מדרש על במדבר 15:14

(Bamidbar 15:14) "And if a stranger sojourn among you": This tells me (as being likened to an Israelite in this regard) only of a proselyte who had converted before (i.e., one who had left Egypt with them). Whence do I derive (the same for) one who converts now? From "and who shall be in your midst throughout your generations." "and he shall offer a fire-offering": kinds of blood (i.e., animal sacrifices involving blood). You say sacrifices involving blood, but perhaps only a meal-offering (which is entirely burned)! — It is, therefore, written "Thus shall he do." Just as you (in the desert offered) kinds of (sacrifices involving) blood, so, converts offer kinds of blood. — (In that case,) why should we not say: Just as Israel (in the desert offered) blood of a beast ([and not of a fowl] viz. Shemot 24:5), so, converts (are inducted only) through the blood of a beast. It is, therefore, written (Bamidbar, Ibid. 16) ("One Torah and one judgment shall there be) for you (and for the stranger who sojourns with you") — To you (in general, as requiring blood for induction into Israel) have I likened him and not to (the specifics [i.e., beast versus fowl] of) your offerings. Rebbi says: Just as Israel entered the covenant only with three things — circumcision, immersion, and acceptance of the offering — so, the proselytes, like them. — But perhaps just as Israel through peace-offerings, so, proselytes, through peace-offerings. It is, therefore, written "And he shall offer a fire-offering, a sweet savor to the L-rd." Come and see: Which kind of blood (sacrifice) is relegated entirely to the fire, nothing remaining of it? Only a fowl burnt-offering ([but in a beast burnt-offering, the skin reverts to the Cohanim]). I might think (that the induction of the proselyte can be satisfied) even with a meal-offering; it is, therefore, written ("As you are) thus (shall the stranger be"). To bring one bird (as a fowl burnt-offering) is impossible. For we do not find a single bird serving as an offering in the entire Torah — whence it was stated: All the bird couples in the Torah — half is a burnt-offering; half a sin-offering, except for that of a proselyte, which is entirely (relegated) to the fire. Variantly: "As you do, thus shall he do": What is the intent of this? For it would follow (otherwise), that since we find the Torah to have differentiated his offering, (Israel bringing [beast] burnt-offerings and peace-offerings, and a proselyte, a fowl burnt-offering), it is, therefore, written "Just as you do (with libations), thus shall he do" — Just as you, six (logs) for a bullock, four for a ram, and three for a lamb, thus the proselytes."