Wednesday, March 12, 2014
By HERB WILSON
One of the best-known quotations from the Greek philosopher Heraclitus is translated as "Everything flows, nothing stands still." We always expect change. Of course the certainty of change applies to birds.
Bird populations inevitably vary. Sometimes birds increase locally because particularly favorable conditions occur. For instance, a gypsy moth outbreak provides food for insect-eating birds, making their nesting success higher than normal. Other events like the 1998 ice storm can wreak havoc on local bird populations.
Bird populations can also change on much larger geographic scales. A widespread outbreak of avian conjunctivitis in the early 1990s resulted in the death of many house finches in the eastern United States. The clearing of bottomland forest in the Deep South certainly was a major contributor to the extinction of Bachman's warbler. Global warming is causing significant shifts in the range and population size of most North American birds.
For migratory birds, changes in population sizes can be difficult to understand. According to data from the Breeding Bird Survey, wood thrushes show a steep decline in the three northern New England states over the past 40 years.
Why? Could it be events happening in New England during the breeding season? Could it be destruction or degradation of wintering habitat in Central America? Could it be events happening during migration? Change may be certain but understanding why change occurs is often hard to understand.
One of most spectacular changes in abundance of a Maine bird concerns a shorebird called the red-necked phalarope, which breed at high latitudes on the tundra in both the New World and Old World. On the breeding ground, phalaropes largely feed on mosquito larvae and other aquatic insects in thaw ponds. The phalaropes feed in an unusual manner by swimming rapidly in a small circle, drawing prey items to the surface of the water.
By rapidly opening its upper and lower bills with the tips in the water, a phalarope causes water to flow up the bill, following a surface tension gradient right into the back of the mouth. The unsuspecting prey items are drawn into the mouth with the water, becoming lunch. A personal conveyor belt.
Red-necked phalaropes can be found on inland bodies of water during migration. Thousands can be found on saline lakes in western North America on their way to wintering grounds. These hardy birds winter at sea where they feed on zooplankton, the collection of small crustaceans and other invertebrates found near the surface of the ocean feeding in turn on single-celled algae (phytoplankton). Phalaropes are well adapted to feed on the plankton with their long, thin bill.
Forty years ago, millions of red-necked phalaropes stopped in the lower Bay of Fundy during their fall migration. The most spectacular concentrations were in the vicinity of Lubec and Eastport. In particular, the phalaropes were found in the roiling waters between New Brunswick's Campobello and Deer islands, and Eastport. These oceanographic conditions produce the Old Sow, the largest whirlpool in the Western Hemisphere.
In late August of 1976 and 1977, 2 million phalaropes were estimated to be feeding in this small area. What a spectacular sight that must have been. In 1983, 300,000 phalaropes were counted. But thereafter the phalaropes disappeared. We don't know where the phalaropes went.
In recent years, phalaropes have been seen in the Eastport-Deer Isle region although in much smaller numbers than the heydays of the 1970s. The phalaropes seem to occur where large numbers of their favored prey, a small crustacean called calanus finmarchicus, are abundant. It is possible that the calanus population crashed in the area in the early 1980s, forcing the phalaropes elsewhere. On a global scale, red-necked phalaropes seem to have a stable population.
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at: