The process of evolution is infinitely creative; the diversity of wildlife, extant and extinct, never ceases to amaze me. But nature also knows a good idea when one turns up, and a great deal of the variety in the natural world reflects not so much fresh innovation as variations on a theme — a basic idea that proves successful and radiates into a number of species that share basic similarities.
This process can be evident at all levels of classification. For example, a dozen or so flycatchers of the genus Empidonax can be found in the U.S., and they are so closely related that even veteran birders have a hard time telling them apart. Some cannot be reliably identified just on the basis of appearance and must be either examined in the hand or heard singing (their songs are quite distinct) if you want to know for sure what they are.
Other examples of a natural variation on a theme are less subtle and can be much easier to appreciate in the field. One such cluster of species is currently at its most numerous and most diverse around the Vineyard. The waterfowl — swans, geese, and ducks — are one of the larger orders of birds, and have colonized all of the habitats on the globe. Evolution has worked with enormous creativity on the basic idea of waterfowl, producing a wealth of specialized and distinctive forms.
At the most obvious level, the waterfowl we see on the Island vary dramatically in size. The mute swan is a massive bird by anybody’s estimation, with large males weighing around 25 pounds. In contrast, the green-winged teal, one of the world’s smallest ducks, weighs in at just over half a pound. These birds are recognizably related, with similar body shapes, webbed feet, and broad bills. But in keeping with their size difference, they occupy very different niches in the environment. Swans inhabit large ponds, relying on their long necks to forage on aquatic vegetation even in fairly deep water. A green-winged teal, in contrast, prefers sheltered settings and is much more inclined to feed on floating plants or shoreline vegetation.
Somewhat less apparent are the diverse forms that waterfowl beaks can take (and the related differences in the diets of the possessors of those beaks). At one extreme, the mergansers have evolved thin, elongated, and slightly flexible bills, which they use to catch small fish and invertebrates. (Catching fish with a merganser’s bill must be like catching fish with chop sticks, but it works fine for these ducks.) Red-breasted mergansers can be one of the more common ducks on our ponds and bays, and the elegant hooded merganser, though less numerous, is also easy to find on the Vineyard in late winter.
If mergansers have the daintiest bills among our waterfowl, the eiders and scoters have the most robust. An eider’s bill, in particular, is an impressive piece of equipment, like a pair of heavy pliers mounted on an equally robust skull. Optimized for eating shellfish, especially mussels, eiders dive to their prey, wrench the shellfish off the rocks they grow on, and swallow them whole.
Many ducks fall into the middle ground between these extremes of size and beak design, possessing broad, flat bills that we rightly think of as typical for ducks. Mallards, teal, scaup, widgeon, and many others deploy bills of this design in pursuit of a generalist’s diet: such a bill is suited for everything from grazing on young vegetation to snapping up small invertebrates. Despite their similar size and structure, these birds all have their own habitat and diet preferences, with particular species preferring either salt or fresh water, diving for prey or feeding in shallow water, and subsisting mainly on plant or animal matter.
Most waterfowl are migratory, meaning that the season influences what species are present here. Relatively few waterfowl nest here: mallards, black ducks, Canada geese, mute swans, and green-winged teal being the most likely. Others, including eiders, scoters, and mergansers, winter in our waters but breed farther north. In late winter (and again in mid- to late fall), breeding species, wintering species, and a number of others that pass through but don’t linger can all be found at once. In early March, a birder can generally find a dozen or 15 species of waterfowl on the Island, and with diligence and a little luck, spotting 20 in a day is possible.
As always with birding, the key to finding all the species present is looking in all the habitats they may occupy, from sheltered freshwater to the open ocean. Or, to put it another way, you need to understand what talents and preferences nature gave to these birds, as their basic duck-ness diversified into the variety of sizes, shapes, and habits observable today.