וְאִם־לֹ֨א תִמְצָ֣א יָדָהּ֮ דֵּ֣י שֶׂה֒ וְלָקְחָ֣ה שְׁתֵּֽי־תֹרִ֗ים א֤וֹ שְׁנֵי֙ בְּנֵ֣י יוֹנָ֔ה אֶחָ֥ד לְעֹלָ֖ה וְאֶחָ֣ד לְחַטָּ֑את וְכִפֶּ֥ר עָלֶ֛יהָ הַכֹּהֵ֖ן וְטָהֵֽרָה׃ (פ)
E se as suas posses não permitirem um cordeiro, então ela tomará duas rolinhas ou dois pombinhos, um como oferta queimada e um como oferta pelo pecado; e o Cohen fará expiação por ela, e ela será pura." {פ}
Mishneh Torah, Sanctification of the New Month
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.
Sefer HaChinukh
It is from the roots of the commandment [that it is] in order that she be aroused by action to give thanks to God, blessed be He, Who saved her from the pangs of her birth, which is a miraculous thing. And also they, may their memory be blessed, said (Niddah 31b) that a woman rushes to swear at the time of her throes that she will no longer [have sexual relations with] a man. And therefore she requires atonement, and hence her sacrifice is called a sin-offering, and it is stated about her, “and the priest shall atone for her” (Leviticus 12:8).