I enjoy drawing out my worlds, adding in the important and unimportant landmarks, nations, cities, mountains - everything. For me, it helps a lot with my writing if I have an actual, physical representation of my world. That, and it's fun. Sometimes I'll create a world for no other reason than to do so. Some of these worlds have made their way into stories I later wrote, or been matched with stories I'd previously written.
As mentioned above by others, there should be a limit to how much you create. For example, the question about migratory patterns: does the migratory pattern of swallow have an effect on the plot? Do the characters follow this path for whatever reason, is there a swallow that has something they need, etc.
The detail to which I go into is a bit more than most, I think. I like my worlds to seem real. I've researched things on continental drift and the effects of local climate on certain terrains, etc. Some of this stuff is self-evident and doesn't require research - rivers follow most direct downward path they can, for example. However, I did not know until recently that deserts are created by rainshadows caused by nearby mountain ranges preventing rain from reaching the area as much as it does more fertile land. Now, because of this, I tend to cringe when I see maps that are environmentally inaccurate, and sometimes can cause my suspension of disbelief to waver. (If there is something inaccurate, though, like a river flowing uphill, it will stand out. This can be used to the advantage of the story.)
I've sort of lost my train of thought... basically, if it's important to the story, develop it. If not, it's not completely necessary - but it can still be fun. Is it especially important to you? If not, I think the diagramming idea mentioned above is a great one. If it is, draw it out to as much detail as you want.
Just beware of World-Builder's disease...