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

Abstract

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.

Brood sex ratios in Merlins reflect characteristics of the associated breeding male and population density

“…Although more subtle, Rosenfield et al . (2015) noted a similar pattern to ours with the relative production of more female than male offspring among after‐second‐year females in their rapidly growing urban Cooper's Hawk Accipiter cooperii population, compared with the surrounding stable rural population; yearling females produced more male offspring regardless of population status.…”

“…2013), we expect older females to pursue brood production strategies similar to those of older males, which is in keeping with reports for other species (Rosenfield et al . 2015, Morandini et al . 2020).…”

Sex ratio and relatedness in the Griffon vulture (Gyps fulvus) population of Serbia

“…In the year 2013, the balance shifted towards males while the year 2020 demonstrated the shift towards females. Strong and viable populations that inhabit a healthy environment with enough resources should exhibit a balanced sex ratio ( Fisher, 1930 ) while it was shown that in the case of the poor condition of parents or deprived resources parents tend to invest in less costly sex ( Brommer et al, 2003 ; Chakarov et al, 2015 ; Ferrer, Newton & Pandolfi, 2009 ; Rosenfield et al, 2015 ; Trivers & Willard, 1973 ). The study of pollutant influence in the environment, like mercury contamination, on the formation of sex in birds demonstrated the skewed balance toward females among fledglings in three different bird species where females are considered as less costly sex: belted kingfisher ( Megaceryle alcyon ), eastern bluebird ( Sialia sialis ) and tree swallow ( Tachycineta bicolor ) ( Bouland et al, 2012 ).…”

Are life history events of a northern breeding population of Cooper's Hawks influenced by changing climate?

“…These study sites were chosen without preconceptions about their suitability for nesting Cooper's Hawks (Bielefeldt et al., ). We note that we have been unable to show in our multidecadal studies that habitat (i.e., rural or urban, conifer plantation or nonplantation, presumptive site quality as indexed by consistency of nesting area use, and high breeding density) is related to size of clutch or brood counts, nesting phenology, annual adult survival, and production of recruits, or fitness in Cooper's Hawks in Wisconsin (Rosenfield & Bielefeldt, ; Rosenfield, Bielefeldt, Sonsthagen, & Booms, ; Rosenfield, Bielefeldt, Rosenfield, Booms, & Bozek, ; Rosenfield et al., ; Rosenfield, Stout et al., ; Rosenfield, Hardin et al., , R.N. Rosenfield, unpubl.…”