Stray Feathers From a Bird Man's Desk

I used to think that the battle of the sexes so ably portrayed by James Thurber was artificial, a man- and/or woman-made thing. But recently I've come to see it as old—probably as old as sex itself in the animal world.

Under the severe tide, "Secondary Sexual Characters and Ecological Competition," in a paper from the Bird Division of the Chicago Museum, I've outlined the possibility of competition for food, between the sexes, being a factor in evolution, responsible in part for characteristics of structure and traits that distinguish them.

In circles that discuss evolution the idea is current that food competition is important between species. It may even be stated as a rule: two species with the same food habits cannot live in the same place. Competition drives one out, unless they have different food habits. These differences seem especially evident when you look at closely related species, and they are accomplished in a variety of ways. A habitat difference is very common. The long-eared owl hunts in the woods—its cousin, the short-eared owl, hunts the meadows; the song sparrow favors the drier shrubbery while its cousin, the swamp sparrow, lives in wetter shrubbery.

THE SIZE FACTOR Sometimes the difference is accomplished by size; take the downy and hairy woodpeckers of our wood lots, very similar except that one is larger and is adapted for larger prey, the other smaller and adapted for smaller food items. Sometimes they feed differently, as the Baltimore oriole, which picks flowers and pecks through their sides, while the orchard oriole probes into flowers as they hang on the branches. Thus more individuals of several species live in an area.

When a pair of birds "sets up housekeeping" and starts "raising a family" they can no longer drift about, looking for easy living and places where food is plentiful. Their wanderings are restricted by having a fixed point, the nest, as their center of interest. Two individuals must draw on the food supply from an area about the nest. Competition would be extreme, and, if there were a scarcity, perhaps critical.

We know how different the sexes may be; how different the rooster is from the hen in our domestic fowl, or the drake and the duck in the mallard, or the red male and the green female of the scarlet tanager. These sexual differences have mostly correlated with display and mating. But logically there should be differences in feeding behavior and adaptations between the sexes.

The basic idea is contained in the old nursery rhyme:

The two birds of a mated pair, limited to a single area, could be expected to have different food preferences or adaptations for getting it. And we find that there are cases of this. The most striking is that of the huia from New Zealand, of which I've written in a Chicago Museum bulletin. Both sexes have similar food preferences, especially wood-inhabiting insects, but they get them in different ways. The male has a short, straight, stout bill for digging out the wood-boring grubs, woodpecker fashion; the female has a much longer, slender, and curved bill for probing into holes for them, creeper fashion. The female may get grubs in wood too hard for the male to chisel. They supplement each other.

DIET VARIATION BY SEX It is possible that further study may show more sexual differences to have a feeding advantage; the larger size of female hawks fitting them to take larger prey; the smaller size of certain female songbirds fitting them for smaller prey, the smaller bills of female hornbills, the straight bill of the male western grebe, and the upturned bill of the female. Perhaps all are of advantage to the species in giving each sex slightly different advantages in getting food.

Selection could have its effect in the populations with most sexual difference in feeding habits being most successful in raising and leaving progeny. Thus, slowly, differences between the sexes would accumulate. However, it must be kept in mind that this sort of evolution would be limited. The drifting apart of the sexes would be checked by the necessity for their coming together periodically for at least a short period, at nesting time.