Photographers Rachel Israel and Robert Wong love the view of all the ducks at different locations from Central Park.

Why black-bellied whistling-ducks pushed northward isn’t clear. But because food plays a role in the range expansion of any bird, the ducks may have headed north to lands with abundant grasses ...

Peter Yaukey, a professor of biology at University of Holy Cross, says the ducks have been migrating northward from Mexico. He says it's hard to tell why the population is expanding.