Goldfinches rely on thistle seed to feed young and starve cowbirds

by Emily Stone Naturalist/Educator Cable Natural History Museum

Ouch! I cursed as something in the handful of weeds pierced my hand. With the coming of the frost, it was time to clean out my garden.

In this corner, a small patch of parsley had gotten overgrown by the gracefully arching seed heads of a grass.

My flesh had found the spiny, well-armed basal leaves of a thistle in the bunch, too. The deep green leaves spread out in a perfect circle, at least a foot in diameter.

Spines adorned the jagged leaf margins menacingly. This year it didn’t produce a flower stalk. Next year it won’t either—because with a quick pry of the pitchfork I dislodged the offensive weed, roots and all.

As I continued to load up the wheelbarrow for a trip to the compost pile, I waded into the overgrown garden border to remove a couple of bushy, secondyear thistle plants, too. (I donned gloves for this task.)

A closer look revealed spiny wings of leaf material along the stems – a sure sign of a non-native thistle, probably bull thistle. Although many of the seeds had already drifted into my green beans, I figured I could keep at least a few more seeds out of the garden.

Each summer I struggle with time management – as soon as I start harvesting, I stop weeding! If I was really responsible (a.k.a. not busy), I’d have taken care of all the late summer weeds before they went to seed. But, when life gets busy and I have to choose, I pick beans.

In some ways, I’m also choosing to share my garden with wildlife. As long as thistles don’t take over an area from native plants or interfere with grazing, they can benefit a surprising number of species.

At least twenty-three species of insects feed on thistle foliage. Countless other insects (and hummingbirds) feed on the nectar and pollen. Even more spiders, birds, reptiles, amphibians and mammals eat the critters that feed on the thistles.

Of course, native thistles will host the most diversity of life. Most need high quality prairie remnants to thrive. Bull thistle may not be ideal, but at least it is a biennial and dies after producing seed in its second year. It can be a problem in disturbed areas, but generally won’t take over quality habitat. Canada thistle (actually from Europe) forms perennial colonies that are much tougher to eradicate.

I suppose one reason I didn’t bother to pull the thistles earlier in the summer – when they still sported fuzzy, fuchsia flower heads – has to do with some childhood nostalgia.

As a native Iowan, I have a special place in my heart for goldfinches (they are our state bird). For as long as I can remember, I’ve been taught that goldfinches have a special relationship with thistles; first by my parents, and then by poet Mary Oliver:

“the finches/wait for midsummer,/ for the long days,/for the brass heat,/ for the seeds to begin to form in the hardening thistles…”

– from “Goldfinches” in New and Selected Poems by Mary Oliver.

I first learned that American goldfinches nest later in the summer from this very poem. While the little yellow birds use the soft fibers of thistledown to line their nests, in truth, it is the energypacked thistle seeds they are waiting for.

If you feed birds, you know how much goldfinches love thistle seeds. The seeds in your feeder aren’t even true thistles. They are seeds from the unrelated African nijer thistle. In the wild, many relatives of thistles in the Aster or Composite family are favored food sources for goldfinches, and most of them are late-summer bloomers.

Thistle is one of the most common, as well as Joe-Pye weed, and sunflowers. Dandelions are an early-blooming exception, and goldfinches may feed heavily on spring dandelion seeds in addition to the seeds you put out.

As seed-eating specialists, finches are among the most strictly vegetarian birds of the world. While many small birds eat seeds, most of them also feed insects to their young.

Finches feed their chicks exclusively on regurgitated seeds! This has an unexpected advantage: when brown-headed cowbirds try to mooch (they are nest parasites who don’t build their own nests, and instead lay their eggs in the nests of other birds), the invading young die after three days on this diet.

With such a strong reliance on thistles for their nesting success, goldfinches need to make sure that their timing is perfect.

One researcher found that male goldfinches exposed to both warm temperatures (a July-like 82 degrees F) and blooming thistles will rev up their testosterone production.

It is pretty incredible that they respond to the flowers as a precursor to seed production, and not just the seeds themselves.

As year-round residents, goldfinches don’t need to hurry up and prepare for migrating thousands of miles. Instead, they can patiently make use of an abundant late-summer food source.

As I hauled the loaded wheelbarrow toward the compost pile – a crown of thorny thistles perched on top – a flock of goldfinches sang me cheerfully along.


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