FOOD FACTORS INTO BIRD MIGRATION 

Wednesday, July 30, 2008

I am sure you have seen them in the last week or two - tree swallows by the hundreds, all lined up on the utility wires, preparatory to the long journey south. I wrote in a column many years ago that the hydro lines were "sagging beneath the weight of so many swallows."

One reader begged to disagree that a bird, weighing only a few grams, could cause a wire to sag, regardless of how many there were. What I was seeing, he explained, was that the line, covered in swallows, was more detailed against the sky, making it appear to sag.

Never one to lose an argument without a fight, I did some calculations. If the space between two hydro poles is 400 feet, that’s 4,800 inches. If the swallows were spaced on the average of four inches apart, that’s 1,200 birds from pole to pole. If each bird weighs 30 grams, that’s 36,000 grams of birds, and when converted to ounces, then to pounds, that translate into 80 pounds of birds. Or, as we used to say on the farm if we were comparing weights - "that’s about the same as a full bag of laying mash for the chickens." And my feeling is, if someone were to suspend an 80-pound bag of laying mash from the hydro wire, that line would surely sag!

Enough said. Sagging, or not, the swallows are gathering, marking the start of the fall migration. A few shorebird species such as least sandpipers and lesser yellowlegs are already passing through, having commenced their long migration from the sub-Arctic tundra to Argentina almost a month ago. More locally, one would be hard pressed to see an upland sandpiper in a few weeks, as they are also among our earlier migrants.

Mostly, it’s a food thing that dictates when birds arrive and leave, or conversely, remain through the winter. Insect eating tree swallows that insist on arriving in late March, much sooner than insectivorous birds should be arriving, have developed a method of being able to switch over to a diet of berries. While likely not as palatable as insects, indigestion is probably a better option than interment. Yellow-rumped warblers also have this knack of switching diets. Those that spend the winter here - and there are a few - swing from weevil, wood borers and insect larvae in the spring, to other food items as the season progresses. When these insects become scarce in the fall, yellow-rumped warblers become experts at the skills of flycatchers, darting after mosquitoes and gnats and houseflies, also hovering and snatching spiders lured out by the warmth of the sun. This is why we always see yellow-rumped warblers around our windows in November, a most unlikely place to see any member of this family. When winter descends, those yellow-rumped warblers remaining to challenge the rigours of winter, change their diet again, doing extremely well in Prince Edward County and Bay of Quinte areas on the fruiting berries of red cedar. And judging from the crop of berries on the red cedars this year, we may see a few of these warblers around again this winter.

Some birds pick up this skill rather quickly and become experts at being opportunists. The hundreds of tundra swans that now linger well into late December at Wolfe Island and Prince Edward County’s South Bay, do so likely because of zebra mussels. Usually well on their way to wintering grounds in Chesapeake Bay by late November, tundra swans now hang around until freeze-up. Lots to eat here - no point migrating just yet. The dramatic increase in scaup, long-tailed ducks (oldsquaws to many of us) and white-winged scoters in the open waters along Lake Ontario every winter has been largely attributable to a corresponding increase in zebra mussels.

After close to 50 years of birding, I still look forward to the convention of swallows on the hydro wires. Now if each swallow eats its weight in insects every day, and 1,200 swallows then roost on the hydro wires, wouldn’t this double their weight? Is this why I see the poles leaning inward?