Wild bergamot seeds and the goldfinch's summer feast

Along the open edges and disturbed meadows near Westminster, the wild bergamot is finishing its bloom cycle. Some plants still carry a fringe of pale lavender flowers, but most heads have gone to seed — the tight, bristled clusters drying in place on stiff upright stems. This is the window that matters for American goldfinches, and right now they are using it.

Wild bergamot is a native mint-family plant, and it grows in dense stands where the soil is dry and the sun is full. It flowers through mid-summer, attracting golden northern bumble bees, two-tailed swallowtails, and a steady rotation of other pollinators. But once pollination is done, the plant shifts its investment. Each spent flower head develops into a cluster of small, dry nutlets packed tightly inside the dried calyx. To a goldfinch, that structure is a seed cache. The bird lands directly on the stem, grips the head with its feet, and works the seeds out with its bill, one by one. If you are near a bergamot stand right now, watch for the stems swaying without wind. That motion is almost always a goldfinch.

American goldfinches are one of the latest-nesting songbirds in North America. Most songbirds time their breeding to coincide with the peak of insect abundance in late spring. Goldfinches do something different: they wait for thistles, bergamot, and other composite and mint-family plants to set seed before they begin nesting. The female lines her nest almost exclusively with plant down, often from thistle and milkweed, and the nestlings are fed a diet of regurgitated seeds rather than insects. That dependency on seed maturity pushes their nesting peak into the heart of summer, which is exactly where we are now. The showy milkweed nearby is already producing pods, and the goldfinches will use that too, both for nest material and eventually for food. The plant community here is giving them everything they need at the same time.

Male goldfinches are in their brightest plumage through the breeding season: a sharp, saturated yellow with black wings and a black cap. That color comes from carotenoid pigments the birds cannot produce themselves. They get it from the seeds and plant matter they eat, particularly the yellow-pigmented compounds in certain flowers and seed oils. A male that has fed well on a diverse seed diet through the spring carries more pigment into his feathers during the summer molt. The brightness you see on a goldfinch working a bergamot head is, in a direct way, a record of what that bird has been eating for the past several months.

The bergamot stands here also carry other visitors worth noting. The golden northern bumble bee, which is a threatened species in this region, works the remaining open flowers with a focused persistence. The plains bee assassin, a native ambush predator, sometimes waits motionless on flower heads for exactly that kind of visitor. Both species are present in this area, and both are tied to the bergamot in different ways: one as a pollinator, one as a predator of pollinators. The goldfinch, arriving weeks later for the seeds, is using a resource the plant produced partly because those earlier visitors did their work.

The bergamot stems stand about waist high in full sun. If you are near a patch of them, look at the dried heads. Many will show small gaps where seeds have already been taken. Some stems will still be moving. The goldfinch's call is a bright, ascending note, often given in flight — four or five syllables that rise and then fall. In the heat of a peak summer afternoon, with the milkweed blooming and the bergamot going to seed, that call is one of the more reliable sounds in this landscape.