So two theories: The seeds stick to birds and the birds fall to the ground, scavengers find them and eat them, the seeds then go through the scavenger's digestive tract (which presumably cleans off the glue) are deposited by the scavengers nearby, but not too close (scavengers wouldn't leave scat at such a great feeding site).

Another theory is, the seeds evolved to stick to branches to be carried across the ocean, but as a side-effect they stick to birds.