Popularly, as an onomatopoeia, this song is called “cluck”. The hen emits it between 1 minute and 15 minutes after laying.
There are numerous theories that try to explain this behavior. The studies conducted do not provide a univocal interpretation and tend, instead, to correlate very different reasons.
To lay an egg, the hen takes between 15 and 90 minutes; time in which she is far from the flock and vulnerable to predators. The song after laying the egg, if we take into consideration a free flock that moves in a limited territory, could have the function of calling the rooster (who, in turn, responds with his own specific song) so that he can help her, escorting her, to rejoin the group. In fact, the rooster, when the hen crow, moves quickly in her direction and performs a specific dance.
This theory is among the most convincing and also helps to explain the “collective nest”. Where a hen lays an egg, other hens often lay their eggs, because it is considered a suitable and safe place, both by the rooster and the hens. It is a form of group cooperation.