No abstract

The function of avian ultraviolet (UV) vision is only just beginning to be understood. One plausible hypothesis is that UV vision enhances the foraging ability of birds. To test this, we carried out behavioural experiments using wild-caught blue tits foraging for cabbage moth and winter moth caterpillars on natural and arti¢cial backgrounds. The light environment in our experiments was manipulated using either UVblocking or UV-transmitting ¢lters. We found that the blue tits tended to ¢nd the ¢rst prey item (out of four) more quickly when UV cues were present. This suggests that UV vision o¡ers bene¢ts to birds when searching for cryptic prey, despite the prey and backgrounds re£ecting relatively little UV. Although there was no direct e¡ect of UVon the time taken to ¢nd all four prey items in a trial, search performance in the absence of UV wavelengths tended to increase over the course of an experiment. This may re£ect changes in the search tactics of the birds. To our knowledge, these are the ¢rst data to suggest that birds use UVcues to detect cryptic insect prey, and have implications for our understanding of protective coloration.

Ultraviolet (UV) vision is well documented for many species of vertebrates. UV cues are known to be used in foraging, navigation and in mate choice. We conducted a series of behavioral experiments to investigate the role of UV perception in mate choice in both female and male guppies (Poecilia reticulata). In our experiments the visual appearance of potential mates was manipulated using either UV transmitting (UVϩ) or UV blocking (UVϪ) filters. Female guppies significantly preferred UVϩ males. Male guppies tended to prefer UVϪ females, but their preferences were marginally nonsignificant. Further experiments investigating the role of luminance, indicate that UV wavelengths are probably being used for color discrimination rather than for detecting differences in brightness. These experiments raise the possibility that UV is used in mate assessment in different ways by male and female guppies. This may reflect the different strategies that the two sexes have in order to maximize reproductive success. To our knowledge, these are the first data showing that UV is used by any fish species in mate selection.

Despite the fact that the vast majority of natural prey items are dispersed in a non-random manner, few studies of frequency-dependent selective predation have explicitly examined the effect of prey dispersion on selectivity. We examined the effect of prey dispersion on the direction and strength of frequency-dependent selection by wild birds feeding on artificial prey (green or brown pastry baits). In a series of four experimental manipulations, we tested for the occurrence of frequency dependence with two different dispersion patterns (random or clumped). Manipulations were carried out at one of two absolute densities (25 prey m or 100 prey m), and were repeated at different sites in Southampton, England and Aljarafe, Spain. Our results suggest that prey dispersion has no effect on either frequency-dependent or -independent preferences. One possible explanation for this is that the birds had virtually complete information about prey frequencies at the relatively high densities used and based their preferences on their overall perception of availability. However, we cannot rule out the possibility that dispersion may influence selectivity when prey are cryptic or available at lower absolute densities. Although there was no effect of dispersion, frequency-dependent selection was, overall, significantly anti-apostatic (i.e. rare baits were more preferred than common baits). This anti-apostatic effect was stronger in Southampton than Aljarafe and stronger at 100 prey m than 25 prey m. The differences in the strength of selection between the two locations was interpreted in terms of (i) whether the avian predators foraged in flocks or not, and (ii) the number of different species present (and, consequently, the variation in preference among individual birds).

Video playback potentially allows the presentation, manipulation, and replication of realistic moving visual stimuli, in a way that is impossible with real animals or static dummies, and difficult even with mechanical models. However, there are special problems attached to the use of this technology; this article concentrates on the problem of accurate colour rendition. Video and television simulate the colour of objects rather than reproduce the spectrum of light that they naturally emit, transmit, or reflect. This simulation is achieved by using relatively narrow waveband light to stimulate the cone cells in the retina in a similar pattern to that produced by the natural object. However, species differ in the spectral tuning of their photoreceptors, so a faithful colour rendition for a human is unlikely to be achieved for another species. This problem is discussed with special reference to birds, a taxon renown for its colourfulness and frequent use in behavioural experiments but which has a very different colour vision from that of humans. We stress that the major pitfalls that can arise when using video playback with avian subjects can also occur in 'normal' behavioural experiments. However, the problems of faithful colour rendition are particularly severe with video, and the major benefits that the technology brings will only be realised under a limited range of circumstances, with careful validation experiments.