Although common here from roughly mid-September through mid-October, blackpoll warblers are very uncommon here in mid-August. Photo by Scott Surner
Birdwatchers make lists. We list bird species seen by day, week, year, or lifetime. We list birds seen in a yard, town, state, or continent. We compete and get a special thrill from finding a stray from far away.
It is a truism of birdwatching that attention to common species is key to finding rarities. The observer thoroughly familiar with the appearance of the locally common ring-billed gull at different ages and seasons is prepared to notice differences of a similar short-billed gull wandering here from the west coast or the equally similar common gull blown from Europe (the “common gull” is indeed common in Europe, but rare in North America). Familiarity with the biology, not just the appearance, of common species leads to even more rare discoveries.
In August 2020, New England birdwatchers were entertained by sooty terns brought from tropical oceans by Hurricane Isaias, and, a few days later, I saw an ovenbird recently fledged from its nest. Which is rarer? Neither the sooty tern nor the ovenbird, both with populations of tens of millions, is globally rare, and each is readily seen in appropriate habitat and season. Years can go between New England sooty terns, but they are expected after a hurricane. Ovenbirds, among the most common nesting birds in spring New England forests, are reliably found here May through September. Ovenbirds, though, do not normally nest in August; an ovenbird fledgling that late signals a rare delayed nesting. Early that August there were about three dozen sooty terns seen in Massachusetts and few, maybe only one, fledgling ovenbirds.
The previous August I saw an adult male blackpoll warbler in Belchertown. This species breeds abundantly across northern North America and is a common spring and fall migrant in Massachusetts. The individual I saw, though, was rare. Although common here from roughly mid-September through mid-October, blackpoll warblers are very uncommon here in mid-August. At the end of their breeding season, blackpoll warblers molt into a relatively drab feathering in which adults and young of both sexes all look similar. The bird I saw was unexpectedly still mostly in adult male breeding plumage. This individual of a common species, late in molt and early in migration, was doubly rare.
Rare behaviors sometimes seen or heard include singing by females of species in which usually only males sing or, conversely, incubation by males of species in which only females usually warm the eggs. Such events are noted as rare only if one knows typical behavior for each species.
A rare behavior much appreciated by birdwatchers is the venturing into the open of a bird normally difficult to see. A neck-straining treetop vireo or warbler visiting an eye-level open spot is a rare treat, as is a well-camouflaged rail of dense marshes that walks out into easy visibility. Another type of observer may note birds that stray from normal habitat — predators, too, take advantage of individuals that become atypically visible.
A December or January eastern phoebe (a flycatcher) is a rare find here, although the same individual would be commonplace in warmer months. Photo by Scott Surner
In winter, another kind of rarity occurs. Birds that would normally be hundreds to thousands of miles south of New England sometimes linger into cold weather. Insect-eating warblers or flycatchers need to find places such as warm water seeps where active insects occur even in mid-winter or switch to fruit or seed diets. A December or January eastern phoebe (a flycatcher) or common yellowthroat (a warbler) is a rare find here, although the same individual would be commonplace in warmer months.
A bird that lands on the wrong continent, shows itself in the wrong habitat, or breeds, molts or migrates at the wrong time has a relatively poor chance of surviving. To the extent that departures from normal have a genetic basis, the failure of such unusual individuals leaves their species genetically unchanged. Alternatively, odd individuals that live to reproduce may contribute to evolutionary change in their species — expanded ranges or behavioral repertoires. Rare events can thus affect evolution.
People have long noticed rare birds; the expression “rara avis” — “rare bird” — was used in ancient Rome for unusual events. The Roman poet Juvenal famously described a worthy wife as being as rare a bird as a black swan. Although the Romans might have prided themselves on a broad empire ruling the “known world,” there was much they didn’t know, including Australia and its locally common black swan. As Mark Twain commented, “The information the ancients didn’t have was very voluminous.” Today, too, rare birds may turn out to be less rare than originally imagined. Birdwatchers who study a secretive species sometimes learn to find it reliably, and behaviors once considered rare, after intensive investigation, sometimes turn out to occur regularly. The thrill of a rare bird, whether rare by range or by behavior, can go beyond competitive listing to a deeper understanding of biology.
David Spector (he/him) is a retired biologist and former board president of the Hitchcock Center for the Environment.
Earth Matters has been a project of the Hitchcock Center for the Environment for 15 years. HCE’s mission is to educate and to inspire action for a healthy planet. Our Living Building and trails are open to all at 845 West St. in Amherst. To learn more, visit hitchcockcenter.org.
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And of course that is why I, and I think most people familiar with wildlife, dislike the term “common” used in animals’ names. But there is also the question of what status we give to species at the limit of their respective “natural” ranges. There are many very common species listed at some level in Ontario’s endangered species legislation simply because they peter out in Ontario…they are not even rare, let alone endangered, just across that imaginary line we call the border…and yet “common” native species are sometimes killed to protect them in Ontario because that is mandated by the legislation.