QUICK TIPS: Do not offer mealworms to bluebirds to feed their nestlings. If food supplies are extremely scarce, consider offering mealworms, suet or fruit.
After an extended period of cold, rainy New England weather, I found an Eastern Bluebird nest in the condition shown in the photo. (Bluebird nests are usually very clean.) This nest, constructed of pine needles, was sopping wet, black, stinky, and looked like a tar pit. Some of the fecal material was white (a typical fecal sac) and the rest of the goo was black.
The nest was in a Gilwood box that contained six nestlings, all of which fledged successfully, although one was a runt and fledged at least a day later than the others.
I had been feeding mealworms, as I usually do for bluebirds that nest in my yard. Some mealworms were stuck in the goo, probably dropped by the parents during feeding. However, I have never seen this glop in a nest for other broods fed mealworms.
Normally on my trail, a bluebird nest post-fledging is spotless on top, and looks almost unused with the exception of being flattened, and sometimes containing blow fly larva. Bluebirds usually continue removing fecal sacs until fledging occurs. This is in contrast to the tarry mess I usually find in a Tree Swallow nest post-fledging. Tree Swallows stop removing fecal sacs 4-8 days before fledging, so there is generally a build up.
A fecal sac is a sort of contained, white "birdie diaper" that comes out immediately after each baby is fed. The baby actually sticks its rump up in the air, "offering" it to the parent for removal. These fecal sacs are usually taken from the nest by the adults and dropped some distance away, sometimes in a single dumping area. (See a hilarious video clip of an older Tree Swallow nestling dropping its load out of the nestbox entrance hole, entitled "Thanks Mom.") Tree Swallows sometimes drop sacks over water, probably to avoid attracting predators.
I had stopped opening this box after Day 12 to avoid premature fledging, and then we had a Nor 'easter. I did observe the parents actively feeding the nestlings, and assumed all was well. However, I am lucky that no babies got pushed into this mess by larger siblings, as they might have gotten stuck in it. The waste can build up into a cement-like substance that can prevent a nestling from fledging. I wonder if these birds fledged sooner than they should have (leaving the runt, which was not well-feathered, to fledge a day or two later) to escape.
Some people don't believe that "fecal glue" exists at all. Having seen it firsthand, I believe. Theories on what cause it include the following.
The build-up occurs when parents are under stress - e.g., a single parent frantically trying to feed a clutch (especially a large one as was the case here) and/or spending so much time trying to find food that they can not attend to removing fecal sacs.
Severe diarrhea. This may occur in bluebird babies fed earthworms, and can result in dehydration and death. The baby birds' undeveloped stomachs apparently can't handle earthworms because of the dirt castings in the worms' gut. Earthworms may be used as a source of food by bluebird parents during bad weather, when nothing else is available. Keith Radel believes earthworms break down the fecal sac. Diarrhea might also occur from feeding excessive amounts of fruit.
A small nestbox floor size may aggravate the problem, as the excrement is more concentrated. (A Gilwood box, which seems to be preferred by bluebirds, only has a 3.5 x 4.25" floor.)
Some people report that this is more common in second or third broods.
If you find babies stuck in fecal glue, gently rinse off the poop with warm water (if it's on the feet, soak them in a small container of warm water), and use Q-tips to gently clean off the rest.
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