Help! Muscovies having horrible hatch rates

We have let 4 nests go this year, our first year with the duckers laying, hoping to have a few more ducks for eggs, get a few drakes for the table and test the girls out to see who plays the best mommy. I've been candling the eggs on a regular basis and could see the ducklings developing, the air sac expanding, etc, but as they neared hatching day, they all slowed down to a crawl and almost all have died before the internal pip so far. Something is obviously going wrong here but I can't figure it out Could really use some advice

So the details:
The first nest was Egghead's, our first layer this spring, with 20 muscovy eggs. She hatched two after apx 45 days and "got rid of" all the rest, one by one, as they went bad.

The second nest was Blondie, 20 eggs including 5 fertile campbells I had purchased on ebay. Around day 40, she hatched one of the campbells overnight, which we found dead before the next morning, then the next week hatched one muscovy which is doing well. She kept sitting for another week and a half, but all the rest eventually went bad and had to be disposed of.

The third nest was a mess - started with Patch sitting, then Surrogate (aka "Mommy too"...can't tell how she garnered her name, can ya?) moved in and started sharing, and a few weeks later, Brownie decided she wanted to be broody and forced her way in. After a week of constant fighting over who was going to sit on the eggs, and at least two eggs being knocked out of the nest and broken during these scuffles, we separated the ducks to find that what had been 20 eggs including the other 5 campbell eggs mixed in had become 38 eggs with none of the campbell eggs. Almost half weren't even starting to develop yet! We separated Patch and Surrogate into two nests, Patch taking the more developed eggs, and kept the other ducks locked out so no one would interfere or lay new eggs into their nests. None of Patch's eggs made it - all went dark and stinky within a few days and had to be disposed of. Surrogate is still sitting on her half of the nest but has been getting rid of one rotten egg a day the past few.

The ducks are free-ranging (plenty of slugs, grasshoppers, clover and misc greens available) with evening feedings of layer pellets when we put them in for the night. We currently have two drakes left of the 4 we had at the beginning of spring, when the girls all started laying. There's definitely been plenty of mating going on, both consensual and not. The 2 first ducklings that hatched are already beginning their first molt and look healthy and strong - Blondie's hatchling is smaller than she should be still imo, but looking healthy and starting to take on some independence finally. The ducks definitely sat on those eggs day and night - they barely came out to bathe in the ponds, and only on warmer days when the eggs wouldn't go cold on them. Temps since the beginning of June have been nice - coldest was maybe upper 40s with hottest into low 90s and most days being in the 70s and low 80s.

I thought at first that maybe it was just the wonky back and forth, hot and cold spring season we get, but once June hit and the weather balanced out, the next hatch still went terrible. Then I thought that maybe it was because they had just started laying this year, but all of the campbell eggs failed as well. Again, because the campbell eggs failed too, it can't be inbreeding causing the problem either. They get plenty of clean water and fresh food, and have a very low stress lifestyle (which consists of lounging around in the sun, bathing, eating their favorites, then bitching at us to feed them half the afternoon).

What the heck are we doing wrong? I'm at a loss. Out of 60 eggs over 3 nests, I can't imagine that 3 ducklings is "good". Waiting to see on this last nest but even that's not looking great at this point. Getting desperate here!

I'm not a duck person, so take this for what it's worth, but is 20 a normal clutch size for ducks? For chickens, that's too many. They can't keep them all uniformly warm and tended to.

They start developing, so you know they're fertile, and it sounds like your hens are sitting pretty consistently, so trying with a smaller number of eggs would be my best suggestion. If nothing else, it's one more variable you can rule out, if the same thing happens.

Interned on a farm with Muscovies last summer.

A few things we did:

A) Enclose the broody mother, alone, once she has a reasonable number of eggs; best done fairly promptly to try and keep the egg age reasonably close.

B) Identify which ducks are good brooders/mothers, and only let those ones sit. IMO the bad brooders/mothers are best eaten, so that eggs of theirs won't be around for the better brooders to hatch out, and thus your ducks should in theory be improving as the generations go...

