Ultraviolet colour variation influences blue tit sex ratios

Brilliant blue and violet structural colours are common plumage ornaments in birds, but their signalling functions are poorly understood(1). This may be because birds also communicate in ultraviolet (UV-A) wavelengths (320-400 nm)(2-5), invisible to humans, but a strong spectral component of many structural colours(6). From a wild population of blue tits-Parus caeruleus, sexually 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 ultraviolet reflectance reversed a positive correlation between reflectance and brood sex ratio observed in control pairs, demonstrating a causal effect of male ultraviolet ornamentation on offspring sex ratio. Ultraviolet reflectance also predicted male survival to the following breeding season, suggesting that it serves as a viability indicator. When taken together with ecological effects (laying date, nesting area), our experiments reveal that an unexpected amount of control exists over the primary sex ratio in birds, suggesting that chromosomal sex determination may not constrain the sex ratios of multiparous vertebrates.