Bearing identical colors and markings, society finches (Lonchura domestica) are difficult to sex. To distinguish males from females, you must consider the vocalizations and courting behavior of the birds, have your veterinarian examine your bird internally or have your pet's sex determined genetically, by sending a biological sample to a suitable laboratory. However, unless you plan to breed the birds, the sex of your bird does not matter, as the care of males and females is identical.
Vocalizations and Courting
Both male and female society finches make chirping noises, but only male society finches sing. Some advanced hobbyists are able to distinguish between the chirping sounds of males and females, by noting the presence of an "R" sound in the female's song. However, appreciating these small differences in chirps requires significant practice and familiarity with the species. Fortunately, most males sing vigorously in the presence of females, allowing their gender to be determined conclusively. During the course of singing, most male society finches engage in courtship displays, during which they fluff up their feathers and excitedly hop about the cage.
As with all other bird species, only female society finches deposit eggs. They will often produce eggs even when housed without males, although the eggs will obviously be infertile. Nevertheless, birds of unknown sex who do not produce eggs cannot be positively identified as male; this method is only suitable for identifying females. Both sexes participate in incubation duties and help feed the hatchlings.
If you are unable to determine your society finch's sex through sounds or behavior, you can safely determine its sex with your veterinarian's help. Historically, veterinarians would examine the reproductive organs of birds by inserting a small camera into the bird's network or air sacs, through which they could visualize the organs directly. However, because this requires anesthetizing the animal, genetic testing -- in which a laboratory examines the DNA of a blood sample, egg fragment or feather -- has become more popular, as it is noninvasive and more affordable.
Some experienced bird keepers will attempt to distinguish between males and females by feeling the pelvic bones of a bird. Female birds typically have wider, more loosely articulated pelvises than males do, but the differences are very subtle. Accordingly, this technique is not very reliable and not recommended for beginners, as it is very easy to break your bird's bones by pressing too hard. However, some males develop bulging vents during the breeding season, which are often readily visible.