Leviticus 1:14 And if the burnt sacrifice for his offering to the LORD be of fowls, then he shall bring his offering of turtledoves, or of young pigeons. - Free Bible Online

      14-17. if the burnt sacrifice . . . be of fowls--The gentle nature and cleanly habits of the dove led to its selection, while all other fowls were rejected, either for the fierceness of their disposition or the grossness of their taste; and in this case, there being from the smallness of the animal no blood for waste, the priest was directed to prepare it at the altar and sprinkle the blood. This was the offering appointed for the poor. The fowls were always offered in pairs, and the reason why Moses ordered two turtledoves or two young pigeons, was not merely to suit the convenience of the offerer, but according as the latter was in season; for pigeons are sometimes quite hard and unfit for eating, at which time turtledoves are very good in Egypt and Palestine. The turtledoves are not restricted to any age because they are always good when they appear in those countries, being birds of passage; but the age of the pigeons is particularly marked that they might not be offered to God at times when they are rejected by men [HARMER]. It is obvious, from the varying scale of these voluntary sacrifices, that the disposition of the offerer was the thing looked to--not the costliness of his offering.

JFB.