Richard Locke posted this Youtube video of a wren feeding its chicks which finishes up with the scene above. Check it out now as we will repeat the final scene below at length. Then, if the last 5 seconds offend you, close this window.
As a reformed gastroenterologist this was too good an opportunity to pass up. As you may have guessed by now, this isn't dinner for a chick but the remains of dinner past. Many of the feeding trips the parents make end in hauling out the waste, the bird equivalent of changing disposable diapers. Fortunately these are biodegradable disposables. The very act of eating can stimulate the gut urge to empty. This is called the gastrocolic reflex in humans and probably something worse in bird language. If you are still with us, Audubon describes this more scientifically.
"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). 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."
OK, maybe not pure science but you get the idea. Fecal sacs are encased in mucus. This requires protein which is a valuable resource. Journey North has a lot more detail on the sacs to describe the process. Once the chicks fledge, they stop wasting protein on making the sac and poop freely anywhere they want.
Richard Louv describe the large numbers of caterpillars a chickadee feeds its young every day. Imagine how deep a pile of feces would pile up in a nest over several days. The parents hauling out the diapers serves multiple purposes. The obvious one is hygiene, reducing the material that support bacteria. Another unproven theory is that it prevents predators from finding the nest by its odor.
If you are still with us, watch this two minute video condensing fecal sac removal by Richard's wren, a phoebe caring for a parasitic cowbird chick,* and a pileated woodpecker. Sometimes the chick actually backs up to the edge of the nest and lets fly over the side. Occasionally the parent consumes the sac.
So why do the parents sometimes eat the fecal material? Baby birds' guts don't start with all the bacteria necessary for complete digestion of the food. There can be a lot of available calories in poop if you hold your nose and birds can't hold their noses but they can reduce the number of hunting trips needed for their own nutrition. Once the bacteria in the feces build up the parents stop eating it, apparently "a matter of taste."
* The phoebe and cowbird footage is by Linda Bower, skillfully photographing poop once again.