Ultraviolet colour variation influences blue tit sex ratios

Brilliant blue and violet structural colours are common plumage ornaments i n birds, but their signalling functions are poorly understood(1). This may be because birds also communicate in ultraviolet (UV-A) wavelengths (320-40 0 nm)(2-5), invisible to humans, but a strong spectral component of many st ructural colours(6). From a wild population of blue tits-Parus caeruleus, s exually dimorphic primarily in the ultraviolet(7,8)-we report experimental evidence that females skew the sex ratio of their offspring in response to the ultraviolet plumage ornamentation of their mates. Masking male ultravio let reflectance reversed a positive correlation between reflectance and bro od sex ratio observed in control pairs, demonstrating a causal effect of ma le ultraviolet ornamentation on offspring sex ratio. Ultraviolet reflectanc e also predicted male survival to the following breeding season, suggesting that it serves as a viability indicator. When taken together with ecologic al effects (laying date, nesting area), our experiments reveal that an unex pected amount of control exists over the primary sex ratio in birds, sugges ting that chromosomal sex determination may not constrain the sex ratios of multiparous vertebrates.