Published in Trumansburg Free Press (Trumansburg, NY), July 13, 2005
|Land & People
Birds and Climate
by Bill ChaissonThe other day a loud liquid “TEA-kettle, TEA-kettle, TEA-kettle” song came bursting through the screen door into my kitchen. When I looked through the screen I could see a brown bird with a prominent white stripe over its eye crouched in the lower branches of a spruce beyond the porch railing: a Carolina wren.
The Breeding Bird Survey (http://www.mbr-pwrc. usgs.gov/bbs/bbs.html) shows the southern Finger Lakes region to be the northern edge of breeding range of this bird; stray birds wander into southern Canada, but rarely breed. The singer in my backyard did not arrive until late June. The Carolina wren is one of the many southern songbirds that has been extending its range northward over several decades of observation by amateur and professional birdwatchers.
My grandmother’s favorite bird was the cardinal. In the early 20th century, when she was young, cardinals were unknown in central Connecticut, where she grew up. When these striking red birds began showing up at her winter feeding station in the 1950s she felt like she was getting an exotic visitor from the tropics. Cardinalis cardinalis is now resident north to southern Canada.
Several other birds, including the familiar tufted titmouse and the less familiar blue-gray gnatcatcher and red-bellied woodpecker, also have been moving north. With the exception of the gnatcatcher, all of these birds are non-migratory. Harsh winters tend to reduce their numbers at the northern edges of their ranges and limit their expansion.
Most people over 40 remember the winters of their childhood as being colder than those we experience now (at least until the last couple of years). This apparent trend leads many to conclude that we are observing global warming and that the expansion of southern birds’ ranges northward is one of the effects. Well, maybe.
Some species, like the cardinal, began expanding northward before the 1960s, when winters in New England were still quite harsh. The cardinal, the titmouse and the red-bellied woodpecker are all regular visitors to bird feeders in the winter and it is generally agreed that the increasing popularity of feeders has enabled these birds to survive colder
|winters than they
could have unaided. All of these birds, including the Carolina wren,
are insect-eaters during the warmer months, but switch to seed-eating
during the winter.
Winter weather in the northeastern United States is dominated by an atmospheric pattern called the Arctic Oscillation. The most influential portion of this circum-arctic phenomenon is the North Atlantic Oscillation or NAO. The strength of westerly winds fluctuates in response to the steepness of the pressure gradient between the “Icelandic low” and the “Azorean high”. A “positive NAO index” is caused by a very steep gradient and leads to warm wet winters in both northwestern Europe and northern United States. This condition predominated for most of the last 30 years.
Recently, however, the gradient has decreased markedly, leading to a “negative NAO index” and harsher winters in areas that have known milder conditions. More information on this and a historical record of the index since 1860 can be found at the website of Columbia University’s Lamont-Doherty Earth Observation (http://www.ldeo. columbia.edu/ NAO/).
The period of observed range expansion by the songbird species is much longer than this decadal fluctuation in climate. There may be a longer-term pattern superimposed on the NAO fluctuation. Some climate scientists have argued that the amplitude of the NAO index has been growing through the 20th century and it has an increased tendency to get “stuck” in the positive position. The late Gerard Bond, a Lamont-Doherty earth scientist, discovered a cyclical change in climate with a period of approximately 1500 years (the “Bond cycle”). The last cold period, between the 15th and 19th centuries, was called “the Little Ice Age” by historians, and climate has been warming since then. The Little Ice Age was the most recent of several “cold snaps” that have occurred in the 10,000 years since the end of the last “real” Ice Age.
The fossil record of birds is very poor because their bones are so lightweight and fragile that they do not preserve well. It is therefore very difficult, if not impossible, to know whether the Carolina wren, the tufted titmouse, the red-bellied woodpecker and the cardinal are expanding back into territory that they abandoned either in the 14th century or during the last interglacial (100,000 years ago), when it was warmer than it is now and there were certainly no bird feeders to be found.
Hibernian Weather Channel Productions
Last revised: January 2, 2006