Floaters

This past week has been spent worrying about one type of bird: the floater. The birds I’m modeling breed in a fixed number of breeding territories, and typically there are more birds born than there are territories to accommodate them. Because there are no territories for them to breed in, they don’t reproduce, but they still hang around, eating food, trying to steal mating territories, and accumulating mercury.

These floaters complicate things in a couple of ways. When there are open territories, who occupies them, floaters or newborns? Can floaters displace birds that already occupy territories? Of course, these things probably vary from species to species and from environ to environ. I’ve spent time coding these features into the model. Professor Cristol recommended that I ignore floaters in my model, but we want our simulations to be as realistic as possible. Up until last week, my model was set up as if each generation, all birds in breeding sites left their territories at the end of the year, and at the beginning of the next year it was total free-for-all for these spots. In a lot of species, though, a bird that occupies a territory one summer stays there over the winter and inhabits the same territory each year after until it dies. So first, I changed the model such that each bird that occupies a territory owns it until it dies. Then I changed the equations such that floaters and newborns compete for open territories left behind by dying birds; right now it’s possible to actually fine-tune what proportion of these spots go to floaters and what proportion go to newborns (and it doesn’t seem right now that that really makes a difference in the end result). Professor Lamar and I also came up with a simple way to have a certain number of the already-nested birds displaced by floaters; this is also controlled by a tunable parameter such that we can have all birds who nest keep their nest to the next year, no birds who gain a nest keep their nests the next year, or somewhere in between.

The real motivation behind this model is to study change in allele frequencies in a population. So, if we are studying a single population in isolation, I don’t think allele frequency changes too much whether nest sites are occupied by floaters or second years because they all come from the same population with the same allele frequencies. That is, it doesn’t matter in an isolated if the birds who are breeding are floaters or newborns because they are all equally likely to have genes for mercury tolerance. But if we broaden our perspective to include other habitats which also have floaters, then things may get interesting. Floaters don’t just hang around the same confined habitat all of the time; they go all over the map to find places to breed. So it’s possible that floaters who are born near the Shenandoah and have genes for mercury tolerance may float away to distant habitats where there is no mercury and start spreading genes for mercury tolerance there (which is bad). The reverse may happen, where floaters from uncontaminated areas may move into territories opened up along the South River, where their hatchlings are endangered by mercury. This question of floaters, then, is really a formal way of thinking about migration. That, I think, is the next step in constructing this model.