Skewed Sex Ratios in Cooper's Hawk Offspring

Rosenfield, Robert N.; Bielefeldt, John

doi:10.2307/4088881

Cited by 45 publications

(41 citation statements)

References 11 publications

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“…And thus Fisher's equilibrium hypothesis would in raptors predict a preponderance of male offspring, the apparent cheaper sex (Rutz ), such as we earlier reported as occurring at conception and in broods for highly dimorphic Cooper's hawk offspring in Wisconsin exclusive of Milwaukee (Rosenfield et al. ).…”

Section: Introductionmentioning

confidence: 82%

“…Fisher's population model is controversial for several reasons and one, for example, is that its outcome of a sex ratio of unity is indistinguishable from that which is expected from random Mendelian (meiotic) sex allocation, but the model is oft‐cited in sex ratio studies and is recognized as having explanatory power (Rosenfield et al. ; Ranta et al. ; Donald ; Perlut et al.…”

Section: Introductionmentioning

confidence: 99%

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

Rosenfield

Stout

Giovanni³

et al. 2015

Ecology and Evolution

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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.

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Section: Introductionmentioning

confidence: 82%

Section: Introductionmentioning

confidence: 99%

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

Rosenfield

Stout

Giovanni³

et al. 2015

Ecology and Evolution

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“…In addition there are a number of studies which provide intriguing evidence for pre-hatching adjustment (e.g. Howe, 1977;Patterson & Emlen, 1980;Dijkstra et al, 1990;Bednarz & Hayden, 1991;Zijlstra et al, 1992;Daan et al, 1996;Dzus et al, 1996;Leonard & Weatherhead, 1996;Rosenfield et al, 1996;Tella et al, 1996;Bradbury et al, 1997;Torres & Drummond, 1999;Koenig et al, 2001), but these suffer from major confounding influences such as nestling mortality prior to the collection of sex ratio data and are not presented here Nishiumi (1998) Ring-billed gull…”

Section: Introductionmentioning

confidence: 99%

Potential mechanisms of avian sex manipulation

2003

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The aim of this review is to consider the potential mechanisms birds may use to manipulate the sex of their progeny, and the possible role played by maternal hormones. Over the past few years there has been a surge of reports documenting the ability of birds to overcome the rigid process of chromosomal sex determination. However, while many of these studies leave us in little doubt that mechanisms allowing birds to achieve this feat do exist, we are only left with tantalizing suggestions as to what the precise mechanism or mechanisms may be. The quest to elucidate them is made no easier by the fact that a variety of environmental conditions have been invoked in relation to sex manipulation, and there is no reason to assume that any particular mechanism is conserved among the vast diversity of species that can achieve it. In fact, a number of intriguing proposals have been put forward. We begin by briefly reviewing some of the most recent examples of this phenomenon before highlighting some of the more plausible mechanisms, drawing on recent work from a variety of taxa. In birds, females are the heterogametic sex and so non-Mendelian segregation of the sex chromosomes could conceivably be under maternal control. Another suggestion is that follicles that ultimately give rise to males and females grow at different rates. Alternatively, the female might selectively abort embryos or 'dump lay' eggs of a particular sex, deny certain ova a chance of ovulation, fertilization or zygote formation, or selectively provision eggs so that there is sex-specific embryonic mortality. The ideas outlined in this review provide good starting points for testing the hypotheses both experimentally (behaviourally and physiologically) and theoretically.

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“…, Arroyo , Sasvari & Nishiumi ), variously reporting male‐biased sex ratios (Rosenfield et al . , Brommer et al . , Sasvari & Nishiumi ), sex ratios at parity (Negro & Hiraldo , Arroyo ), female‐biased sex ratios (Tella et al .…”

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confidence: 99%

Variation in offspring sex ratio of a long‐lived sexually dimorphic raptor, the Eastern Imperial Eagle Aquila heliaca

Katzner

Jackson

Ivy

et al. 2014

Ibis

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Sex ratio theory attempts to explain observed variation in offspring sex ratio at both the population and the brood levels. In the context of low‐fecundity organisms producing high‐investment offspring, the drivers of adaptive variation in sex ratio are incompletely understood. For raptors that display reverse sexual dimorphism (RSD), preferential allocation of resources to the putatively cheaper sex (male) may be a response to environmental, social or demographic stressors. To assess the extent of skew in offspring sex ratios and to evaluate possible dietary, environmental and demographic correlates of such skew to long‐lived RSD avian species, we evaluated the offspring sex ratio of 219 chicks from 119 broods in 30 territories of Eastern Imperial Eagles Aquila heliaca across 7 years and four regions at a nature reserve in Kazakhstan. Only in one region in 1 year of our study did the offspring sex ratio differ significantly from parity (10 males : 1 female in 11 territories). Whereas offspring sex ratios were independent of dietary diversity, precipitation, temperature and productivity, we found that year had a moderate effect on brood sex ratio within territories. Our results provide limited evidence of brood sex manipulation in these populations of Eastern Imperial Eagles, and no mechanistic insight into predictions associated with it. Stochastic variation is likely to explain much of the fluctuation we observed in sex ratios, but our observations are also consistent with the hypothesis that sex‐ratio manipulation may occur irregularly, in concurrence with atypical environmental or demographic conditions that fluctuate at a time scale longer than that of our 7‐year study.

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Skewed Sex Ratios in Cooper's Hawk Offspring

Cited by 45 publications

References 11 publications

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

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

Potential mechanisms of avian sex manipulation

Variation in offspring sex ratio of a long‐lived sexually dimorphic raptor, the Eastern Imperial Eagle Aquila heliaca

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