I have notcied that at my shul, whoever gives the siyum on the fast of the first-born each year before Pesach is always a first-born himself. Is this a generally practiced custom? I cannot imagine a reason why this would have to be the case.
I recently heard that Rabbi Elyashiv Zatzal always made a Siyum on Erev Pesach. Once a fellow asked him that he was not a Bechor as his mother had prior miscarriages so why does he make a Siyum every year. He said he knows he is not a Bechor he does it since the Rav is supposed to make a Siyum.
It is quite likely that often a Bechor makes the Siyum since he requires it.