The Medrash Tanchumah (Bereishis 10) says: “When Kayin killed Hevel he was left lying there, and Kayin didn’t know what to do with him. Hashem then arranged that two tohar birds came on the scene, one bird killed the other, and the live bird then dug a hole and buried the dead bird, from here Kayin learnt what to do, and he dug a hole and buried Hevel. Because of this incident birds’ merit to have their blood covered after they are slaughtered (kisuy hadam).
However, from the Pirkei D’Rabbi Eliezer (Perek 21) it seems that Adam HaRishon buried Hevel. The Pirkei D’Rabbi Eliezer writes: “The dog which guarded Hevel’s sheep guarded Hevel’s corpse from being eaten by animals and bird, and Adam and his wife were sitting there crying and mourning over their loss – and didn’t know what to do. Then a raven who had lost his friend said I will teach them what to do. The raven dragged his friend to Adam and Chavah and then buried it in front of them. Adam then said: I will do like what the raven did. He took Hevel’s body and buried it in the ground.