Yellow Warbler Nestlings & Jackson’s New Project

Eight day old Yellow Warbler nestlings, begging for food  photo by Jackson Bloch

Eight day old Yellow Warbler nestlings, begging for food
photo by Jackson Bloch

Every Sunday after dinner, the Bird List of every bird ever seen on Kent Island is read with the solemnity of ritual. Whoever has seen the bird that was called from the list shouts out, and the bird is checked off, or, very rarely, added to the list. While I am unlikely to recognize a Hooded Merganser or a Black-crowned Night Heron, I have made it my goal to see the nestlings of every Bowdoin Scientific Station study species. So far I have seen (and held) the nestlings of Herring Gulls, Tree Swallows, and as of yesterday, Yellow Warblers, with Savannah Sparrows, Petrels, and Guillemot nestlings still to go. Many bird species have breeding grounds on Kent Island, but they are invisible to the untrained eye, with one exception. Herring Gull nests are everywhere, from the shoreline to the fields on the interior, and now that they have hatched, the chicks are everywhere underfoot, stumbling like drunks in the undergrowth. I never would have been able to glimpse the well-hidden nestlings of smaller bird species, however, without going into the field with scientists.

On Thursday Jackson invited me to watch him band the four Yellow Warblers from the nest that he found in the woods right behind the shower. He reached into the bushes and emerged with two of the smallest, most delicate birds that I have ever seen. Jackson explained that they were six days old, though they looked much younger than the week-old Tree Swallow nestlings I had examined a few days before. The Warbler nestlings were bald except for two little tufts of fuzz that drew attention to their wizened, pinkish faces. They barely had feathers, and the underlying architecture of their wings was exposed, as were their tendons, their fat, and the contents of their almost transparent bellies. Like the Tree Swallows, their faces were dominated by wide beaks, the receptacles for the insects that their parents spend most of the day hunting and feeding to their young. Jackson had remarked on the short tarsus of the Tree Swallow, which made them more difficult to band, but these nestlings had almost comically long legs. Tree Swallows spend most of their day in the air, hence their huge wingspan, while the long legs of the Yellow Warbler are adapted to perching.

Yellow Warbler fledgling photo by Jackson Bloch

Yellow Warbler fledgling
photo by Jackson Bloch

Yellow warblers fledge and leave their small nests at about ten days old, much sooner than tree swallows, which are in no hurry to leave their spacious bird boxes. Jackson explained that leaving the nest so soon is a survival strategy, because the offspring are dispersed and less likely to be killed all at once by a predator. Once they leave the nest, the fledglings, which are gray except for the yellow that edges their wing feathers, stay close to the nest, and their parents continue to feed them. Jackson has watched them foraging in a huge gooseberry bush for butterflies, mosquitos, and other insects in the North Field.

Besides banding nestlings and observing fledglings, Jackson is also putting recorders in the nests to capture the nestlings’ begging calls. Like Ben, Drew, and Liam, Jackson has had to adapt his project to field conditions, though luckily his previous research has not been wasted. His original goal was to record the incubation calls of females to see if they were encoding a “password” that their young repeated in the nest as a way of identifying intruders. When he wasn’t hearing the females’ incubation calls, he decided to shift his project’s focus on the development of the nestlings’ begging calls, and the structural differences between calls in different ambient noise conditions. “It was actually a seamless transition,” Jackson remarked, saying that his methods of close observation and recording in the nest are the same as for his previous project. He has already noticed that the nestlings’ calls increase in frequency as they get older, and hypothesizes that the calls differ in frequency range based on the ambient noise around the nest, such as the (always grating) sounds of Herring Gulls. Thanks to Jackson, I can now recognize the chipping of fledgling Yellow Warblers as well as the distinctive calls of their parents. The heightened satisfaction that I get from knowing exactly where the bird calls I hear all on Kent Island come from instead of dismissing them under the umbrella of birdsong as I would have in the past is remarkable.

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