Adjectival form: migration strategy or migratory strategy?

They are equivalent. Migratory is an adjective meaning (in this use) "relating to migration" while when used as an attributive noun the noun migration has the meaning "relating to migration".

As such either is correct. Reasons one might want to favour one over the other include:

1. How commonly which is used. Along with your paper count, consider this:
2. Other possible readings. Since migratory also has a meaning denoting an animal that migrates, it's possible that someone might think of it only in that sense and read it wrong. (This seems very unlikely, I'm mentioning it a a general point, rather than specific here).
3. Subtle nuance from such things as one word being Latinate when the other is Germanic (equal here), one being shorter and punchier, etc.
4. Style guides explicitly favouring one approach over the other.
5. You just like one more than the other.

Point two is the only one with a clear and objective difference in this case (I can't speak for how you'd answer point 5 and don't know which if any style guides you are using). Just how important this is varies. Generally, if the rarer form is still understandable, then it's normally perfectly fine to use it. However it can be the case that a given use is so common as to almost form its own well-known compound meaning, in which case it's to be preferred when that meaning is appropriate but just as much so to be avoided if that meaning is in fact not quite what you are talking about.