Heisig says this in RTK 3 about Owl (梟):

"Note that the 'tail feathers' of the bird disappear (or are replaced by) the
tree. This is the only occasion in the kanji in which the element for
bird is so altered."

But hasn't the bird primitive been altered that way in Island (島)?

I believe that "OWL" and "ISLAND" are the only two that vary the "BIRD" character with the fire excluded.

Anyway, if you want to remember these variants, here are my stories for it if it helps.

"Owl" - "Owl (Bird)" blows out the "fire" to take nap on "tree"

"Island" - "Bird" puts out the "fire" on mountain on tropical island, then flew to it.

Perhaps it is the difference between replacing the four strokes with mountain (Notice how in 島 how the mountain is wrapped by the remains of tail feather?) whereas in 梟 the tail is more so scrunched up and lets the bird seemingly 'rest' on top of the tree element.

[ファブリス]

lankydan Wrote:"Owl" - "Owl (Bird)" blows out the "fire" to take nap on "tree"

Ohhh.. nice one!

My "island" story is more like an image, and worked really well : I just imagined the tip of a mountain peaking through clouds, which is the only part a bird flying high sees... and thus the bird at the top of the mountain thinks it's an island (well I guess birds aren't that dumb but the image works anyway )

烏 - crow is also a variation of bird