By BILL CHAISSON
By Bill Chaisson
After spending some time looking through NH Birds online, especially this time of year, you could be forgiven for asking, “Why do field guides bother with range maps?” Apparently birds do not read field guides and show up wherever and whenever they feel like it. Recent sightings in New Hampshire include a Tennessee warbler at Gilmanton, a black-throated blue warbler in New Castle, a rose-breasted grosbeak in Pittsfield, and — this is truly strange — a white-winged dove in Concord.
The first three can be explained as migrants who are merely dawdling to enjoy the unseasonably warm weather of the past week (apparently choosing to ignore the snow before that), but the dove is a species that most New Englanders know from a Stevie Nicks song, not an observation in the wild.
The white-winged dove (Zenaida asiatica) shares the same genus with and resembles our common mourning dove. It is bulkier, with a fan-shaped tail and, true to its vernacular name, has large patches of white on its wings. These are also visible as broad white bands along the bottom of the wings when the birds are perched.
Though largely confined to the desert Southwest, there is also a population in south Florida. David Sibley’s range map for this species shows green dots up the east coast all the way to Newfoundland. This has been a busy and prolonged hurricane season in the southern U.S (we are now well into the Greek alphabet when it comes to naming the storms), which may have blown some Texas or Florida birds northward. Birdwatchers are perversely enamored of hurricanes because they sweep exotics before them or drive pelagic birds shoreward as they barrel up the Gulf Stream.
Non-hurricane, higher-latitude storms in the North Atlantic are also known to blow European birds across the pond. Earlier this month birding circles were set in motion by the appearance of a common cuckoo in Snake Den State Park at Johnston, Rhode Island. The Providence Journal article of Thursday, Nov. 5, featured a beautiful photograph of the wayward bird, leaving its identity quite unambiguous. Unlike the white-winged dove, the cuckoo is a strenuous migrant, changing hemispheres on a seasonal basis.
The published photographs show the lost cuckoo to be a juvenile, which somewhat explains its diversion from its normal path to southern Africa. While many migrating birds have organs in their heads that are sensitive to the lines of force in the Earth’s magnetic field, experience is also a factor in successfully completing a migration. This is a lone migrant; so it is unlikely to extend the range of its species around the Northern Hemisphere. As it is a brood parasite, most people hope it won’t.
A famous storm-tossed range extension is that of the cattle egret. Bubulcus ibis is a widespread species in the Old World, found from sub-Saharan Africa to southeast Asia with other populations scattered through the Mediterranean and the Middle East. In 1933 an entire flock of cattle egrets went off course during their migration and crossed the Atlantic to the Guianas on the northern coast of South America. The species had been sighted there as early as 1877, but in the 1930s it began breeding and then extended its range westward along the coast toward Central America. By the 1960s it was established across a wide swath of North America.
It is not known to breed in New Hampshire, but it has been seen here since 1948. It is now spotted regularly in the spring and more irregularly in the fall. Its North American numbers spiked in the 1960s, but have declined by 50% since 1966. In spite of the decrease in the North American population, the species continues to extend its range, both here and elsewhere in the world.
The distribution maps we see in bird guides usually show range polygons with hard edges, but the maps at eBird show you a truer picture. They are shaded to reflect the density of sightings and the color varies with the time of year a species is found in a given part of its range. But even these maps are mere snapshots. They show the statistical nature of distribution in space and eBird also has very cool animated maps that show the cyclical advance and retreat of migratory species. But it is more difficult to show the changes in bird distribution through historical time. That is one reason why I collect old bird guides.
Last week I saw a Carolina wren in Wilmot Flat. These are not migratory birds, so this one will likely be present through the winter, sustained by the bird feeders in the neighborhood. In my 1917 “Birds of America,” the northern edge of its range is given as the lower Hudson and Connecticut valleys. In the New Hampshire atlas of breeding birds, published in 1996, it is not listed at all. In the “Sibley Guide to Birds” from 2000, the range of this species is shown to extend to Cape Cod with a flurry of green dots covering the rest of New England. In “Birds of New Hampshire” from 2013, it is called “an uncommon resident in the southern half of the state, having arrived in the last half century from the south.”
This is the phenomenon of dispersal, which is ongoing for nearly all species not confined to islands or by strict dietary requirements. While dispersal is often associated with increasing populations, the cattle egret provides a counter-example. The populations of all forms of life are dynamic, constantly in flux in space and time. They may move on their own, like white wagtails establishing a beachhead in Alaska, or with human help, like house finches released from cages by customs officials… or coronavirus, hitchhiking around the globe in human hosts. Sometimes you hope they stay, but like the cuckoo and the virus, sometimes you hope they don’t.
Bill Chaisson, who has been a birdwatcher since age 11, is a former editor of the Eagle Times. He now works for the Town of Wilmot and lives in Sutton.
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