Have you ever noticed that there are pairs of birds that look a lot alike but are different sizes? How about Downy and Hairy Woodpeckers? Or Lesser and Greater Yellowlegs. Long-billed and Short-billed Dowitchers. And Cooper’s and Sharp-shinned Hawks.
These pairs are called “sibling species” and are presumed to have been one former species in the past that split into two. Lesser and Greater Yellowlegs, North American shorebirds, inhabit the same geographical areas during breeding and migration. They make their living by probing the muck of wetlands for invertebrates. The Lesser Yellowlegs has a bill about as long as its head while the very similar but taller Greater Yellowlegs has a bill at least 1/3 longer. The Greater Yellowlegs eats larger prey like frogs and crayfish and skims the water’s surface in search of fish, which the Lesser Yellowlegs, eating smaller items, never does. The Cooper’s and Sharp-shinned Hawks share their looks and geography but differ in size, the Cooper’s being 25 percent larger. In addition, the average size of prey of Cooper’s Hawks, mostly medium-sized birds like robins and starlings is more than double than that of the prey of Sharp-shinned Hawks which prey on smaller songbirds. In India, four kingfishers in order from smallest to largest are the Blue-capped, Collared, Black-capped, and Brown-winged all live a mangrove habitat of eastern India, but they differ in food items and behavior – the height of their feeding perch, distance covered on foraging forays, and the size of prey, all reflective of the birds’ body sizes. The larger the bird, the bigger and higher the perch, the farther it flies to feed, and the larger the food items.
In his classic 1959 paper, G. Evelyn Hutchinson, often considered the father of modern ecology, posed an explanation as to why there are such differences in size and why there is a limit to diversity. Hutchinson proposed that there needs to be about a 1:1.3 difference in the morphology of birds; in other words, around a one-third difference in size between species. This is sometimes called the Hutchinsonian ratio. Lesser and Greater Yellowlegs and the Sharp-shinned and Cooper’s Hawks show approximately that difference between species. In the case of the Indian kingfishers, the largest is 1.25 times larger than the next largest bird which is 1.12 times larger than the third largest bird, which in turn is 1.5 times as large as the smallest bird. So it sort of became a rule of thumb that organisms that have similar niches can only coexist if they are about one-third different from each other, but the idea is still controversial.
There is a limit to diversity – there may be sufficient resources for only one kind of nuthatch, yellowlegs, hawk, or kingfisher, but if two or more similar species evolve a sufficient difference in size, they can both share a habitat with overlapping niches. But it appears they have to be different by about a third in one or more aspects. Other examples of similar species differing in size are the Lesser Spotted and Greater Spotted Woodpeckers, the Merlin and Peregrine Falcon, the Whimbrel and Eurasian Curlew and the American and Fish Crows.
What’s interesting about this ratio is that you can find similar differences in human-made objects. Examine the sizes of a row of terra cotta flower pots the next time you are in your local nursery or department store – every size is about 1/3 different than the next size. Same thing with sauce pans or ceramic casserole dishes; there may be something else going on here.