Ian C. T. Nisbet Jeremy J. Hatch Patricia Szczys Jeffrey A. Spendelow Brian G. Palestis 2012 A difficulty in the study of monomorphic species is the inability of observers to visually distinguish females from males. Based on a sample of 745 known-sex birds nesting at Bird Island, MA, USA, a discriminant function analysis (DFA) was used to sex Roseate Terns (<i>Sterna dougallii</i>) of the Northwest Atlantic population using morphological measurements. DFA using only the total length of the head (including the bill) correctly identified the sex of approximately 86% of the terns, which increased to 88% if both members of a pair were measured. Including additional measurements increased these percentages slightly, to 87% and 90%, respectively. These levels of accuracy are generally higher than those reported for other species of terns. Because female-female pairs are frequent in this population, one cannot assume that the member of a pair with the larger head is a male, and additional discriminant functions were developed to help separate female-female from male-female pairs. application/pdf 10.1675/063.035.0312 en The Waterbird Society Morphometric sexing of Northwest Atlantic Roseate Terns article