Like animals, there is safety in numbers. Mallard ducks like to congregate together like this because it helps protect against predators. They can also help in foraging for food by sharing information and also flushing out prey together. Also, by staying together like this, there is an obviously greater opportunity for mating and other social activities like establishing hierarchies and learning from one another. I also find personally it gives me some comfort seeing so many together, as it tends to indicate a healthy ecosystem in which to thrive.
