570 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 © 2014 International Society of Zoological Sciences, Institute of Zoology/ Chinese Academy of Sciences and Wiley Publishing Asia Pty Ltd Integrative Zoology 2014; 9: 570–582 doi: 10.1111/1749-4877.12091 ORIGINAL ARTICLE Geographic segregation and evidence of density-dependent changes in sex ratios in an abundant colonial waterbird Brian S. DORR, 1 Katie C. HANSON-DORR, 1 Travis L. DeVAULT, 2 Alban GUILLAUMET 3, * and Scott C. BARRAS 4 1 United States Department of Agriculture, Wildlife Services (USDA-WS), National Wildlife Research Center, Mississippi Field Station, Mississippi State, Mississippi, USA, 2 USDA-WS, National Wildlife Research Center, Ohio Field Station, Sandusky, Ohio, USA, 3 Department of Wildlife, Fisheries and Aquaculture, Mississippi State University, Mississippi State, Mississippi, USA and 4 USDA-WS, Moseley, Virginia, USA 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 inluence segregation of sexes and sex ratios, many of which originate with den- sity-dependent processes. Departure from 50:50 sex ratios of double-crested cormorants (Phalacrocorax auri- tus) 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 speciically-designed ran- domization 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 locks were slightly biased toward females. Cormorants were partly segregated by sex on the wintering grounds, with signiicantly more males found in ar- eas with intensive channel catish aquaculture. The null hypothesis that females produced a balanced sex ra- tio 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 popula- tions stabilized, the sex ratio was more equal. This examination of sex ratios indicates that different manage- ment methods and locations result in sex-biased culling of cormorants. Sex-biased culling in cormorants could make population reduction efforts more eficient and reduce overall take. We suggest further research to exam- ine density-dependent effects on primary sex ratios documented here. Key words: Ashmole’s halo, management, Phalacrocorax auritus, population growth, sex ratio theory Correspondence: Brian S. Dorr, USDA-WS National Wildlife Research Center, PO Box 6099, Mississippi State, MS 39762, USA. Email: brian.s.dorr@aphis.usda.gov *Present address: University of Hawaii, Ofice of Research/ PIPES, Hilo, HI, USA. INTRODUCTION In wild bird populations, tertiary sex ratios are de- ined as the sex ratio of breeding adults, but are general- ly measured as the sex ratio of all non-juvenile individ- uals (Mayr 1939). Determination of the true tertiary sex