Saturday, June 18, 2011

Convergent Evolution

Bird #1
Bird Number 1: Sitting by my river, I watched him working the opposite bank, teetering and bobbing as he went from stone to stone. He is a brownish bird, with a white breast and prominent dark spots on his breast. He is seldom found far from water, but it can be a river bank, a tiny creek, a lake, or a muddy pond. I remember seeing him one time when I was walking a friend’s farm in Pennsylvania. He stood ankle deep in a muddy puddle surrounded by dense brush and shaded by the forest canopy, bobbing his tail with each step. The one I was watching from my chair by the river suddenly took flight, his “peet-weet, peet-weet” giving me the signal to follow as he flew upstream to the next big rock where bowed repeatedly in my direction.

Bird #2
Bird Number 2 bobs and teeters, just like Bird Number 1. Bird Number 2 is larger than Bird Number 1: 8.5 inches verses 7:5 inches. Instead of the prominent spots, Bird Number 2 has a faint brown band around the neck and a white breast. It also is a bobber and teeterer. Both of these birds are tail waggers. The tail moves busily up and down as these birds perform their bowing gait. I see Bird Number 2 in puddles and ponds during May. It only stops in our neighborhoods very briefly before continuing its migration to the northern reaches of our continent to breed.

Bird #3
I saw Bird Number 3 was on the edge of a beaver pond in Stratton town. At 6 inches, this bird is smaller than the other two. But, it looked remarkably similar. Its back was dark olive-brown, its breast streaked and its sides buffy. The stripe above its eye was also buffy. The source of its folkname is obvious as I watched it move along a prone log: this was the “water wagtail.” However, the difference between this bird and the first two is most apparent when it sings: a loud, ringing, “twit twit twit sweet sweet sweet chew chew chew” carries through the forest, rivaling the winter wren for the power and clarity of its song.

Bird #4
Bird Number 4 is a Vermont breeding bird, like Number 1 and 3. In our neighborhoods it likes flowing streams, leaving the standing or sluggish waters to Bird Number 3. Bird Number 4 is almost identical to the Bird Number 3, except the sides and the eye stripe are whiter. Like the previous birds it wags its tail incessantly as it walks along the damp stream edges. Its folk names is also “water wagtail.” When he finds a singing perch he lets loose with three clear slurred whistles and a jumble of twittering notes.

So what are these four similar looking birds who have similar habits (tail wagging and bobbing), and very similar habitats?

Bird #2 - Spotted Sandpiper
Birds 1 and 2 are sandpipers, the Spotted Sandpiper and Solitary Sandpiper, respectively. They belong to the order, Charadriiformes, which includes the sandpipers, plovers, phalaropes, gulls, terns, alcids, and other birds closely associated with the water.

Birds 3 and 4 are waterthrushes - the Northern Waterthrush and Louisiana Waterthrush, respectively. They belong to the order “Passeriformes” - the perching birds. If you are able to watch either waterthrush for a period of time, you may occasionally see him leave the downed log or the damp pondside and sing his resounding, musical notes while perching on a low tree branch.

Bird #4 - Louisiana Waterthrush (J.J.Audubon)
Given their appearance, you might then conclude that the Northern and Louisiana Waterthrushes are related to the other thrushes, like the Wood Thrush or Hermit Thrush. Their names certainly suggest such a relationship. Alas, that is not the case. They are wood warblers, disguised as thrushes and with an extreme fondness for water, like sandpipers.

Confused? Don’t be. Classifying species is an art form masquerading as science, although in fairness to the biologists who try to do the classifying, DNA testing is making species classification much more scientific that it use to be.

Though not related, these two sandpipers and two waterthrushes occupy similar habitat, have similar habits, and are similar in appearance. “Convergent evolution” is the term used when unrelated species have, through time, come to look alike, think alike, and live alike.

In a very broad sense, the development of the forearm into a wing in bats and birds is an example of convergent evolution. In the Northern Hemisphere there is the auk family (e.g. Atlantic Puffin) while in the Southern Hemisphere there are the penguins with a similar life style and similar habitats - another example of convergent evolution.

Along our streamsides and ponds we may be able to see convergent evolution up close in the resident Spotted Sandpiper and the transient Solitary Sandpiper, and in the warblers which don’t look like warblers: Northern Waterthrush and Louisiana Waterthrush.

Bird #3 - Northern Waterthrush (Belize)
Edward Forbush, the early twentieth century Massachusetts ornithologist, provides this description of the northern waterthrush: “Watch him now, and see how prettily he walks, rustling among the fallen leaves where he threads his way like a mouse, or wading even up to his knees in the shallow miniature lakes, like a Sandpiper by the sea-shore, all intent in quest of the aquatic insects, worms, and tiny molluscs and crustaceans that form his varied food. But as he rambles on in this gliding course, the mincing steps are constantly arrested, and the dainty stroller poises in a curious way to see-saw on his legs, quite like a Spotted Sandpiper.”

Bird #1 - Spotted Sandpiper (non-breeding)
But lest we think that the Northern Waterthrush and Spotted Sandpiper are just imitations of one another, Forbush writes about the latter: “The Spotted Sandpiper swims and dives readily. It can dive from the surface of the water or from full flight, at need. Under water it progresses by using its wings, which it spreads quite widely, and in shallow water it can go to the bottom and run a short distance with head held low and tail raised like a Water Ouzel or Dipper.”  Spotty and the American Dipper is yet another example of convergent evolution.

Learning about the habits and relationships of the birds can be very illuminating. Watch the closely and see what you notice. Start with the birds at your feeders. Good Birding!

4 comments:

Bill S. said...

Very interesting post. It probably happens in people and animals as well.

eileeninmd said...

Great birds and photos. Very nice post, Chris! Happy birding and have a great weekend.

forestal said...

Wonderful post Chris, very informative.

dan

Kay said...

Fun quiz! I didn't realize the Spotted Sandpiper dives in the water. Interesting.
Kay

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