What Are Fecal Sacs? Bird Diapers, Basically

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, only a handful of studies have been done on the subject. "It just seems obvious that the parents are going to have to remove feces, so no one has paid attention,” Murphy says.

Despite this dearth of research, evidence suggests that fecal sacs have a number of uses. Here, Murphy and Juan Diego Ibáñez-Álamo, an ecologist at the University of Groningen who has been studying fecal sacs for a decade, walk us through the potential benefits these poop bombs have to offer. 

Keeping the Nest Clean

A fecal sac is essentially a diaper, says Murphy. “It provides the parents with a very self-contained structure that allows them to easily pick up feces and remove them from the nest. It’s a way of getting rid of all this material that might otherwise smell and decompose.”   

According to research by Ibáñez-Álamo, the membrane might also help keep the birds healthy. “There’s a physical isolation from potentially dangerous bacteria or microorganisms in the feces that might otherwise affect the parents or the nestlings,” he says.

Recycling a Nutritional Snack

Some birds have evolved a more efficient method of dealing with fecal sacs: They swallow them. 

Why? The best guess—the one with the most research—is that birds eat fecal sacs because nestling poop serves as a nutritional treat (a trait known as coprophagia). “Parents will eat the feces because the nestlings cannot completely digest the food that they eat,” Ibáñez-Álamo says. “There is still energy and nutrients available in those sacs.”

Now, some adult birds eat the sacs throughout the whole nesting period, but others only eat them when the chicks are young. Both Ibáñez-Álamo and Murphy believe this might have to do with the lack of nutrients in an older chick's sac. "The young's digestive tract is becoming more efficient and leaving less and less potentially usable material in their feces," Murphy says. 

As for whether the sacs can be nutritious while also containing harmful bacteria, Ibáñez-Álamo thinks so: The mucous covering might protect a parent's feathers or skin from bacteria that their guts can handle no problem.  

And if all that wasn't enough, there might be even more to this behavior. Transporting the poopy packages outside of the nest could attract predators, Ibáñez-Álamo says, and also leaves the chicks alone and vulnerable. Eating the sacs keeps the nest hidden and parents close to their young.

Preventing Detection by Predators

Finally, poop-free nests might also be less noticeable by predators that are drawn to the sight—or more likely the scent—of feces. There is even a theory that birds such as Common Grackles will deposit the sacs in rivers and swimming pools so that any evidence of nearby newborns will disintegrate quickly.

Murphy says predator avoidance is likely, and that some species' clever user of water—or frustrating, if you're a pool owner—could also be a possibility. “Most of the time when birds are dropping them in the pool, it’s probably related to getting their sacs as far away as possible,” he says.   

Ibáñez-Álamo isn't so sure; he says there’s little proof to support the idea of predator evasion being a main driver for fecal sacs, and as with research in this field overall, further studies are needed. “While for some species that might be true, the evidence we have is inconclusive,” he says.