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ALWAYS IN SEASON: Goldfinches delay nesting, delight birders

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Picking a bird of the week can be risky, especially at this time of year, when new species blow through — sometimes literally — in a single day. What’s abundant on one day may be absent the next, and what hasn’t been seen so far this year may be common tomorrow.

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Add to this the delays inherent to producing a newspaper, and the subject of a weekly column about birds may be old news before any reader sees it.

This happened a couple of weeks ago, when the yellow-rumped warbler was the bird of the week. Shouldn’t be hard to find them anywhere around here, I wrote with confidence.

The warblers thought differently. Before the ink was dry, most of them had moved on, leaving only hard-to-find remnants.

This is typical of warblers, most of which are passage migrants here. Only a handful of species remain to nest in our area.

There is a colorful bird that graces our backyards throughout the summer season, though, and this species has been present in large numbers throughout our area. The population of American goldfinches will thin out as the season progresses, but many will hang around throughout the summer — and some will spend the winter with us, too.

Last week’s column introduced the idea of dimorphism, which is used to describe species in which the species do not look alike, such as last week’s subject, the northern shoveler.

Goldfinches exhibit a greater level of dimorphism. Not only are the sexes different, but the birds have different plumage in winter than in summer. As the ornithologists put it in a monograph on the species published by the American Ornithologist Union, goldfinches are both sexually and seasonally dimorphic.

This helps account for their sudden appearance in the spring. The birds have been around, they just look different — much more colorful and therefore much more obvious.

A male goldfinch in breeding plumage is bright yellow with a black cap and black and white markings in the wings. He is unmistakable, really.

Individuals vary in brightness; one at my feeders last week was very pale, ghostly almost, while the majority were vivid yellow.

Females are plainer, rather yellowish-green or gray in color, but with the wing stripes.

The size and shape of the bill are giveaways, too. Goldfinches are seed eaters, and they have relatively large, conical bills, the better to crack seeds. In summer, males have bright orange bills, so they look rather front heavy.

Through much of May, goldfinches have been incredibly abundant. On some mornings, they hung in the trees like ripe lemons. On the ground, they looked like a field of dandelions below the bird feeders.

Goldfinches can afford to loaf about at this season. They are among the latest to nest of all North American birds. They won’t get about the business of pair formation until late June, and nesting commonly occurs in July into August.

It’s not completely clear why the goldfinches wait to nest. Here are a couple of theories:

The birds may be waiting for nesting material. They favor the down from blooming thistles to line their nests, and this is dry and absorbent, potentially offering goldfinches a healthier nesting environment than would be available if they nested earlier.

Or, perhaps nesting is timed to take advantage of the crop of thistle seeds that is available in late August. Abundant food easily gathered would increase the chances that chicks would survive.

Late nesting also avoids nest dumpers, such as the brown-headed cowbird. Cowbirds are impatient. They leave our area soon after depositing their eggs in the nests of other species.

Or, perhaps the goldfinches need time to recover from the stress of a spring molt in order to build up the energy needed to face the business of perpetuating the species.

In any case, late nesting is clearly a successful evolutionary strategy, because goldfinches are abundant, despite the fact that late nesting means they usually can raise only one brood a year — a factor that should limit reproductive success.

In any case, this adaptation is to our benefit, because it means this colorful and interesting species is abundant, obvious and easily identified — the clear choice for bird of the week.

Jacobs is a retired publisher of the Herald. Reach him at mjacobs@polarcomm.com.

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