John James Audubon, the pre-eminent 19th-century painter of birds, considered the snowy egret to be one of America’s surpassingly beautiful species. The egret, he noted, was also abundant.

The great egret, a majestic yet stressed bird, strides through the city streets with an air of desperation. Its snow-white feathers, typically a vision of elegance, now appear dull and ruffled ...