Can ultraviolet cues function as aposematic signals?

The fact that birds are sensitive to ultraviolet light (UV, 320-400 nm) has been largely ignored by previous studies of aposematism. Therefore, in the present article we investigated whether great tits preferred ultraviolet-reflecting colors compared to colors without UV reflection and whether UV cues alone could function as aposematic signals. We were able to manipulate prey visibility in UV light by changing the UV reflectance of prey items as well as altering the lighting conditions. In order to perform a preference experiment we used three pairs of colors (green+UV vs. green, gray+UV vs. gray, yellow+UV vs. yellow) on a black background. The birds ate both UV types equally for all three colors. Thus, there was no avoidance of the UV-reflecting prey. Next we tested the possibility that UV reflection may affect avoidance learning. We used either green or green+UV as a signal for unpalatability. In this set-up the difference in UV did not allow avoidance learning to occur. Our experiment with great tits does not support the hypothesis that UV cues alone might work effectively as aposematic signals.