http://www.sciencemag.org/news/2019/09/common-pesticide-makes-migrating-birds-anorexic

White-crowned sparrows stop eating after they ingest small amounts of neonicotinoid pesticides.

When birds migrate, timing is everything. Fly too late, and they miss the peak season for finding good food, a good mate, or a good nest site. But that’s just what may happen to migrants unlucky enough to eat pesticide-laced seeds, new research shows. Toxicologists studying white-crowned sparrows have shown that these large, grayish birds become anorexic after eating neonicotinoid pesticides, causing them to lose weight and delay their southward journeys. The study might apply to other birds as well—and help explain the dramatic songbird decline of recent decades, researchers say.