“In evolutionary terms, these far-flung wanderers might be viewed as testing the limits,” Mr. Kaufman said.

The anhinga in Brooklyn may be on its own, but there were earlier indications that the species had been making forays much farther north. Days before the sighting in Brooklyn, Timothy Wing spotted another anhinga outside his car window in Rome, N.Y., about 180 miles north of New York City.

“Out of the corner of my eye, I saw what I assumed was a double-crested cormorant sitting on a log in the canal on my left,” said Mr. Wing, a bird enthusiast. “The color for the head and neck was much lighter than a typical cormorant, and it didn’t seem right.”

He pulled over and took a closer look with a spare set of binoculars he keeps in his car.

“To my amazement, I saw multiple anhingas sitting on a log, and many others up in the trees along the opposite bank of the canal,” he said.