Avian ultraviolet vision and frequency-dependent seed preferences.

It is well established that ultraviolet sensitivity plays an important role in the visually guided behaviour of birds. From a foraging perspective, evidence now exists that ultraviolet wavelengths are used by birds when foraging for insects, berries, seeds and mammals. Here, we present the results of two laboratory experiments that test the effect of removing (i) ultraviolet wavelengths and (ii) wavebands in the human-visible region on the frequency-dependent seed preferences of zebra finches (Taeniopygia guttata). Although the seeds and backgrounds used in our experiments reflected mainly at long wavelengths, we found that removal of ultraviolet wavelengths significantly changed the strength and direction of frequency dependence compared with full-spectrum illumination. We also found that the removal of ultraviolet wavelengths (300-400 nm) did not affect the strength of frequency dependence compared with the removal of short wavelengths (approximately 400-500 nm), medium wavelengths (approximately 500-600 nm) or long wavelengths (approximately 600-700 nm). Since frequency-dependent selection has direct consequences for the stability of prey populations and the spectral quality of ambient light is known to vary considerably with climate, time of day and local habitat geometry, our results suggest that ultraviolet wavelengths might play an important role in the dynamics of plant populations. However, we urge caution about overestimating the importance of ultraviolet wavelengths compared with wavelengths in the human-visible spectrum.