Finches with their hefty seed-crackers; warblers with their forceps made slender for extracting small insects hidden among leaves and stems; raptors with their curved hooks for tearing; shorebirds with their probes, straight or curved, which help them extract foods buried on a beach or mudflat. Novice birders quickly learn that the wild diversity of bird beaks is among the most reliable means of quickly determining to what family, and often even what species, a bird belongs. When you’re faced with the bewildering array of avian life in a fall marsh or spring woodlot, that certitude is a comfort, something solid to rest on. See this article's accompanying photo gallery But it’s a bit misleading, too. Birds’ beaks are, in fact, always changing. They’re not static over the course of an individual bird’s lifetime, and they’re certainly not fixed as bird species respond to—and instigate—changes in their environment. Yes, the basic order taught in Birding 101 is there...