Zoo and wildlife management faces a problem with bird sexing, as many bird taxa have indiscernible gender differences in size and coloration. Problematic groups are geese, cranes, rails, raptors, owls, parrots, doves, auks, shearwaters and some passerines. Commonly accepted invasive sexing techniques based on genetics, laparoscopy, morphometric and on cloacal inspection, are all needed in bird capturing and handling. Capturing and subsequent manipulations may be inapplicable for free-ranging birds, whereas distant voice-based sexing is relevant for many species. This review evaluates the potential for noninvasive sexing by separate calls or duet calls, for adult birds of 69 species from 16 orders and for chicks of 11 species from 7 orders. For adult birds of 25 species, a single call per individual was sufficient for 100 % reliable sexing by ear or using spectrographic analysis. For chicks, the potential for voice-based sexing seems to be very limited. For birds calling rarely or unpredictably, we propose a simple way of provoking vocalization using playbacks of species-specific calls that are available from sound libraries. We conclude that sexing by voice may represent a feasible alternative to the classical sexing techniques, both in the wild and in captivity.

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Acoustic-based monitoring has proved useful for many birds and seems promising for the endangered red-crowned crane. However, its validity in crane conservation is unclear in the absence of knowledge concerning the long-term stability of pair-specific duets. The redcrowned crane is monogamous and long-lived, with stable pair bonding both within and between years. Pair mates perform loud duets-a succession of male and female calls emitted with definite temporal coordination. We examined the stability of duets for five captive pairs over five years (2003)(2004)(2005)(2006)(2007) on the basis of analysis of the syllables within the duets. MANOVA showed that the effect of pair identity on syllable characteristics was always stronger than the effect of the year of recording. Cross-validation of duets from 2004, 2005, 2006, and 2007 with discriminant analysis (DFA) functions derived, respectively, from pooled samples from resulted in comparably high percentages of correct classification into pairs. The pairs could be reliably identified by their duets and pair-specific differences in syllable characteristics were stable with time. These data suggest acoustic monitoring is a feasible alternative to more invasive methods of identification.