It was a gorgeous fall day, the slight chill in the air lessened by the bright sun shining from a sky streaked with wispy cirrus “mare’s tails” clouds. I was walking during my lunch break at work, hoping to see some interesting birds.

I wasn’t disappointed. The pathway I was on was bordered by a small area of mowed grass, which gave way to chest-high plants such as goldenrod (which had gone by), brilliant purple asters and spent Queen Anne’s lace, as well as what looked like Canada bluegrass, its tall seed heads nodding in the breeze. Behind this swath of vegetation, young white birch, oak, and white pine grew, overshadowed by their older relatives as the young growth gave way to mature forest.

Even from a distance, it was obvious there was a lot of bird activity. Small songbirds were making continuous forays into the weed banquet from the protective cover of the young trees. As I drew closer I could make out the call notes of common goldfinches, but at first I didn’t recognize those of the other birds. They made repeated sallies from their perches to catch flying insects as well, and this should also have given me a clue to their identity — but apparently I was a bit rusty. It wasn’t until I saw the yellow patch at the base of their tails that I realized they were yellow-rumped warblers — affectionately known among birdwatchers as “butterbutts.”

The individuals I saw were immature birds, lacking the crisp black-and-white patterns around the face and chest of the adult male, or the lighter, but still-sharp coloring of an adult female. Instead, they sported pale buff plumage overall, except on their chests, sides and underbodies, which were a buffy white color, with pale yellow patches and faint streaking on their sides. Their yellow rump patch, which became visible when they flew, was especially bright and noticeable by comparison.

Hearing their short, sharp call notes — “chek” — made me realize why they had sounded so familiar — they had always been one of the few warblers whose call notes I could easily recognize. A few young males occasionally uttered brief, quiet, almost whispered snatches of the adult male song. This, too, is easy to recognize, as the song is two-parted, with the second part most often falling in pitch.

This group of young birds was likely on migration, and was fueling up for the next leg of its journey. Among warblers, yellow-rumped have a distinct advantage over their kin. First, they are generalists, meaning they use a wide variety of habitats in which to forage, as well as employ a few different foraging techniques. Second, they are able to switch their diet from that of insects in the warm months to mostly fruit in the winter months. In this they have a special edge, as they are the only species of warbler that can digest the waxes contained in bayberries.

The bayberry plant is a very hardy, semi-evergreen shrub that grows on coastal sand flats and in tidal marshes. It is able to withstand poor soil conditions and salt spray from the ocean. It provides cover and nesting sites for a variety of birds, as well as food. Because yellow-rumped warblers can feed on the berries, they have the ability to winter along the more temperate coast as far north as Nova Scotia, if the fruit crop is good and weather is not severe.

Most yellow-rumped warbler populations migrate. Because of their versatile diet and foraging behaviors, they are one of the first warblers to arrive in spring and the last to leave in autumn. And along the coast, as mentioned, they can be a pleasant surprise find where bayberry plants are plentiful, well into winter.