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

Abstract

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.

The influence of geographical location, host maturity and sex on intestinal helminth communities of the double-crested cormorantPhalacrocorax auritusfrom the eastern United States

“…The high variety of parasites found in P. auritus confirms the generalist feeding habits of this waterbird, with over 250 fish species documented in its range (Dorr et al , 2014b). Shared parasites among sampling localities and between seasons suggest that the life cycles of many parasites of P. auritus persist throughout the range of their definitive host.…”

“…Although identification of cestodes to species from frozen P. auritus was unsuccessful, we suggest that this is a previously underrepresented parasite group in cormorants and additional studies on unfrozen host samples could help elucidate their identities and distributions. The high variety of parasites found in P. auritus confirms the generalist feeding habits of this waterbird, with over 250 fish species documented in its range (Dorr et al, 2014b). Shared parasites among sampling localities and between seasons suggest that the life cycles of many parasites of P. auritus persist throughout the range of their definitive host.…”

“…), suggesting that foraging and feeding between parents could be more asymmetrical than previously reported. Similarly, Dorr et al (2014b) reported geographic segregation of sexes on the wintering grounds in cormorants, presumably influenced by sex-specific prey and habitat preferences. Differences in parasite communities among cormorants on the wintering grounds, where males forage more frequently at aquaculture facilities than females (Craig et al , 2016), were expected.…”

“…These opportunistic pursuit-diving birds forage on the most abundant fishes between 2 and 25 cm in length (Campo et al , 1993; Kirsch, 1995; Fenech et al , 2004). In natural systems, the most abundant fishes can vary seasonally (Gido & Matthews, 2000; Anderson et al , 2004; Coleman & Richmond, 2007; Dorr et al , 2014b) and fish assemblages and their parasites can vary among water bodies. Consequently, cormorants eat an assortment of fishes and parasites as they move from one foraging ground to another.…”

Reproductive Characteristics of Double-Crested Cormorants (Phalacrocorax auritus) in the Eastern United States: Demographic Information for an Intensely Managed Species

“…Our findings indicate that the culling of cormorants directly from breeding colonies will remove greater numbers of breeding cormorants than culling from foraging flocks away from colonies. However, the im-pacts of culling with respect to population modeling efforts are further complicated by evidence that on colonies sex of culled cormorants is biased toward males (Bédard et al 1999;Dorr et al 2014b). These factors combined could lead to more rapid reduction in cormorant numbers than would be expected given proportional culling of breeders and non-breeders and sexes.…”