I'm not sure if this is what I would expect a White Ibis to eat ... but this is what this individual had.
I spent a good 20 minutes with this Ibis. In that time it would 'tenderize' the frog with it's beak, then take it to the water and slash it around. After about 15 minutes, it decided it wasn't going to be able to swallow it (or at least that's what its actions led me to believe), so it left it by the water foraging for it's regular diet.
Shooting Info:
Canon 5D3 | 500 f/4 II + 1.4x III
1/640" | f/5.6 | ISO 1600