Mishnà su Levitico 5:8

Hatat [birds] are on one side, and olot [birds] are on the other and an unassigned [pair] is in the middle: If from the middle pair one bird flew to this side, and one bird flew to this side, then he has not lost anything, because he [the priest] says that the bird that flew [from the middle] towards the hataot is a hatat and the bird that flew towards the olot is a olah. If one [from each side] returns to the middle, then [all] those in the middle must be left to die, but those [left on either side] can be offered up as hataot or as olot respectively. If again a bird [from the middle] returned and flew away to the sides, then all must be left to die. One cannot pair turtle-doves with pigeons or pigeons with turtle-doves. How is this so? If a woman has brought a turtle-dove as her hatat and a pigeon as her olah, she must then bring another turtle-dove as her olah; If her olah had been a turtle-dove and her hatat a pigeon, then she must bring another pigeon as her olah. Ben Azzai says: we go after the first [offering]. If a woman brought her hatat and then died, her heirs must bring her olah; [But if she first brought] her olah and then died, her heirs need not bring her hatat.