Okay, we know that the adults are working tirelessly at their own expense to feed the nestlings. As in this video, you’ve probably noticed in several earlier ones that the adult typically feeds the chicks then waits around for a moment before grabbing something white from a nestling’s behind and eating it. You probably thought, “hey, did she just eat that nestling’s poop?” You’d be right.
Songbird nestlings produce what’s called a fecal sac: a mucous membrane surrounding the poop (and urine, since birds mix it all up before it leaves their all-purpose hole—the cloaca). This keeps it all contained like a diaper.
They likely do this for a few reasons. First, it keeps the house clean. You may have noticed that the nest isn’t disgusting, right? Fecal sacs make it easier for the adult to collect the poop from the nestlings and carry it away from the nest, preventing unsanitary conditions at home.
We also talked about predation risk from begging nestlings. But imagine how much easier it would be to locate a nest if there was a bunch of bird poop all over the place! Birds may carry fecal sacs away to ensure that the nest goes unnoticed.
Okay, so my catbirds occasionally carried the fecal sacs away, but they were mostly eating them. Since we also just discussed how exhausting this must be for these busy adults, can we also appreciate that they might want a snack? Nestling digestion isn’t 100% efficient, so there are still a lot of nutrients in their poop (like from this blueberry in the video below). Parents may be eating the fecal sacs for a little snack on the go!