After spending several hours trying the AFK method in Elder's Recess, and quick traveling between camps in Rotten Vale, I finally found one in area 15 of the Vale. What worked for me was checking both Mosswine in Rotten Vale on an Expedition, leaving and repeating. Found it on my third go with this method.

I got mine from Rotten Vale because it got stuck on the wall. I didnt even know of its existence, so I thought "The hell is that noise?" and saw this ball of fur (or more like feathers) spazzing out.

All you are just lucky so far that little ahole nowhere to be seen and allready spent multiple hours teleporting, returning afking. This is pure rng bull*****and I everybody else who is fked by it.

It can also spawn on the Raider Ride Gastodon that can be summoned in the Recess, just happened to me

I've spend about 5 hours jumping between Rotten & Recess ( in expedition) without any success, however, after midnight I tried 4 times the optional quest "Lavasion, Monster of lava" 7* and the first 2 had the bird in the same group of Gastodon. There might be a limited number of birds you can see per day.

With iceborne is there really any trick to increase your chances? Any special event quest or new route? I'm just curious, something similar to the grandfather magrel downy crake quest.

Some advanced players told me to get all the cooking ingredients, then you can always get zookeeper buff from group vouchers in tbr canteen. Its took me over 10 attempts, but I found on rotten vale

I actually heard a Bristly Crake chirp while I was near some Gastodons in the Guiding Lands!

I just caught one of these for the first time and it was a giant gold crown, so thats epic. I named him Manuel.

just in case this helps anyone: i was listening for the same chirp as the downy crake. these make weird, weird noises that are different than bird chirps. lol. but i've caught one on a kestodon in the recess and one in the honey cave in rotten vale so far, the rotten vale one was during the acidic glavy mission and the recess one was just on an exp during sunrise iirc