 File) when the wavelength at 50% transmission (T50) is known.…”

“…While molecular methods and microspectrophotometry are increasingly used to determine the physical properties of visual systems (e.g., [21, 22]), complete characterizations are available for relatively few species. Furthermore, groups of closely related species have rarely been compared (see [23] for an exception).…”

“…Studies comparing dicromats to trichromats [37, 38] and trichromats to tetrachromats [39, 40] have quantified how these visual systems differ in their ability to tell apart patches with different spectral properties. Others have tested the effect of different light environments [22, 41–43], photoreceptors sensitivities [22, 44], photoreceptor densities [44, 45], oil droplet characteristics [44, 46–48], ocular media [22], and receptor signal-to-noise ratio [44]. While these studies have been very informative when considered together, the use of different visual system starting points and non-standardized methods of presenting results have made it difficult to compare the relative effect of each parameter within a single context.…”