Seasonal change in sexual differences in nestling size and survival: a framework to evaluate sex-dependent environmental sensitivity in the wild

Background Sexual size dimorphism (SSD) occurs in a wide range of species in birds and other animals, but the magnitude of SSD often varies with environmental conditions. In general, in the developmental stages, the larger sex is more vulnerable to adverse environmental conditions because the larger sex requires more energy than the smaller sex. However, this may not hold true for birds with large brood sizes; the larger sex can acquire more food by suppressing the smaller sex. In addition, most previous studies have been experimental, such as by manipulating clutch size and ectoparasites, which may not reflect natural conditions.

Methods In the present study, we propose a general framework to assess sexual differences in environmental sensitivity in natural populations. Because environmental conditions change throughout the breeding season, seasonal changes of nestling SSD and sex ratio should reflect sexual differences in environmental sensitivity. We applied this approach to a large dataset (1555 nestlings over 5 years) of Japanese Tits (Parus minor). In this population, the male nestling is generally larger than the female (5% SSD in body weight).

Results We found that the magnitude of SSD (weight, tarsus, wing) and fledgling sex ratio increased both in the beginning and the end of the breeding season.

Conclusion Our study suggested that female nestlings are more valuable to poor environmental conditions in the relatively fecund species. This study underscores the importance of brood size on sexual differences in environmental stochasticity and our framework encourages comparative analysis among different bird species.