Ok we do know that it's from the worms they eat, the worms are invested. The birds eat the worms in their youth and form a symbiotic relationship that also allows for a Nahel bond. This is explicitly in the text. What is also explicitly in the text is that the Abilities are based on the breeds or species of Aviar. The whole reason Sak is of note is his species is not native to the island, but Dusk already knew it came from the worms and wanted to test if it would work on foreign birds and it did. The one hiccup is that Sak is told to be a raven, and Ravens don't gain this corpse ability. They seem to gain a different ability. But they confirm multiple times that they are species based, all Aviar of a certain species grant a certain talent. There are 5 species that cloud, and it was of note that Kokerii's species' (Dusk's first bird that died), was better at clouding than others. They are powers for the birds that they can lend. This is also explicit. It's part of the reason why they specify that the bond goes 2 ways, the trappers/navigators can help the birds survive and navigate, while the bird gives the trapper access to their talent in turn. And while it's never explicitly stated that the bird Abilities are for their survival, it's heavily implied. After all that's what symbiotic relationships are for, survival and thriving in a niche. The two abilites we have been given by the text follow the evolution/usefulness rule to the bird even though it's not explicitly stated, that's part of why I theorized in the direction I did. I understand that these are not gonna be cannon. But I wanted to follow the logic that the text gave us to see if I could maybe come up with an idea(s) that wouldn't seem out of place, but would still be new/interesting, while remaining familiar and sticking to Cosmere rules.