Local offspring density and sex ratio affect sex allocation in the great tit

The expected fitness gain for offspring of a given sex may depend on local population sex ratio and density. Knowing the influence of such social factors on brood sex ratios may contribute considerably to the understanding of sex allocation in higher vertebrates. For 3 consecutive years, we manipulated sex ratios and densities of juvenile great tits (Parus major) within forest plots and investigated how next year's brood sex ratios were affected. For yearling female breeders, we considered the treatment of the plot of fledging and settlement and for experienced female breeders the treatment of the breeding plot. Experienced females produced more female offspring at increased plot densities and more males at reduced densities. Yearling female's brood sex ratio was not affected by the density treatment but was positively related to the natural density in the plot of fledging. The plot sex ratio treatment affected sex allocation differently for yearling and experienced females. Yearling females produced more females when fledged from male-biased plots and more males when fledged from female-biased plots. Experienced females produced more females in control plots and unbiased brood sex ratios in male- and female-biased plots. Our results show that experimental changes in local sex ratio and density generate social or environmental cues that are used in sex allocation. We cannot show that sex allocation was adaptive. However, natural and experimental plot density and experimental plot sex ratio related differently to sex allocation for yearling and experienced females, which suggest that several sex allocation scenarios might act simultaneously.