In order to gather eggs you meed to build this building in the swamp, anywhere else will not work. Best way to get them is get a behemoth gate and make two rooms on either side using stone fence foundations and doorways, but do not place the actual door in the door way this tricks the game into thinking they are in an open space (if they are in a closed area they will not lay eggs). These doorways need to be three tall or the can clime over and escape. Once you get your cage, be aware when you drop them in they can still stick there heads through and bite you. Now that you have your snakes in your pit, leave out of render distance for about two minutes then come back, once you come back, there is a chance an egg was laid. Open the behemoth gate and lead the snakes to the other side then lock them in there by closing the gate. You can now safely retrieve your egg.
The dossier says:
"Found only in the dark caves of the island"
They are already wrong! It should say something more like:
"Found everywhere but the dark caves of the island, this creature will gladly ruin your day by jumping out and killing you as soon as you even think about going to the swamp. Nearly impossible to tame because it has to be tamed via passive which is very difficult to do. Worst of all you need to feed them fertilized eggs. Why can't it e a little easier? Up-vote of you agree!