Players are able to build nests by acquiring twigs from the ground or in trash bins. Other players may also help in building the nest in the same manner.
Eagles make massive nests, while Robins for example, make very small nests. The size of the bird also affects the size of the egg and chick.
Two mates will often help build the nest and the female is usually the one to sit on the egg. However, if the nest is abandoned, it may be lost.
In Hillscape, a floating nest can be seen just above the river. This is possibly an error made by Gamevial during the Hillscape's production.
This was not done with hacks, but a floating nest was seen at the Snowscape. The player who may have built the nest was also walking in midair. This may be related to another glitch where if you build a nest on the roof of a snowscape house, the house will disappear but you and the nest are still on the now invisible roof. This can also be seen by other birds in the server.