Wild Neighbors: Seedy Business: Diet Secrets of the Goldfinch

By Joe Eaton
Thursday July 16, 2009 - 11:19:00 AM
Male (left) and female American goldfinches at thistle feeder.
Ken Thomas
Male (left) and female American goldfinches at thistle feeder.

Last weekend Ron and I were on the front porch late in the afternoon when we heard an unfamiliar bird call: not the towhee, not the wren, not a lesser goldfinch; something different. The source was revealed when an adult male American goldfinch flew out of the mulberry tree and onto a utility wire, followed by a second bird, greenish and frowsy-looking, fluttering its wings and repeating the odd call. It was a fledgling begging its father for food. 

We don’t see that many American goldfinches in the neighborhood; lesser goldfinches have always dominated the feeder traffic. And this was the first youngster of either species we’d noticed this year.  

These “wild canaries” have a reputation as late-season nesters, due in part to the pervasive Eastern bias of North American bird books. Kenn Kaufman, in his admirable Lives of North American Birds, says: “Nesting begins late in season in many areas, with most nesting activity during July and August.” 

But that doesn’t apply to California. I needed to dig into my collection of breeding bird atlases to flesh out the details. 

(Breeding bird atlases are usually done on a county basis, with several consecutive years of field surveys covering as much of the county as possible. In Humboldt County, this was complicated by the inhospitability of marijuana growers; timber companies were more cooperative. Surveyors use a number of behavioral cues to rate the likelihood of nesting from possible to confirmed. Contra Costa’s BBA results are online, at; Alameda’s atlas data is still unpublished.) 

It seems that American goldfinches in northern California begin nesting in April or May, which is not out of line with most of our songbirds. In Napa County, they were seen carrying nesting material as early as April 9. They may produce more than one brood per season, which would account for the fledglings seen in Monterey County on Sept. 7 and Humboldt on Sept. 22. 

Why does the California population get an earlier start? American goldfinches are dedicated seed-eaters. One dietary study in California reported a diet of 95 percent vegetable matter on an annual basis. Most of that would be the seeds of thistles and other plants in the composite family, although the finches will also eat flower and leaf buds, young leaves, and more rarely fruit and berries. “American goldfinches in California initiate nests in late April or May, probably because of the earlier maturation of seed-bearing plants in our winter-wet Mediterranean climate,” writes Dave Shuford in the Marin County BBA. 

Goldfinches—and I believe this goes for the lesser and the nomadic Lawrence’s as well—are unusual even among finches in raising their young on seeds, a trait they share with the house finch. The male brings a cropful of seeds to the female on the nest, who parcels it out among the kids. Regurgitation is involved. Most other finches, including the closely related pine siskin, switch to insects during the spring and summer, giving their offspring a hefty dose of protein. But young goldfinches do fine on seeds alone. 

It would be interesting to know whether the popularity of bird feeding has affected the timing of goldfinch reproduction. Collectively, Americans put out a lot of thistle seed for these finches. Despite the extra resources, eastern goldfinches seem to be programmed to wait for the wild seed crop. There’s been speculation that nesting is triggered by the first flowering of thistles and other composites. 

The seed diet has one advantage you might not anticipate. American goldfinches, like most songbirds with open cup-shaped nests, are vulnerable to nest parasitism by brown-headed cowbirds. Female cowbirds dump their eggs in the nests of other species, leaving the hosts to hatch them and raise the voracious young. 

The cowbird chicks are usually larger than the host’s young and tend to starve them out. Unlike European cuckoos, in which lineages have evolved to specialize on particular host species, the brown-headed cowbird is a broad-spectrum parasite; any accessible nest will do. 

The trouble with goldfinches as hosts, thought, is that young cowbirds don’t thrive on all-seed diets. Field studies indicate that few cowbird chicks survive more than three days in a goldfinch nest. That gives the young goldfinches better odds of making it. 

At one Ontario study site, no cowbirds were raised to fledging by parasitized goldfinches. But female cowbirds, who don’t stick around to monitor the fate of their progeny, are unaware of this, and persist in wasting their eggs in goldfinch nests.