Ultraviolet sensitivity in birds: consequences on foraging and mate choice

Abstract

In addition to human-visible waveband (400-700 nm) birds are sensitive to near ultraviolet (UV) waveband (UV-A; 320-400 nm). The aim of this thesis was to contribute to the understanding of importance of UV sensitivity in foraging and mate choice in birds. First I conducted two separate laboratory experiments with redwings (Turdus iliacus) and with black grouse (Tetrao tetrix) to test whether UV wavelengths are used when birds are feeding on bilberries (Vaccinium myrtillus). The colour preferences of birds varied due to different UV light levels in the light environment. Both experiments show that UV may be a good advertisement colour in light environments rich in short wavelength light. In the other three papers I investigated the importance of UV cues in intraspecies signalling in pied flycatchers (Ficedula hypoleuca) and in blue tits (Parus caeruleus). According to the field study, adults had higher UV reflectance levels than yearlings, and males had higher UV reflectance levels than females. Proportional UV reflectance of dorsal side of pied flycatchers was negatively related to male arrival time to the breeding sites. This was independent of male brownness. Thus, UV reflectance of plumage may be related to male quality in pied flycatchers. The results of the outdoor aviary mate choice experiment suggest that females use UV cues of plumage when choosing mates, at least when males are displaying simultaneously and the territory quality is controlled for. In the last study I did not find any strong relationship between human-invisible colour and parental quality. However, long-time field studies are needed to investigate this relationship. The results of this thesis show that birds can detect UV cues of light environment and UV cues of plumage. Therefore, the UV sensitivity of birds should be taken into account especially when studying plumage colours and conducting colour manipulations and conducting experiments in artificial light conditions.