@Riger I forgot that feature existed...
Welp, speculation time!
Since he didn't realize the eggs weren't quail eggs, the buff probably wasn't distinct (to him).
It's also entirely possible he didn't look at the initial buff window, so it could be a good buff.
As for buffs a phoenix's eggs might give, maybe something related to fire, rebirth, or healing? It could also give a buff like "easily recognized by birds".
There's not much to go off of.