The Mishna on today’s daf teaches that if a woman takes an oath that she will bring a nest of birds as a sacrifice should she give birth to a boy, should she, in fact, give birth to a boy, she will be obligated to bring two “nests” – one that she is obligated to bring upon giving birth (see Vayikra 12:6-8), and the other as fulfillment of her oath. The kohen who receives the birds from the woman must bring three of them on the upper part of the altar, above the ḥut ha-sikra (see Zevaḥim daf 26), and one on the lower part. If the kohen did not discuss the matter with her and brought two on the upper part of the altar and two on the lower part, she will have to bring one more bird to be brought on the upper part.
The issue dealt with in this Mishna is the requirement to bring a sin offering on the lower part of the altar and a burnt offering on the upper part. The woman’s obligation upon giving birth is to bring one of each type of sacrifice. In the Mishna’s case, the woman accepted upon herself an obligation to also bring two more burnt offerings, which must be done on the upper part of the altar.
Most sacrifices of fowl in the Temple came in pairs, which is how they were sold in the Jerusalem marketplace as a single nest with two birds in it. Nevertheless there were, on occasion, situations where a single bird was brought, as described in the Mishna, or, for example, in a case where a prosperous woman might choose to bring a lamb as her burnt offering and a pigeon or turtledove as her sin offering.
It should be noted that both men and women could bring sacrifices from fowl, although the Mishnayot in Massekhet Kinnim all discuss situations of women bringing these korbanot, since the majority of these types of sacrifices were brought by women who had given birth.