There are lots of possible reasons for a high mortality rate among baby birds. Storms can blow them out of their nests. Cowbirds and cuckoos lay their eggs in the nests of other species. The cowbird or cuckoo hatches first and pushes the other babies out of the nest. Occasionally a nest is simply unstable and falls apart. Malicious or bored kids could be knocking the nests out of the trees.

I think if it were truly West Nile virus, you'd be seeing dead adult birds as well as babies—but I'm no expert.

An unusually large number of bird droppings? Some kinds of berries give birds diarrhea. Buckthorn does this. (But buckthorn berries aren't ripe yet in my part of the world.)

Maybe people are feeling paranoid and therefore paying more attention to birds than they used to.