A field technique to identify sex is critical for facilitating conservation of species where declines are potentially unequal between the sexes. Although morphometrics are often used to sex individuals in the field, for many tropical and seabird species there is extensive morphometric overlap between the sexes. Accurate sexing of individual birds for research and conservation is thus reliant on genetic analysis, which is not instantly available. In this study we use an endangered tropical bird, the Ma’oma’o (Mao) (Gymnomyza samoensis), as a case study to investigate reliable methods for accurately determining sex in the field for a species with extensive sexual morphometric overlap. We provide the first comprehensive description of its vocalizations and morphology and examine whether individuals of this apparently sexually monomorphic species can be accurately sexed using three features: morphometrics, eye color, and vocalizations. Acoustic analysis, which measured the central frequency of Ma’oma’o alarm calls, allowed sex of all sampled individuals to be correctly identified and was the most accurate mechanism for sexing Ma’oma’o in the field. Our results also indicated that despite high morphometric overlap binomial generalised linear models enabled 54% of the Ma’oma’o to be sexed with 95% confidence in the hand. Individuals that had a high probability of being incorrectly sexed could also be identified. Eye color did not allow strict delineation of the sexes, although the only birds with blue eyes were adult males. We propose that using vocalizations to differentiate sex should be investigated further in other bird species. Not only can it provide an accurate method which does not require capture, but it may also be useful when combined with automatic sound recorders for monitoring sex ratios in bird populations where the greater decline of one sex is suspected.