Geographic segregation and evidence of density-dependent changes in sex ratios in an abundant colonial waterbird

Demographic information,such as geographic segregation of sexes and sex ratio data,is needed to develop,model and evaluate conservation and management strategies for wildlife.A variety of physiological,behavioral and environmental factors can influence segregation of sexes and sex ratios,many of which originate with den­sity-dependent processes.Departure from 50:50 sex ratios of double-crested cormorants(Phalacrocorax auri­tus)collected during control efforts in breeding and wintering areas across their eastern range of the USA were evaluated using using a Z-test as well as Stouffer’s weighted Z-tests.In addition,a specifically-designed ran­domization test was used to evaluate density-dependent effects on primary sex ratios in cormorants from egg collections and colony nest count data over a 21-year period.Cormorants collected from breeding colonies were strongly male-biased,whereas cormorants collected from feeding flocks were slightly biased toward females.Cormorants were partly segregated by sex on the wintering grounds,with significantly more males found in ar­eas with intensive channel catfish aquaculture.The null hypothesis that females produced a balanced sex ra­tio independent of number of nesting cormorants was rejected:more male embryos were produced during rapid population growth,whereas at maximum nesting number more female embryos were produced.Once popula­tions stabilized,the sex ratio was more equal.This examination of sex ratios indicates that different manage­ment methods and locations result in sex-biased culling of cormorants.Sex-biased culling in cormorants could make population reduction efforts more efficient and reduce overall take.We suggest further research to exam­ine density-dependent effects on primary sex ratios documented here.

Existing theories do not explain sex ratio variation at birth in monomorphic roe deer (Capreolus capreolus)

The phenomenon of skewed sex ratios at birth has been reported in many ungulate species.So far,no consistent trend has emerged for roe deer(Capreolus capreolus),because male-biased,female-biased and equal sex ratios at birth have all been found.Nevertheless,both the Trivers–Willard hypothesis and the theory of local resource competition have gained support.Despite the great number of studies carried out regarding the ecology of roe deer,too many aspects remain unclear,and contradictory results have been produced with respect to several crucial elements.Without further research,the discussion on which theory applies will therefore remain inconclusive.We put forward the argument that eventually the theories of Trivers–Willard and local resource competition can be considered as being not essentially different.After all,both theories explain the observed skewed sex ratios as being due to the effect of the progeny s sex on the mother s body condition and hence her reproductive success in subsequent years.Furthermore,neither theory is likely to prove to be suitable for roe deer,as several assumptions are unlikely to be met.In roe deer,skewed ratios probably only have a temporal character.As a matter of fact,several observations of skewed sex ratios in birds and mammals did not withstand the accumulation of further data,as sex ratios that were initially believed to be biased turned out to be equal in the long term.This is likely to be the case in roe deer as well.We hypothesize that roe deer,as r-strategists,will produce as many offspring as possible,regardless of sex.