They come from ducks and geese which spend most of their time in and around water. Some diving birds can dive hundreds of feet below the surface (the record as I understand it is 690 feet by a Brünnich’s guillemot). It seems ironic that down/feather bags are not recommended for wet environments - I believe because they lose their loft and insulating properties when wet and take a long time to recover. I’m just curious why that is the case. Is it because the live birds are constantly preening and keep their feathers oily or something?
Just a guess but we have an African Grey parrot and an cockatiel. They create this dust as a waterproofing and need rinsed a lot with water. Probably something to do with feathers being on a live bird vs processed and clean.
They don’t maintain their loft when wet
Ducks and geese have a gland at he base of their tail (lower back) the use the secretions to waterproof their outer feathers and the down stays fluffy underneath to keep them warm. When the “fluffy” gets wet it collapses and you loose the warmth. So yes, you are correct about the insulating properties.
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I worked for some years in the camping gear trade and got a lot of manufacturer training in materials and construction of same.
On the bird, the down plumule is attached under each feather and provides a sort of lubricant, keeping overlapping feathers from hanging up on each other, also maintains trapped air insulation when the bird “fluffs” it’s feather coat. Essentially the interlocked feathers coated with the bird’s expressed oils act like the fabric shell of a down jacket or sleeping bag, protecting the down from getting saturated in immersion or rain so it can maintain its structural integrity and trap air. When down gets wet, the fibers don’t so much get soaked as they adhere to each other because of the surface tension and other properties of moisture and clump together, collapsing the structure. The fabric shell being wet exacerbates the problem by sagging and sticking to itself.
Some natural oils are removed in processing down but in quality products, some remains which does help it stay fluffy. Repeated washings and especially improper dry cleaning (have to use Stoddard solvent, not perchlorethylene) will strip the oils out completely, shortening the functional life of down.
Also storing down filled items compressed will squish the plumules permanently and they won’t rebound as well once unpacked and fluffed. Eventuallly, with age, the individual fibers that are part of the plumule will start to disintegrate and detach from the clump and the product the down is stuffed into will lose loft. After enough time you will only have a bag of dust,
There were some scandals back in the big surge in demand for down gear back in the 70’s and 80’s when some of the big name brands were sold so-called “couchet down” by Asian vendors. Couchet down is down that is recycled from used bedding (down comforters and duvets were widely used throughout Asia and Europe long before they caught on here in the US so there is a lot of the stuff). So not only is it older material but usually already degraded from the frequent washing that bedding requires. I remember one of my first down sleeping bags was a Snow Lion that was recalled after that lot was found to have been made with a batch of couchet. I had wondered why it started to go flat after less than a year – was glad I was working at the outfitter and was able to get a warranty replacement. There are better safeguards nowadays to assure down is a “virgin” product.
Though synthetic insulations are not as light or compressible as down (nor as lush to sleep cocooned within) they do maintain their much more interlocked structure than down when wet or compressed. They can also be damaged by sustained compression. We used to demonstrate to customers who bought polyester insulated sleeping bags that the proper way to pack them down them for backpacking and for storage was to grab random handfuls and stuff like laundry into the stuffsack or storage bag. Folding and rolling them would result in flat spots along the fold lines that would eventually not rebound. ( One of the sales staff at the first outfitter I worked for was a big bruiser of a guy with massive hands and he used to stuff sleeping bags into small stuffsacks in seconds using only his thumbs, like somebody turning socks inside out.)
I do love a down sleeping bag for comfort and breathability. Back when I used to do a lot of cold weather backpacking and mountaineering I solved the problem of moisture by using a two bag system. Winter tent camping means a lot of frost condensation that tends to get into the bedding. I used a 20 F degree rated Trailwise down mummy plus an oversized barrel shaped no-hood poly insulated bag rated to about 45 F. Nested together they gave me a sleeping arrangement that was good to well below zero (my overnight record for camping “rough” was -20 F in January in the White Mountains). The polyester “overbag” protected the down bag from moisture. And if I awoke too warm I could wriggle out of the overbag and sleep on top of it.
Both bags compressed down to two stuffsacks that were about 8" x 15" and far easier to stuff inside my backpack than the huge compression bag I had previously had to use for my -20 F rated down expedition bag that had to be strapped to the outside.
Plus I could take the overbag in my summit/day pack for excursions when we did not plan to sleep out overnight but could have to bivouac in an emergency – also as a safety backup if someone got hurt and we needed to enwrap them for rescue. I used the down mummy on its own for early Spring and Fall trips and the poly bag alone for summer backpack and canoe trips.
