After an indiscretion in his personal history, Nathan caused much grief for Athene, and this betrayal was not taken well.  For his part, not having realized the scope of what he had agreed to, Nathan turned himself in and submitted to punishment.  After some negotiation, he was sentenced to spend a year as an owl with a crippled right wing, with possible parole after a shorter period if he was willing to be honest about the reason he was shamed and cursed.  He couldn't talk during this, so he had to rely on Dawn to communicate for him.  It was a time of hardship and frustration for him.

And it also taught him a lot, and brought him back down to earth.  Pride goes before the fall, they say...but defeat is in not getting back up after falling down.

In any event, Dawn didn't want to remain homebound the entire time, and that wouldn't fulfill the parole condition anyways, so they were honest about it, and he went out as an owl on several occasions, even submitting to being an example in a lecture about Athene by a priest.  As a result, he was paroled after several months, and restored to human form, with a number of lessons learned.

However, since then, he's been unable to lie to someone he loves, and when he tries, his voice cracks to an owl screech and his right hand throbs, as a reminder of his lessons.