Federal agents slaughtered more than 250 geese and dozens of babies in Prospect Park after corralling the adults and their young into a small pen early Thursday morning, boxing them up, and tossing them in a gas chamber.

The horror-movie-style measure was part of the city’s ongoing effort to exterminate the feathered creatures within a seven-mile radius from both LaGuardia and JFK airports — a radius that was expanded last year to include Prospect Park, putting all its birds in the path of a federally mandated massacre.