Hardy warbler acrobatic forager ‘Butterbutt’ uses variety of strategies

As I walked along the waterfront, I kept glimpsing at a small bird as it flew ahead of me. It appeared to be searching among the rocks and driftwood for food, and at first I assumed it was a European house sparrow, of which there are many in the area. I had often seen these birds searching for small seeds in this location.

However, I quickly realized it was not a house sparrow, or even a sparrow at all. It was too small, and its method of hunting was not the peck and scratch typical of sparrows. Instead, it resembled a leaf caught by eddies of wind, first blown up into the air, then settling down again lightly, only to rise up once again to be blown about.

As its acrobatic foraging antics brought it closer to me, I saw it was obviously some kind of warbler. I also realized it was after small flying insects reawakened by yet another warm spell; the bird would sally from its rock or driftwood perch to snatch an insect out of the air, then alight again on another perch. It was so quick and buoyant in the air that it really did resemble a small leaf being picked up and blown about by the offshore wind.

Looking closely, I searched for clues to the bird’s identity; although its foraging technique had already tipped me off, I wanted to be sure.

The bird had faint streaks along the sides of its white chest; it had a solid brown back, with two white-wing bars and a white eye-ring. Finally, I saw the definitive clue to its identity as it hovered aloft for a brief second: the yellow patch at the base of its tail. It was a yellow-rumped warbler.

All of our other warblers and Neotropical songbirds have long since departed their breeding grounds for the winter. Yellow-rumped warblers are hardier than their brethren, though, and this is due to a number of factors.

Yellow-rumped warblers employ different strategies in food gathering; the first is the method I described above: sallying from perches to snatch flying insects from the air. The second is much more sedate: The bird simply clings to a branch while it gleans insects from the surrounding leaves and twigs.

When foraging for insects within a tree, yellow-rumps utilize all areas of a tree; this is contrary to what other birds do, as most tend to exploit particular “niches” – the lower branches only, for example, or only the outer branches higher up in the tree. This type of behavior tends to reduce competition for food among similar species of birds.

Additionally, these warblers have also been observed clinging to the vertical surfaces of tree trunks, searching for insects in bark crevices, as do woodpeckers, creepers, and nuthatches. This gives them yet another edge.

These warblers also utilize a wide variety of general habitats, especially outside of the breeding season. At this time, they also expand their diets to include winter fruit, including the fruit of the bayberry plant, an abundant, native shrub of the East Coast of North America. According to the “Birds of North America” species account, yellow-rumps are the only warblers that are able to digest the waxes of this fruit; this has enabled them to winter as far north, in coastal areas, as Nova Scotia.

As I watched the lone yellow-rump (also called a “butterbutt” by birdwatchers) that day among the shoreline rocks, I wondered where its companions were. A few days before, I had seen a small flock of them foraging among the long dune grasses and enjoyed hearing their call notes as they kept track of one another. I hoped I’d see more of them, given the unseasonably warm weather we’ve been having, as well as the birds’ hardiness and resourcefulness.

BDN bird columnist Chris Corio can be reached at bdnsports@bangordailynews.net