Also, bird migration patterns are changing, therefore they must also be socially constructed.

When you use "socially constructed" to mean "influenced in any way at all by society," it becomes meaningless. Its strong meaning, "entirely constructed by society with no natural basis at all," is a useful idea.

I don't know that any serious person has claimed that attitudes about weight and homosexuality are unchanging human universals. Historical literature shows a variety of attitudes to both. Societies around the world have a range of attitudes. A post-modernist who claims to have discovered this idea is being disingenuous.