Aggressive male ducks

[staceyp1023]

Outside in the courtyard of my 7th grade classroom there is a female duck sitting on her nest of eggs. Lately, 3 male ducks have been ganging up on her, trying to move her from the nest. At first I thought this was some weird mating thing (yeah, lots of fun when 30 7th graders are watching), but now I think it's more aggressive. They bite her and try to run her off, and literally block her return to the nest. There is another female duck with hatched ducklings at this point too, so I am wondering if it is a terratorial thing? Also, there are many crows in the area that want to eat the eggs.

My questions are: Why are the male ducks doing this? And how can I scare the crows away but not the ducks? I feel like I am an accomplice to murder by crows!

[HWKE]

The male ducks don't believe the eggs are theirs, so they try to prevent them from hatching in order to father the next brood and preserve their own line. It happens a lot in the animal kingdom.