Well when a special ability "acts" like a spell of a certain level then it simply mimics the spell. Otherwise, you have to change the way the special ability works. If the silence only lasts as long as the crowing does, then you need to say the ability works like the spell except that its duration is for a long as the crowing lasts and you need to put time limit in place. How long can it crow for? Does 1d4+1 rounds sound sufficient?