C) Up to ~12 eggs per brooding duck. I've read that the max for Muscovies is 16, which seems reasonable for such a sizable bird.

Despite the above, we had varied success; the first couple ducks that sat hatched out 7 and 9 of their ~12 eggs. Later on we had some destroy all their eggs, or only hatch out 2... However, the process of identifying the good mothers was still under way; hopefully this year things have been more consistent, but I haven't heard.

Thanks to both of you - we were reading on how many eggs to let them run with earlier this year and found some were saying 16 to 20 while others were saying up to 25 was ok. We even saw reports of some ducks hatching out nests of 28 to 30! Didn't really see any recommendations of less so we went with 20 per nest. Will definitely try with less eggs per duck next year because this was a really, really disappointing experience (not to mention really stinky). I personally don't want to go through that again, nor do I want to subject the duckers to weeks of sitting for such little gain as it seems to be pretty rough on them as well.

In my meanderings last night, I did come across a page with troubleshooting for incubators that referenced humidity levels in the last 2-3 days before hatch - specifically how humidity being too low or high could kill the chick/duckling, either making it difficult to pierce a dried out, leathery membrane into the air sac resulting in suffocation or causing the duckling to drown due to water accumulating in the air sac. Where the ducks have been exposed to the elements a bit, under cover but not in any sort of climate controlled setting, I'm wondering if that might have had something to do with it all as well. High humidity levels for weeks at a time is sort of a given here during the late spring and early summer. Not sure what I could do to help with that, but if that's a likely culprit some brainstorming on normalizing humidity levels might be in order.

After watching feral muscovies hatching out 12+ ducklings every spring in Florida on the roofs of camper trailers, fully exposed to the elements, seeing only a 5% hatch rate from my girls seems unnatural and therefore my fault.

Some advice .... leave them alone. Pulling eggs from under ducks and candling them, messing about in the nest, it is not natural for ducks to put up with that and it interfers with setting. It also deposits skin oil on the shells. Try some fewer eggs. That way natural setting will allow you to check which of them does the best job and you can select based on that. Sure we all want to know if things are going well and that works great in an incubator where nothing will be disturbed except the level of humidity in the incubator. When settling down for the home stretch, humidity is very important for a successful hatch and the duck knows the best way to maintain it.

We fence off the broody ducks once they have about a dozen eggs, and as suggested we also tend to leave them to it. Often you can see the more succesfull mothers covering up the nests when they leave.
Also the material and location of the nests seems to have some impact (we're putting in some boxes for this reason).

We're officially done for the season as of today - Surrogate, aka Mommy Too, had been sitting on those last eggs what seems like forever and was down to just 4 left that she hadn't tossed herself, all of which had gone dark and stinky. We took them away from her today.

From the pointers in this thread and several hours of google searches, the issues we had were almost certainly three-fold: high humidity/moisture problems from poor nest siting, too many eggs per nest, and us "stupid humans" messing with things. Because the roof of the shed/duck house we set up last year had performed so poorly and let too much water get to the cordwood walls, we had to tear everything down to rebuild which meant the ducks all had to combat more extreme temperature swings and moisture. The nesting material (hay) was even molding at times which definitely says a lot. Add in people messing with things, touching and moving eggs, disturbing their routines and I'll have to just be happy we got the three ducklings we did this first time around. 3 adorable ducklings are better than no adorable ducklings any day

Assuming things go well with the new shed and duck house build, the ducks will have MUCH better temperature and humidity control for next season's nests. We'll definitely be limiting the number of eggs to 12 per nest and will try to stay completely hands-off once the brooding begins, only interfering when absolutely necessary. We've learned from this season that Blondie and Egghead both beat the odds and hatched something from their nests, and with Surrogate showing such discipline, it would be a shame to not let her go broody again next year, so these will be the ones let to go broody. Others, like Patch and Brownie, were off the wall - screaming from dawn to dusk for food, not bathing properly, abandoning their own nests to steal another's....not going to let that happen again!

Thanks again to everyone for the help on this. Will have to update this thread next season with updates, and hopefully good ones!