Although if you play Ars Magcia and haven't seen this movie, I'd be a little surprised.
There's a thread on RPG.net asking about movies with an Ars Magcia feel. Ladyhawk came up. I mentioned, as I'm wont to do, that the curse in the movie cannot be done with Hermetic magic. This was certainly true in 4th edition, but I suspect with some of the Mysteries and other virtues (Fairy magic, maybe Infernal?) that it might be possible without a hand-wave explenation. But my skill at non-Hermetic rulesmongering is pretty low.
So help me out here. In the supplements is there a way to do a Ladyhawk curse: to change someone into an animal from sundown to sunrise for an ongoing period of time until some special condition is met (I can't remember what it was).
With the exception of the ending condition you could do it Hermetically with an enchanted item, but not a spell.
The Merinita Spell Timing Minor Mystery (from Houses of Hermes: Mystery Cults) does exactly what you want. In a nutshell, to make a spell recur daily, learn this mystery, make it a ritual and add 4 magnitudes.
Then add a dispelling effect based on another Duration given by the same mystery - essentially when a certain Condition happens, the effect takes place, and you are done.
Incidentally, the condition was confronting the evil bishop who cast the curse during a (sun) eclipse, though it was worded in some fancier way.
Note that hermetic magic is not very good at this sort of stuff. Gruagach magic (from Hedge Magic), Infernal magic (from Realms of Power: Faerie), and, to a lesser extent, Volkvy magic (from Realms of Power: Faerie) are much better at dishing out curses like this one. Personally, to replicate the movie, I'd make it an infernal curse by the (infernally corrupted) bishop.
I'd say, there are various ways to do this, at least the 'hawk from every sunrise to every sunset' part:
1st: Enchant the victim. Ok, this is no fast way, but I can't find any paragraph in the rules claiming you can't enchant living beings. MuCo(An), 1 use daily, range: self, duration sun, enviromental trigger (sunrise). Unfortunately you pay a season to get your result.
2nd: If I remember correctly (serf's Parma) there is a passage in the rules that claims that formulaic spells don't have to follow the r/t/d-categories as closely as spontanous magic. Perhaps it would be possible to create a ritual spell with a durration of year+1 or 2 magnitudes to have this on/off effect for year duration (unfortunately no permanent Muto-effects allowed in 5th ed.). I'd leave this decision to the GM.
3rd: Even if the GM was adamant on this developping such a duration would most certainly only be a minor breakthrough, so any competent Magus who wants to cast something in that line is probably abele to produce the neccessary knowledge given time.
Btw., ezzelino, I am pretty sure that RoP:Faerie only deals with Faerie Magic, whereas Infernal Magic appears in RoP: the Infernal.