One of the activities that might be restricted on erev Pesah involves an egg farmer who prepares nests or coops for his birds. The Mishna teaches that hens can be put on eggs to warm them for hatching on the 14th of Nisan; similarly, if a hen has abandoned her post on the eggs she can be returned to it, or if the hen dies another can be brought as a replacement. These activities are not true melakhot, but they do involve a certain amount of hard work to accomplish.
Gemara: Now, the mishna stated that placing a brooding hen to sit on eggs is permitted; is it necessary to mention that restoring a hen to its brooding place is permitted? Abaye said: In the last clause of the mishna we have arrived at the of the intermediate days of the Festival, when placing a hen to sit on eggs is prohibited, yet one may nevertheless restore a hen that fled, as failure to do so will cause him to incur a loss. On the fourteenth of Nisan, one may even place a hen to brood ab initio.
Rav Huna follows this by teaching that the bird can only be returned to the nest within three days of leaving d’akati lo parah tzimra minei – because the hen has not yet lost its warmth, but after that the hen cannot be returned.
When birds, including chickens, sit on their nests and warm their eggs, it is not a simple act of rest for them. “Brooding” (degira in modern Hebrew) involves a complex hormonal change in the chicken that gets the bird to sit for weeks on end in a single place, ready to fend off any attackers. This condition is brought about by a number of factors, including the season and the physical touch and feel of eggs against the body. In many birds, the body temperature rises (some specific areas on the body become warmer) as part of the hormonal change. In the event that the bird loses its natural inclination towards brooding, returning it to the nest within a short time may successfully restart its instinct for remaining on the nest, due to the contact with the eggs. If the bird is not returned shortly after leaving the nest, it may not be possible to return it to brooding.