Like all newborns, baby birds poop. A lot. But if you peer into most species’ nests, there’s little to no evidence of it. So, where does the doo-doo go? It seems diaper duty is one of the most unique, understudied, and, frankly, underappreciated behaviors among birds. Oh, and, it’s one of the most disgusting, too—at least from a human's point of view. Perhaps you’ve glimpsed the process before: A nestling turns its rear end to mom or dad and ejects a floppy white bag of poop encased in mucous: a fecal sac. The parent then flies away to dispose of it. Or scarfs it down as a snack. Fecal sacs, which only nestlings produce, are common among passerines like robins, bluebirds, and other “altricial” birds (species that require around-the-clock parental care at birth). But even though nasty videos of fecal sacs abound, scientists still know relatively little about them. According to Michael Murphy, a biologist at Portland State University and a fecal-sac expert...