One December while home in Pennsylvania for the holidays, ornithologist David Yeany II heard an Evening Grosbeak’s buzzy chirp. Hoping to enliven the yard with the bird’s cheerful chatter and striking hues, he and his dad spent an afternoon building a platform feeder from scrap wood. The next morning, nine oversized finches—nicknamed “grocery beaks” for their insatiable sunflower-seed appetite—were munching away with their impressive namesake bills. In the past, Northeast birders could expect to spot Evening Grosbeaks every two or three winters. In years when food gets scarce in their breeding grounds in Canada’s boreal forest, grosbeaks will range much farther south in events called irruptions. “Seeing them brings a certain kind of magic to our backyards in the wintertime,” says Matt Young, president and founder of the Finch Research Network (FiRN). But since 1970, the once common species has sharply declined in the East, making irruptions less frequent and...