The gemara states that if we can remove the Mitzvas Lo Sa'aseh from Shiluach HaKen, then if a Metzora wishes to take a mother bird for his Taharas Metzora it would set up a scenario of Mitzvas Aseh against another Mitzvas Aseh and there is a good reason to believe that the Mitzvas Aseh of Taharas Metzora would be docheh the Mitzvas Aseh of Shiluach HaKen. I have 2 questions:
Is there a requirement of "B'idna" when one Aseh is docheh another Aseh just like we find this requirement by an Aseh being docheh a Lo Sa'aseh? If so, wouldn't we consider the Taharas Metzora not coming until after the bird has been taken (since taking the bird itself would be a hechsher mitzvah at best until the actual mitzvah occurs when the bird is shechted or sent away) in which case the rule of one Aseh being docheh the other Aseh would not apply and we should say that the mitzvah that is currently at hand (Shiluach HaKen) should overrule the future mitzvah of Taharas Metzora?
I would think that there is a mitzvah of Shiluach HaKen that applies every time a person runs into a situation of a mother bird sitting with its baby birds, even several times a day. Yet, the mitzvah of Taharas Metzora is a one-time mitzvah - once it is completed the former metzora need not bring another bird unless he becomes a metzora again. If so, why wouldn't we say that if a person who needs Taharas Metzora comes across a situation of Shiluach HaKen, he should be required to fulfill the mitzvah of Shiluach HaKen and wait until he comes across another bird that is in a situation that does not require Shiluach HaKen, since there is nothing obligating him to fulfill his mitzvas Aseh of Taharas Metzora with this bird specifically, unlike the mitzvah of Shiluach HaKen which requires him to fulfill the mitzvah with this bird specifically?
Tosfos in Pesachim (59a) proves from this Gemara that the principle of "Aseh Docheh Aseh" (as opposed to "Aseh Docheh Lo Sa'aseh") does not need b'Idna.
The Gemara must be discussing a case in which there is no other bird presently available for the Metzora.