In 2010, while living in Vermont, I often examined nest-boxes being used by various pairs of birds—house wrens, great crested flycatchers, European starlings, black-capped chickadees, and tree swallows. The swallows had won out over a pair of chickadees in a contest for the same box, and they furnished the nest with white feathers but, curiously, did so only after
their eggs had been laid—not before, as per usual avian protocol.
The female began incubating her six eggs on May 28, 2010, and the nest eventually contained 110 white feathers. The eggs hatched on June 8, and by the 24th the young were feathered out in their ash-gray garb. At that point the nestlings were clambering up from the bottom of the nest to perch, one at a time, in the nest-box entrance hole. There they intercepted each parent as it brought food, fluttering up to its offspring, sometimes hardly stopping. The adult bird simply passed off the food while on the wing.
As this was going on, the adult swallows ignored me and Hugo, my yellow Labrador mutt. But on June 26, they suddenly became noisy and started to dive at Hugo and me, and even made a pass at a kingbird that was pulling threads out of a now-empty oriole nest in the neighboring tall black-cherry tree. When a blue jay landed in the cherry tree near the nest-box, one of the swallows dived at it while noisily chattering an alarm.
Then, for the first time, I saw the male take a short break from his usual nest-tending routine; he landed (twice) on the nest-box, his head and back feathers glistening greenish blue to black, depending on the angle of light. I had never seen one of the swallows stop to land there since after the eggs had been laid, but now both of them landed, and also hovered in front of the nest entrance, but without passing off food. This new behavior was puzzling because earlier, the two had consistently come singly and flown directly to and from the box entrance, always to deliver a meal.
Early the next day, the 27th, the baby leaning out of the nest-box was cheeping nonstop, and at 10:10 a.m. it leaned out and almost fell, but, fluttering hard, managed to hold on to the edge of the box by its feet. It tried to get back in, but another nestling had by then taken its place and was blocking the entrance, so, unable to hang on, the first baby fell straight down into the grass and weeds, a distance of two meters. It sat still there for a few minutes before clambering, with rapid flutters, back up into the vegetation. It then perched on the garden fence directly underneath the nest-box.
A Cooper’s hawk came out of the woods and perched in a tree nearby. It had, I suspected, been attracted by the baby swallows’ hunger cries—the continuous two-syllable chil-it calls, which had been increasing in volume. The baby swallow was a “sitting duck” that would, I thought, have been taken by now, had I not been near enough to discourage the hawk.
The pair of swallows then resumed flying to and from the box, but I had the impression they were feeding the babies less frequently. Could that really be true? To make sure, I needed numbers, and soon found they were making one trip, on average, every 3.5 minutes, which seemed about half as frequent as before. The baby that had fallen out of the nest continued meanwhile to call out like the others. It also preened, shook itself, and then to my surprise flew off in seemingly competent flight, landing in a nearby ash tree.
The Cooper’s hawk returned later, making a quick flyby, and a broad-winged hawk soared overhead. One of the adult swallows chittered loudly and flew up to meet it, looking tiny next to the high-flying hawk. But the swallow was safe, being more agile in flight than any hawk.
The volume of the young swallows’ peeping increased, and now another leaned farther out of the nest-box to meet its approaching parent, even when that bird was still a hundred meters away. However, the young swallow did not react to other birds flying near. Apparently it could recognize its parent. This too was surprising to me.
The begging of the swallow chicks grew more strident as the food deliveries decreased in frequency, but the babies were unable to stimulate their parents to return faster. Feeding nearly came to a stop from 2:30 to 5:48 p.m.; only one food delivery took place during that period of more than three hours. Except during bad weather, I had never before observed such a long interval without food coming to the nest.