Determining the sex of geese is not easy. You can be identify some geese by colour, with males of one colour and females of another, but many geese are virtually identical in many ways until they are sexually mature. Even then, the safest way to determine the gender of a bird is to observe it during mating, when the male mounts the female. Except for vent sexing, you can see most differences in secondary sexual characteristics.
Sexing Young Geese
It is nearly impossible to determine the sex of very young geese. Most geese reach sexual maturity between 2 and 3 years of age, with most reaching maturity at 3 years. Once the geese have reached maturity, they can be vented, which requires turning the bird upside down, causing its back to arch, then expelling the sex organs by applying pressure against the vent with the thumb and index finger. Careful venting is necessary to prevent injury to the sex organ, especially of a young male.
Vent sexing is usually 100-percent accurate and the preferred way to determine gender. It must be done by an experienced handler to avoid permanent injury.
Physical Characteristics of Male Geese
Ganders are usually easier to differentiate because they tend to be larger than females. Their heads tend to be bigger and broader. They also tend to hold themselves taller than the female, and their necks tend to be longer and more masculine. These differences may be more difficult to see in European breeds of geese, while less difficult in Chinas and Africans, which are more easily distinguished because the males tend to have larger knobs on their heads than the females.
Physical Characteristics of Female Geese
Female geese have smaller and narrower heads than ganders and are slightly smaller in stature. Their necks are more narrow and graceful, like that of a swan. Of course, there is always the problem of the extremely large female (usually a good problem) or the diminutive male (not a good problem). The drawback of comparing smaller and larger is that you can't do it with just one goose.
Behavioural Habits of Male Geese
Males that have reached sexual maturity show more aggressive and protective behaviours than females. The male will often extend his neck between an intruder and his mate, especially just prior to and during breeding season. This behaviour tends to diminish during the rest of the year.
The male regular voice (as opposed to its alarmed voice) tends to be higher pitched than the female's, although some ganders have a more in-between pitch.
The biggest tip-off to the sex of the goose is its mating position. The male will always mount the female.
Behavioural Habits of Female Geese
A female goose will show less protective behaviours than the gander. She also has a deeper natural tone of voice. The biggest indicator of a female goose is her ability to lay eggs.
Unusual Sexing Techniques
Some breeders separate the fully grown geese from the ganders by eye colour. The "Journal of Heredity" states that adult ganders have blue eyes while female geese have brown eyes, a trait known as colour dimorphism.