I personally think they are amazing, even the ducklings. We don't really know if the ducks are responsible, but we used to have a ton of biting flies out by our work shed which is where we put the duck run. The last two years, since we got the ducks, we haven't had a problem (knock on wood
). My theory is that the flies are attracted to the smelly duck poop and they are then within reach for the ducks to just snap them up. I know tooo much information.
We also have less weeds in the grassy patch next to the shed -- and, fewer of my flowers are devoured by slugs. You'll still have to give them some feed though.
Ummm.... yeah, they're good fly catchers--but you're not thinking of making them catch all their own food, are you? Ducks, like any domesticated animal, have significant care requirements. They need predator protection and they need a good quality feed, plenty of water, and shelter in the winter. You also need to be prepared for waste management if you have very many in a limited area.
Sorry if I read your question wrong.
They are quite good at catching all kinds of critters--flies, moths, butterflies, mosquitoes, gnats, japanese beetles, etc. The problem with the flies, I have found, is that their poop creates such a wonderful breeding ground that no matter how many flies they eat, having the ducks means we have more flies than we did before.