Ultraviolet colour vision and ornamentation in bluethroats

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Abstract
Many birds see in the ultraviolet (300–400 nm), but there is limited evidence for colour communication (signalling by spectral shape independently of brightness) in this ‘hidden’ waveband. Such data are critical for the understanding of extravagant plumage colours, some of which show considerable UV reflectance. We investigated UV colour vision in female social responses to the male UV/violet ornament in bluethroats, Luscinia s. svecica. In an outdoor aviary at the breeding grounds, 16 females were each presented with a unique pair of males of equal age. In UVR (UV reduction) males, sunblock chemicals reduced only the UV reflectance and thereby the spectral shape (colour) of the throat ornament. In NR (neutral reduction) males, an achromatic pigment in the same base solvent (preen gland fat) was used for a corresponding but uniform brightness reduction. Both colour and brightness effects were invisible to human eyes, and were monitored by spectrometry. In 13 of the 16 trials, the female associated most with the NR male, a preference that implies that UV colour vision is used in mate choice by female bluethroats. Reflectance differences between one–year–old and older males were significant only in UV, suggestive of a UV colour cue in age–related mate preferences.