Not all situations of tum’ah are created equal.
Depending on the source of the tum’ah, bones may or may not transfer ritual defilement. For example, according to the Mishna, someone who touches the bones of a dead person or of defiled sacrifices will always become ritually defiled; someone who touches the bones of a neveila (see above, daf 73) or of a sheretz (see above, daf 122) will become ritually defiled only if the bone contains edible marrow and the bone has a crack or opening in it. This is because human bones are deemed tameh (see Bamidbar 19:16) while regarding animals only edible flesh – including bone marrow – will give off tum’ah (see Vayikra 11:36, 39).
The Gemara relates that there are also differences between ways that ritual defilement can be contracted. For example, aside from coming into physical contact with the dead animal, tum’ah can also be transferred by carrying the animal, even if one does not touch it (see Vayikra 11:40).
This discussion leads Rav Avya Sabba to ask Rabba bar Rav Huna whether according to Rabbi Yishmael, a closed marrow bone would convey tum’ah by carrying, even though the marrow cannot be touched. Rather than answering him Rabba bar Huna replied: “See, there is a raven flying past,” (i.e. he changed the subject to avoid the question). Afterwards, Rabba’s son, Rava asked him why he refused to answer raised by such a great man as Rav Avya. Rabba answered by referring to a passage in Shir HaShirim (2:5): “Today I am in the state best described by the verse ‘Let me lean against the stout trunks, let me couch among the apple trees.’
A number of explanations have been put forward to explain this reference. Rashi suggests that it was the Shabbat of one of the holidays and Rabba bar Rav Huna had just finished a lengthy lecture and he needed to rest before taking questions. According to the Ge’onim, by mentioning this passage Rabba was hinting to the fact that he had just been appointed head of the academy in Sura and was too exhausted to enter into detailed discussion of the topic at that moment.