Sexual size dimorphism and offspring vulnerability in birds
This chapter uses recent experimental and observational studies of birds to explore patterns of sex-specific offspring vulnerability (increased mortality and reduced fledging mass under poor conditions) in relation to sexual size dimorphism (SSD). The results show size-dependent modulation of male fledgling mass but size-independent mass reduction in females. Overall, growth is more phenotypically plastic in males than in females. Comparisons of fledging mass reached in ‘good’ and ‘poor’ environments suggest that having to grow large is mainly disadvantageous when coupled with the male phenotype. Differences in environmental sensitivity between the two sexes during ontogeny, either in the form of increased mortality or reduced body size, will tend to reduce dimorphism during development, affecting adult SSD. These results suggest that environmental conditions during ontogeny contribute significantly to variation in SSD within bird species, particularly when comparisons are made among environments or between generations.
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