These birds as all know carry out their annual migrations to various human populated locations around the world.
So the following scenario is possible:
1. Humans keep relying on non-renewable sources.
2. Use of these sources leads to rapid greenhouse emissions as is the case at the moment.
3. Sunlight gets blocked and the north and south pole start feeling the heat.
4. Ice melts.
5. Frozen influenze strains from years and years ago come to life, ready to get in contact with a living creature.
6. Bird A comes along doing its usual thing.
7. Bird A catches the strain.
8. Bird A thinks about migrating, kinda getting cold here it thinks.
9. Flies to one of the unlucky countries, I'm guessing Mexico.
10. Takes a dump on one of the pedestrains aka Patient 0, boom.
11. Pandemic starts.
So people, another damn good reason to start acting in an environment friendly manner and engaging with groups that are supporting the environment.
This Frankenstein-like story can really come to life.
Full source at: http://www.wired.com/wiredscience/2009/05/iceplague/