Early‐life demographic processes do not drive adult sex ratio biases and mating systems in sympatric coucal species

Abstract

Abstract Sex differences in early‐life survival can drive skewed adult sex ratios (ASR), which play an important role in mating tactics and parental sex roles. Among birds, cuckoos exhibit the largest diversity in mating systems and thus represent an interesting system to study sex‐specific demography. Here we investigate the early‐life survival pathways shaping ASR using long‐term field data and sex‐ and stage‐specific demographic modelling of black coucals and white‐browed coucals—two sympatric Centropus cuckoo species differing in mating system and the extent of sexual size dimorphism. We hypothesized that sexual size dimorphism would lead to differential juvenile mortality due to the greater energy demands and predation risk of the larger sex during maturation. We expected higher early‐life mortality among large‐bodied females in the sexually dimorphic and classically polyandrous black coucal, whereas no bias should exist in the less dimorphic and monogamous white‐browed coucal. Both species had balanced hatching sex ratios, but in black coucals, female survival (μ = 0.64 ± 0.061 SD) was lower during the ‘groundling’ phase (i.e. a flightless 3‐week period following nest departure) than male survival (0.70 ± 0.066). Yet, this sex difference in survival was not strong enough to account for the species' male‐biased ASR where 70% of the adult population is male. In white‐browed coucals, survival during maturation exhibited high inter‐individual variation but lacked clear sex differences, reflecting the emergence of a balanced ASR. In summary, we found no evidence that the strongly male‐biased ASR of black coucals is driven by sex‐specific survival during maturation. In black coucals, the male‐biased ASR must therefore stem from processes occurring later in life: possibly, larger females are more likely to die during migration and/or the recruitment of young females is delayed due to competitive exclusion by more experienced females. Our results differ from studies on shorebirds, in which sex‐specific mortality of precocial young is related to ASR and mating dynamics. Hence, early‐life sex biases in survival of precocial shorebirds cannot be generalized to other taxonomic groups. Factors other than early‐life sex biases in survival are likely to be important in shaping adult sex ratios and the associated mating dynamics and social behaviours in altricial coucals. Read the free Plain Language Summary for this article on the Journal blog.