Gender Roles on the Tundra

Here's a visitor to Waukegan Beach this morning:

Dunlin (Calidris alpina), Waukegan Beach, Lake Co, IL 5/7/2014

This is a Dunlin (Calidris alpina), in it's breeding plumage. It's a cosmopolitan shorebird, nesting in high Arctic tundra all the way around the North Pole, and wintering on tropical coasts all over the world. (1) As might be expected, there are quite a few subspecies around the world, with Pyle listing 5 that occur in N. America and mentioning 3 more that don't.

Identifying these guys to species is easy, with the long bill, rufous back and black belly. Figuring out if this one is male or female is much harder, especially if you're not sure which subspecies you're dealing with. Clearly the birds can manage it, of course, and if you're on the breeding grounds, you will usually find that the male is the smaller of a pair. This pattern is called reverse size dimorphism, since in mammals and birds its not the expected situation. In many other groups of animals (and even plants), though, females are normally bigger -- in spiders and some anglerfish, the difference can be truly impressive.

When we do find something odd, it's always interesting to figure out why it happens. In this case, reverse size dimorphism is quite common in shorebirds (sandpipers, plovers, etc.) even though it's rare in other species. One hypothesis for this is known as the Aerial Display Hypothesis, which argues that males are smaller because small body size enables more impressive courtship displays. Blomqvist, et al. used this species to test the idea, and found that, indeed, smaller males could perform longer displays with more hovering. (2) What they don't seem to have showed is that those longer displays led to more breeding success, but it's still an intriguing bit of support for an interesting idea. Owls, hawks, and falcons all show reverse size dimorphism as well, and in hawks and falcons, aerial displays are important both for courtship and for territorial defense. That suggests that this hypothesis could apply to more than just Dunlin.

(1) Hayman, P., Marchant, J., Prater, T. (1986) Shorebirds -- An Identification Guide. Houghton-Mifflin, Boston, MA. 

(2) BLOMQVIST, D., JOHANSSON, O. C., UNGER, U., LARSSON, M., & FLODIN, L. Å. (1997). Male aerial display and reversed sexual size dimorphism in the dunlin. Animal Behaviour, 54(5), 1291-1299.