I liked the system so much I sold the big down winter bag to a guy who took it to the Andes and Himalayas with him. Being as I had sewn it myself from a Holubar kit (one of the companies like Altra and Frostline that made DIY gear kits in the 1970’s) that was a bit of a personal sacrifice.
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Once a down bag gets wet it loses its insulating properties and feels like wet paper mache. They are hard to dry out.
I agree that preening and oiling of feathers on a live bird retains the water repellency of the plumage.
I took a down bag to Alaska once to go over the Chilkoot Trail. We had 4 days of steady rain. My bag got wet, real wet. We camped near the top of the pass. Above treeline and no chance for a fire. Sleet and high winds. I was afraid to go to sleep. I was fearful of freezing to death. I got up at first light which was before 0400. It was August 31.
If you beat on it it dries out and fluffs
Hard to do in the field its why old canvas sneakers are useful in the dryer
I carry down sleeping bags excusively but they are double dry bagged
Got one wet once snd it was not a fun night
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The older solution was wool, which is heavy as hell but actually does retain some degree of warming value when wet. That small factoid is the reason I did not end up in a medical tent at a major rock concert when I was much younger. I wasn’t thinking too clearly the next day, but I woke up.
Good quality polyester fleece, like brand name Polartech, is a good backup for sleeping warmth. Close to wool in wet condition insulation on its own or as a sleeping bag adjunct. I have a couple of liner weight sleeping bags made of the stuff that unzip to be used as blanket wraps. The heavier 200 weight one also folds up into its own stitched flap for packing or to use as a pillow. Very handy in camp, though the stuff is vulnerable to having camp fire sparks burn holes in it. I take the 100 weight one alongin my carry-on whenever I fly anymore — I have been stranded for hours and even overnight in enough airports to always want to have somefhing I can sleep inside in my kit.
I use down bags for kayak camping and backpacking in the Rockies, just need to have tents that are storm proof, and mine are.
If you beat on a wet down bag nothing happens except breaking up the wet clumps of feathers into smaller wet clumps of feathers.
I use down sleeping bags in wet environments, such as downriver tripping, all of the time. But if the bag should become totally soaked, you are pretty much screwed. You won’t get it dry short of hanging it out in the sun for days or a large front loading drier.
Protecting a down bag from rain or from becoming soaked from immersion just isn’t that difficult. There are lots of options for waterproof dry bags, packs, and duffels that will work for that purpose. Set your tent up before you take the bag out of its waterproof enclosure and make sure that tents seams are well-sealed before you set out.
But on a longer trip in humid conditions a down bag can start to take on a certain amount of moisture from condensation. This is especially so if you use a tent with a single wall design. In my experience, a decent down bag will still retain its insulating properties even on a week long or longer trip, even when subjected to a moderate amount of condensation, however.
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I have used down bags almost exclusively as a backpacker. I do have a -20F rated synthetic that I have used for winter camping where it is calling for rain. A bit heavy though. I use my +20F down bag on most all my camping from kayaks and canoes. I have a cheap Walmart Ozark fleece rectangle that zips open like a blanket that I paid $20 for. I use it in warm weather with a microfiber sheet.
If you are going to dry your bag in the dryer with an old shoe , make sure your spouse is not home. Last time I tried it I go a lecture worse than when I seasoned my dutch oven in the oven and set off the smoke detector. 
I took a twin microfiber bed sheet folded it in half long ways and stitched the bottom and half way up the side. Works great in a fleece bag.
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Been using clean sneakers and down bags since starting canoe tripping in 1966.
The operative word is…clean.
My 19 year old grandsons white sneakers of a color now resembling a mold… no…
My wife uses something called “down bumpers” they are loud, but since the dryer is down in the basement it is not too bad. Our neighbor plays his drums in his basement… which is about as loud.
Clean didn’t enter in to it. It was the banging around. She was sure I was going to break the dryer.
I use a down bag virtually most of the time. I have a military goretex bivy that I slide it into. Also holds a self inflating pad. Cold weather, I zip it all up, other times when it’s warmer I keep it unzipped.
I prefer the wool felt dryer balls and use them for ALL my dryer loads because they fluff the clothes and linens as they tumble and they dry faster and softer. The softball sized balls are cheap (less than $10 for a package of 2 or 3 in TJ Maxx or Target). They do not make as much racket as sneakers or those hard plastic knobbed dryer balls and are not as hard on the fabric. I keep little dropper bottles of lavender, orange and almond aromatic oils on the dryer and sometimes put a tiny drop of one of them on the felt before running the load for a bit of pleasant scent.