Editor's Note: Regular "on the Wing" columnist Mike Burke is recovering from a recent surgery.
During spring in the 1980s, I spent many boat hours on the Choptank River studying osprey breeding biology. As a curious naturalist, I then expanded my field season to learn about the great variety of seasonal natural events on the Bay.
When I began autumn field work on the lower Choptank in 1988, I discovered noisy flocks of common loons. This section of river is a migratory stopover. Attracted by the short-term abundance of "peanut menhaden," juvenile fish nurtured in countless creeks over the summer, loons arrive here in large numbers from late October into early November.
The loons cooperate in flocks to feed on schooling menhaden. In turbid water green with plankton, they dive under the schools to push the fish toward the surface for visibility and ease of predation. In those situations, gulls (as many as five species) take the panicked peanuts at the surface.
The gulls' action cues me to the location of feeding loon flocks. If the peanuts are deeper, perhaps in somewhat clearer water, the loons dive for them. There are few gulls, but still plenty of "yip" calls among the loons as they choreograph their social pursuit of the densely schooling fish.
Loons on the surface form a shallow arc: The visible portion of their living net. During such intense feeding, only a third of the loons may be on the surface at any given time, and one never sees a menhaden in a loon's bill: The bird just sucks each of these streamlined fish down and continues to feed.
To get a good loon count, one may have to wait as long as 30–45 minutes until the entire flock is breathing, resting and digesting on the surface.
Loons can best choreograph this hunt under low-wind or calm conditions.
Flock-feeding is noisy, providing splendid cue for the loon student and for other loons. I often hear the yips —like a pack of terrier dogs but more musical — from 1–2 miles away, and I assume the loons have more acute hearing than I do. This sound cue functions to build flock size when concentrated prey is abundant.
During the late 1980s, the last period when peanut menhaden showed predictable annual autumn abundance in the Bay, I commonly saw feeding flocks of 100–300 loons, and occasionally 500 birds on the Choptank and Potomac rivers. The largest flock I documented was about 600, roughly linear, stretched over perhaps a kilometer, a "living net" across the mouth of Nomini Bay, off the lower Potomac. I think of this as a nonhuman "intercept fishery" of seasonally mobile prey.
Early in the loons' autumn stopover, the prime sites are the lower reaches of some Bay tributaries: the Miles Choptank, Little Choptank rivers and nearby Eastern Bay, and sometimes, the Patuxent, Potomac and Rappahannock. Peanuts are concentrated in these sites, plus the shoreline topography offers some cover from the Chesapeake chop.
I study the loons when it is calm, but if the wind has blown hard the previous day I usually find the loons feeding off what had been the lee shore, especially on a huge expanse of water such as the lower Potomac.
In recent years of reduced Chesapeake peanut menhaden production (measured by the Maryland Department of Natural Resources), I have been lucky to find a flock of 100 loons.
As November proceeds, Bay water temperature drops fast and fish such as the peanuts are migrating out. The loons track them down the Bay. From mid-November to mid-December, I have found the waters off New Point Comfort in Mathews County, VA, to be a prime lower Bay feeding site where flocking loons and gannets practice "intercept fishery" on schooling menhaden and other species.
Some loons winter in the lower Bay, but many of them track migrating fish to the milder climes southwest of Cape Lookout, NC. There, winter's coldwater incursions are blocked by the Outer Banks and Gulf Stream. The banks end at Cape Lookout, but a wide continental shelf remains, with the Gulf Stream meandering along its outer edge, all the way down to central Florida.
Huge numbers of fish winter and breed in these stable shelf waters; and commensurate numbers of loons and gannets arrive to feed on them. The adult loons go flightless for a full wing molt during much of this time, and they dive deep over shelf waters to feed on fish down to the bottom — which is also a habitat of wintering menhaden.
The adult common loons' spring migration up Chesapeake Bay is their next local visit. By April, they have molted back into elegant breeding plumage. They fly overland to the Great Lakes, then onward to their nesting lakes in the North Country of the United States and Canada.
Loons have a heavy wing-loading, and this marathon flight requires sustained strength and thermoregulation. Departing in the cool temperatures of early morning, they stay over the Bay or the Potomac as long as possible, and only migrate in stable weather.
Such overwater April flights can be viewed from Cove Point and North Beach in Calvert County, MD; from Sandy Point State Park in Anne Arundel County, MD; from boats out in the main Bay; and probably by looking west from Poplar Island, where migrating loons would be illuminated by the rising sun.
Driving west across the Bay Bridge one April morning, I witnessed a small migratory burst of loons flying over the main span, probably headed up the Susquehanna Valley to Lake Ontario.