Go out on any fall day when there are insects buzzing about, and you’ll see a unit of feathered fighter jets swooping and diving after them. These avian aerialists—swifts and swallows—may look similar and exploit the same niche, but they’re not related at all. They only resemble each other because of convergent evolution, which means they’ve developed some of the same physical traits to adapt to their shared environment. So how does the enterprising birder tell them apart? By paying attention to details and behavior. First, check the color. Flashes of blue, green, orange, or iridescence are characteristic of swallows. Swifts are mostly soft gray. There are a few exceptions to this rule, however: The White-throated Swift has a chalky belly, while Northern Rough-winged and Bank Swallows are brown overall. While you’re at it, study the subject’s wings, too. Are they uniformly long, skinny, and fluttering? That’s a swift. Are they relatively thick near the body...