On a brisk Maine afternoon, amidst dog walkers and families pushing strollers, a semi-circle of people with craned necks congregate in one corner of Deering Oaks Park. Through binoculars pressed to upturned eyes, they earnestly search the crowns of trees, pointing and consulting in hushed tones as more onlookers join the growing crowd of about two dozen. “Oo, oo, there it is,” says a man from Vermont who is in town for business and currently on his lunch break in this 55-acre park in Portland, Maine. He power-walks to a nearby Norway spruce and looks up. A newcomer to the clump of people asks, “Have you seen it?” as he whips out his binoculars. There’s no need to explain what “it” is. Everyone is here to see the same thing: A neotropical Great Black Hawk, the first wild individual of its species ever spotted in the United States. With a home range that spans from South America to Mexico, this long-legged raptor typically spends its days wrangling snakes and...