Rubber Nest Eggs can be used to encourage hens to start laying or getting them to lay in a particular area.
Along the same lines if you want a chicken to go broody and get her to incubate eggs you can also start the process by placing fake eggs into the nesting area.
Alternatively if you have a hen that goes broody but you know she is not sitting on fertile eggs you can replace her real eggs with fake eggs - she may try to defend her eggs so take care when doing this.
