Situation: A terrestrial environment on a megaworld (artificial construct). I'm thinking of how invasive species tend to dominate the ecosystems they move into--how about the ultimate invasive species?
The westbird really likes to go west. It has no territory, it is a parasitic egglayer so it doesn't need to nest. It is a soaring hunter by preference but it can live on vegetation if it has to. It keeps moving west so it's always moving into new territory whose local life isn't adapted to competing with that variety of westbird.
Given the size of the world pretty substantial evolution will occur before a given westbird's descendants complete a trip around the planet so they never actually return to an environment. Of course there are more westbirds but westbirds at a location at different times have no more genetic access than populations in different places normally do--thus they will be somewhat different, providing an ever-changing enemy for the local flora and fauna.
The weakness of the westbird is that it must be viable in all substantial environments because it will pass through them in it's eternal migration. Harsh environments will no doubt kill most of the westbirds that cross but the survivors reproduce rapidly when they reach a more hospitable place.
The big concerns I have are massive deserts (think what happens to a westbird that encounters a mega-Sahara) and large oceans (although I think this can be covered by making the birds not age so they don't die out when they hit water they can't cross in a lifetime.)