Let me introduce you to a couple of foot tremblers. This may sound a bit odd, but bear with me.
The story starts on a trail in New Zealand in the southern beech forest. The twisted limbs of these evergreen trees each end in a modest tuft of leaves. The leaves are small, less than a half-inch long in one species, and the forest has an ancient, almost musty, feel. A modest number of birds dwell here, including yellow-fronted parakeets, mohuas, brown creepers (not ours), and riflemen, a mite of a bird reminiscent of our winter (now Pacific) wren.
On the forest floor there is a bird called the New Zealand robin. It looks a little like our American robin in shape and behavior but is smaller and dressed in varying shades of gray. The two are not closely related at all. The American robin is a thrush while the New Zealand robin is an Australasian robin — a group restricted to southeast Asia, south Pacific islands and Australia — and is more closely related to crows and ravens.
The New Zealand robin is a remarkably tame bird of the forest floor, where it seeks invertebrates among leaf litter. It will approach closely, hoping you will disturb some tasty insect with your feet. This familiarity allowed me to observe its feeding behavior in detail. As one bird moved in and about my feet, it would pause, shift its weight to one foot and begin to vibrate the other foot almost imperceptibly. The vibrations were very rapid. I had to look closely to be certain of what I was seeing. After a couple of seconds it would pause before hopping a couple of steps and repeating the effort. Occasionally it would stoop to snap up some tiny invertebrate that made the mistake of moving in response.
Here in the Rogue Valley, we have a different bird of the forest floor, the hermit thrush. It has one of the most beautiful songs of any bird, and it feeds on both insects and berries. When feeding on insects, it both pounces on them from low perches and hops along the ground in American robin fashion. In winter they descend from the hills to backyards and woodlands of the valley floor. The black breast spots and ruddy tail distinguish this thrush from similar species.
On a frosty morning a week ago, I watched a hermit thrush foraging in my backyard. It moved deliberately about the edge of the lawn and among the litter under shrubs. Remembering the New Zealand robin, I watched closely.
Like the robin, it shifted its weight to one foot while the other foot subtly trembled. After a few moments it hopped a few steps and repeated the behavior, snapping up flushed invertebrates.
Remarkable. A half a world away an unrelated bird displayed the same behavior as the New Zealand robin. They have both arrived at the same strategy for finding food.
Similar habitats produce similar behaviors to exploit similar resources. This is nothing new to an ecologist, but the duplication of this specialized behavior is still quite impressive. Except for a friendly New Zealand bird I may have never detected the same behavior in the hermit thrush. I wonder what other behaviors I am missing in my backyard.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.