This really should be a spell-like ability; it should be a Standard Action; and it should be limited to a number of times per day. A silence spell cast by a 10th level caster lasts 10 minutes so why does the crow need to continue to crow? It doesn't have to maintain the spell. Rather, it should be able to use this ability 2 times per day (since you already based it on Con 14).Crow at Midnight (Ex): A coalblack **** that crows does not produce sound, but an intense silence. This acts as a Silence spell by a 10th level caster. The **** can continue the crow as a standard action, and can continue for a number of rounds equal to it's Constitution modifier.