I'm trying to find a short story. Here is what I remember about the plot:
- Shapeshifting aliens try to invade planet Earth. They have tried multiple times in the past, but every single time they lost contact with the attack team immediately after it landed on Earth
- The story is told from the point of view of one such alien, part of a later invasion force
- He lands on Earth, and notices the extreme biodiversity of the planet (compared to his own)
- He soon figures out that the previous teams were not dead. They just found a niche in the ecosystem in which they could be an animal (or a plant, or anything really) that had the kind of life they always wished to have
- However, this particular alien is a pilot. He loves flying. There is nothing in our ecosystem (or so he thinks) that could help him with this
- The story ends with the Earth army storming the hiding place of the alien. They find nothing, other than a large bird (a goose?) slowly flying away. It looks like there was a niche for our particular alien, after all...
I read this story a long time ago (more than 10 years), but it is probably older than that. I have no idea about the original publication date. Most likely, I read it in one of Asimov's anthologies, but I don't know if he was the author or just the publisher.