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Although they remain fundamentally the same, the flu virus is an RNA virus. This means it is very unstable and changing all the time. In the replication process it makes lots of mistakes.
If the 'misprint' changes the surface coat enough it can be unrecognizable to the immune system. This is called antigenic drift. and explains why Influenza comes around again and again. Flu is almost never the same and so the antibodies built up from past infections one year can fail to identify the new season's flu (though as long as the change isn't too drastic, we should have partial immunity to it and this explains why the 'winter flu' is a relatively mild disease in most people.
However, flu viruses have another trick up their sleeve. Every now and then , different flu viruses colonise the same cell. When this happens, their RNA can swap giant sections like people playing 'happy families'. So the virus swaps traits. If this reshuffling involves the genes that make the virus' coat this can have DRAMATIC consequences. This is called antigenic SHIFT. A virus that had the kind of coat suitable only for entering bird cells might in this way suddenly acquire the genes for a coat to unlock human cells. It has, in effect, jumped the species barrier in a single leap. If so, a new pandemic is on the cards.
In birds, until recently, the flu virus was fairly harmless. However, in pigs and humans the flu virus is not so harmless. The flu in pigs and humans is probably a relatively new disease (no more than 500 years old) arising as a result of the way humans, pigs, and birds became crowded together in farms and villages. When all were living far apart, an antigenic shift might have produced countless varieities of flu similar to the human flu, but they would have died out without a suitable host......with close quarters, an antigenic shift might readily find a host and survive to have offspring.
Because they are completely foreign to the human immune system, these viruses are highly pathogenic (they can kill their host)
It used to be thought that bird flu viruses acquired the ability to infect humans through pigs. Pig's lungs, unusually, can be infected by both human and bird flu viruses. It is thought that the pandemics of the 20th century began when genes were swapped in pig cells. Hence, the term 'swine flu'.
This theory is being questioned ans some researchers have suggestes that the 1918 Pandemic actually started on chicken farms in Kansas.
You should look into the enzymes neuraminidase and the protein hemagglutinin which make up the H?N?. Each have a different role to play in the ability to infect certain cells (human or otherwise).
Right now, the H5N1 is a pandemic among birds and related animals. It is still primarily a bird disease as it hasn't mutated enough to be able to infect the human respiratory tract easily. There have been about 300 cases of bird to human transmission and very few cases of apparently subsequent human to human transmission. All of these cases required extremely close contact with birds or extremely close and prolonged contact with someone who picked it up from a bird. (airborne, like the flu).
H5N1 MAY be the strain that makes the jump to easy transmission or it may be another of the Influenza A strains. We don't know. Though, H5N1, seems the most likely at the moment.
Hope this helps
EDIT: I forgot to ACTUALLY answer your question:
The mechanism of transmission from birds to humans would be through their feces or respiratory secretions (at the moment). If it were to become a pandemic in humans it would most likely be spread by aerosol (through coughs and sneezes, through mutual touching of surfaces, and the exchange of bodily fluids)
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