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I think you mean humans do not catch flu from birds.
However, pretty much all flus are "Bird flus."
They usually come from China because in rural China humans typically cohabitate with pigs around deep water "Ponds" where different species of fish live at different levels, and where birds come and pigs drink/bathe etc.
Pigs have the ability to contract flu from both humans and birds, and also to pass it back to humans.
The human and avian flu viruses can get genetically entangled in the pigs body, coming out with a hybrid flu that may have originated in birds but can now be contracted by humans.
No evidence
There is no evidence bird flu is currently more easily spread human-to-human, although there is always a chance the virus may evolve to more easily transmit among humans. Most human infections with bird flu can be traced to unprotected close contact with ill or dead animals.