משנה על ויקרא 5:7

A bird hatat is performed below [the red line], but a beast hatat is performed above [the red line]. A bird olah is performed above, but a beast olah below. If he changed this procedure with either, then the offering is disqualified. The seder [ordered ritual] in the case of kinnim is as follows: In the case of obligatory offerings, one [bird] is a hatat and one an olah. In the case of vows and freewill offerings, however, all are olot. What constitutes a vow? When one says: "It is incumbent upon me to bring an olah." And what constitutes a freewill-offering? When one says: "Behold, this shall be an olah." What is the [practical] difference between vows and freewill offerings? In the case of vows, if they die or are stolen, one is responsible for their replacement; But in the case of freewill offerings, if they die or are stolen, one is not responsible for their replacement.