Zebra finches are highly social and mate for life. However, they tend to be shy and fearful around humans, so they should be kept in pairs to ensure their psychological welfare. Consequently, most zebra finches will lay fertile eggs at some point during their lives. Owners should think carefully before choosing to incubate and hatch these eggs. Hatchlings require intense care and will need adequate space as adults. If you think your zebra finch may be gravid, there are several signs she is about to lay an egg.
Watch for nesting behaviour. Male and female zebra finches help to build a nest in which to lay and incubate the eggs. If you have provided your finches with a nesting box, they may start spending more time in it. They may also begin gathering random bits of material from the cage to add to the nesting box is available, finches will build their own nests with pieces of substrate, paper and anything else available in the cage.
Look for changes in the behaviour of both birds. Females may become restless just prior to egg laying and males typically become more territorial and even aggressive.
Wait for egg laying to begin. Zebra finches typically lay their eggs within a week after mating. They lay one egg a day and have clutches of four to eight eggs. After the second or third egg is laid, they will likely begin sitting on the nest to incubate the eggs.
Observe the eggs to determine if they are fertile. Fertilised zebra finch eggs typically darken a few days after egg laying. It is safe to handle the eggs to check their colour, but you should avoid shaking the eggs.