Nice photos, and great (and accurate!) observations on birds and food. We used to keep ducks because we lived on a small river, and there was always tremendous rivalry between the ducks and the different breeds of ducks.

It was interesting because the ducks were generally very sweet and gentle birds you could pick up and they would just sit there peacefully... but when there was food involved, they could become quite aggressive... and as you showed, even attacked their own mates over a bit of food. It's probably just instinct...