Among the labors permitted on Erev Pesach, the Mishnah lists setting hens on their eggs: "Moshivin shovachin le'tarnegolim be'arba'ah asar" - one may set a hen upon the eggs she has laid so that they will develop into chicks, and according to Rabban Gamliel's teaching this may be done on the fourteenth of Nissan.
The commentators disagree about the scope of this permission:
Even after midday - since this is not really considered labor.
Only before midday - in places where labor was prohibited on account of custom.
A hen that fled: the law of Chol HaMoed:
The Mishnah continues: "Ve'tarnegoles she'barchah, machzirin osah li'mkomah" - and a hen that fled may be returned to its place. The Gemara explains that this ruling does not deal with Erev Pesach but with Chol HaMoed itself. For if it were speaking of the fourteenth, we would already have learned it from the first clause: if one may set hens upon eggs in the first place on the fourteenth, then certainly one may return a hen that had already been sitting there and fled. Necessarily, then, the Mishnah is teaching an additional novelty: that even on Chol HaMoed this is permitted.
The Gemara further explains that the permission to return the hen on Chol HaMoed depends on two conditions:
Within three days of her flight - after that the warmth of the eggs has already dissipated, and returning her no longer helps.
After she has sat on them for three days - for then they are no longer fit to be eaten; they need to develop, and have already begun to do so. It turns out that if the hen is not returned there is a real loss of the eggs, and in such a situation she may be returned on Chol HaMoed.
If she died: if the hen died, one may bring another hen and set her in its place. There is an opinion that this last ruling was said not with regard to Chol HaMoed but only with regard to the fourteenth, but on Chol HaMoed one may not set an entirely new hen there.
Raking out the manure from beneath the animal's feet:
On the fourteenth of Nissan one may rake out and clear away the manure that is beneath an animal's feet and throw it outside, and according to most opinions even after midday. In any case, one does not carry it off and take it out entirely to the dunghill; one only moves it aside.
Taking utensils to the craftsman and bringing them back:
We further learned: "Molichin u'mevi'in kelim mi'beis ha'uman, af al pi she'einan letzorech ha'mo'ed" - one may take utensils to the craftsman's house and bring them back, even though they are not needed for the festival. One may bring utensils in for repair and take back utensils that have already been repaired, and this applies all through the fourteenth, even though they are not needed for Yom Tov. For there is no prohibited labor here at all, merely bringing the utensils to one's home or returning them to the craftsman's home or shop.
On Chol HaMoed, however, one may not bring utensils in for repair, even though they will not be repaired until after Yom Tov, and even if they are needed for the festival. So too, one may not bring back from the craftsman's house utensils that are not needed for the festival.