I've been lucky enough lately to see a number of monarch butterflies -- or one of their imitators, which I can't keep straight from monarchs nor can I keep straight which kind of mimics they are. I enjoy seeing any butterflies, which is why it pains me that I see them so rarely in my own yard. Despite nearly zero pesticide use & plantings of all sorts of host and nectar plants, neither this house nor the previous one has seen many butterflies -- lots of dragonflies & bumblebees (and far too many mosquitos), but no butterflies.
Monarchs are amazing creatures on many scales (including their own scales!), but perhaps most amazing is their migration -- each year they schlep off to Mexico for the winter. Most amazingly is the fact that the monarchs which fly south for the winter clearly are homing in on a location they have never before visited -- it was their ancestors a few generations back who flew back. How they do this is still being worked out, but clearly the core of the guidance information must be inherited. Environmental triggers are apparently critical as well; the Wikipedia article notes that monarchs which have taken up residence in mild climes such as Bermuda do not migrate.
I once had an amazing monarch experience. We were going into the city one fall day, and I noted on one of the parkways a number of monarchs flitting across. While we waited for the Orange Line at Wellington, monarchs seemed to pass down the track at a rate of one every half minute or so. For once I didn't mind the long wait for a weekend train. Perhaps it should be rechristened the Orange&Black Line?
As I mused before, one interesting question is how structured are these populations. Are the monarchs I see this year mostly descendants of monarchs who summered here last year, or is everything scrambled? Of course, my solution to this is simple: sequence! With sequencing cheap, one could survey a lot of monarchs (perhaps from museum collections) to find a pool of polymorphisms, which could then be typed on even larger numbers of specimens using chips, directed sequencing or other SNP typing methods. One pleasant side-product would be a draft genome of the monarch.