draconym:

The parents are conducting shift work during the incubation period.
During any given shift change at the nest, the relieving adult will land
in the nest and nudge the incubator to take over duties. If nudging
doesn’t work, then more aggressive moves such as walking on the tail
feathers or back of the unrelenting incubator is conducted. If still no
movement, the reliever will snuggle against the incubator and wait for
an attitude change.

(article from the US Fish and Wildlife Service)

(article from The Dodo–which is not my favorite source, but it links to many other articles)