הלכה אויף ויקרא 15:29

There are things for which we don't make a leap year at all; but we do make them a support for a year that needs to become a leap [year] because of the season or because of the Spring [state of the barley] and the fruits of the trees. And these are them: Because of the goats and sheep that are yet to be born or which are few. And because of the chicks that have not [yet grown enough to have] flown. We do not make a leap year because of these – in order that the goats and sheep should be accessible for the Pesach sacrifices; and [that] the chicks be accessible for the offering of being seen (reiah) or for those who are obligated in a sacrifice of fowl. But we do make them a support for the year.