Does breeding population trajectory and age of nesting females influence disparate nestling sex ratios in two populations of Cooper's hawks?

Offspring sex ratios at the termination of parental care should theoretically be skewed toward the less expensive sex, which in most avian species would be females, the smaller gender. Among birds, however, raptors offer an unusual dynamic because they exhibit reversed size dimorphism with females b...

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Bibliographic Details
Published in:Ecology and evolution 2015-09, Vol.5 (18), p.4037-4048
Main Authors: Rosenfield, Robert N., Stout, William E., Giovanni, Matthew D., Levine, Noah H., Cava, Jenna A., Hardin, Madeline G., Haynes, Taylor G.
Format: Article
Language:English
Subjects:
Accipiter cooperii
Adults
Age
Birds
Birds of prey
Breeding
Comparative studies
Cooper's hawk
Females
Gender
Hypotheses
life history
Males
maternal age
Monogamy
Nesting
Offspring
offspring sex ratios
Original Research
Population growth
Population studies
primary sex ratio
reversed sexual size dimorphism
Sex
Sex ratio
sex ratio theory
Sexes
Sexual dimorphism
Studies
Theory
Trajectories
Unity
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Summary:Offspring sex ratios at the termination of parental care should theoretically be skewed toward the less expensive sex, which in most avian species would be females, the smaller gender. Among birds, however, raptors offer an unusual dynamic because they exhibit reversed size dimorphism with females being larger than males. And thus theory would predict a preponderance of male offspring. Results for raptors and birds in general have been varied although population‐level estimates of sex ratios in avian offspring are generally at unity. Adaptive adjustment of sex ratios in avian offspring is difficult to predict perhaps in part due to a lack of life‐history details and short‐term investigations that cannot account for precision or repeatability of sex ratios across time. We conducted a novel comparative study of sex ratios in nestling Cooper's hawks (Accipiter cooperii) in two study populations across breeding generations during 11 years in Wisconsin, 2001–2011. One breeding population recently colonized metropolitan Milwaukee and exhibited rapidly increasing population growth, while the ex‐Milwaukee breeding population was stable. Following life‐history trade‐off theory and our prediction regarding this socially monogamous species in which reversed sexual size dimorphism is extreme, first‐time breeding one‐year‐old, second‐year females in both study populations produced a preponderance of the smaller and cheaper sex, males, whereas ASY (after‐second‐year), ≥2‐year‐old females in Milwaukee produced a nestling sex ratio near unity and predictably therefore a greater proportion of females compared to ASY females in ex‐Milwaukee who produced a preponderance of males. Adjustment of sex ratios in both study populations occurred at conception. Life histories and selective pressures related to breeding population trajectory in two age cohorts of nesting female Cooper's hawk likely vary, and it is possible that these differences influenced the sex ratios we documented for two age cohorts of female Cooper's hawks in Wisconsin. We conducted a novel comparatie study of sex ratios in nestling Cooper's Hawks (Accipiter cooperii) in two study populations across breeding generations in Wisconsin, 2001–2011. Following life‐history trade‐off theory and our prediction regarding this socially monogamous species, first‐time breeding one‐year‐old females in both study populations produced a preponderance of the smaller and cheaper sex, males. Whereas older females in Milwaukee pr
ISSN:2045-7758
2045-7758
DOI:10.1002/ece3.1674