Why Do Goldfinches Nest So Late? Thistles, Seeds and Down

When do goldfinches nest?

Most North American songbirds build nests in April, May or June. The American goldfinch waits. Pairs may form in spring, and males are singing and in full yellow plumage by May, but most females do not start building until late June to late July. Some nests still hold eggs in late August, and a few chicks are still being fed in September.

This makes the American goldfinch one of the latest-nesting birds in North America and the latest of its finch family on the continent.

The female builds the nest alone over about six days, usually in an upright fork of a shrub or small tree. The male often flies with her as she collects materials but leaves the building to her.

Seeds for the chicks

The main reason for the late start is diet. Most songbirds, even those that eat seeds as adults, feed their nestlings insects, because growing chicks need lots of protein. Goldfinches are different. They are among the strictest vegetarians of all birds, and they feed their young mostly on seeds.

The parents eat seeds, partly digest them and bring them back up as a soft paste for the chicks. To do this, they need a large, reliable seed supply close to the nest. In much of North America that supply peaks in mid to late summer, when thistles, sunflowers, asters, dandelions, grasses and other plants of fields and roadsides set seed.

By nesting then, goldfinches match the hungriest part of the chicks’ lives with the time when seeds are easiest to find.

Thistle down for the nest

The goldfinch nest is a small masterpiece. The outer cup is made of plant fibers and thin strips of bark, bound together with spider silk and caterpillar silk. The inside is lined with plant down, the fluffy seed fibers of thistles, milkweed and cattails.

This down only appears once these plants have flowered and set seed, which again happens in summer. The finished nest is so tightly woven that it can hold water. That keeps the eggs and chicks warm, but in a heavy storm, nestlings can drown if a parent does not shelter them.

The link with thistles is so strong that the old genus name for goldfinches, Carduelis, comes from the Latin word for thistle.

How the late season works out

The male feeds the female while she incubates, so she can stay on the eggs almost constantly. After the chicks fledge, he keeps feeding them while they learn to find seeds on their own.

Why cowbird chicks rarely survive

The brown-headed cowbird is a brood parasite: it lays its eggs in other birds’ nests and lets them raise its young. Cowbirds sometimes lay in goldfinch nests, but this rarely works out for them.

Part of the reason is timing. By late July, many cowbirds have finished laying, so fewer goldfinch nests receive cowbird eggs. One study found cowbird eggs in about 9% of goldfinch nests.

The bigger reason is diet. Cowbird chicks normally grow on insects. Fed on seeds by goldfinch parents, they do not get enough protein, and studies have found that they rarely survive long enough to leave the nest. The goldfinch’s seed diet works as an accidental defense.

Late nesting and the molt

Nesting late also shapes the goldfinch’s year. After the chicks are independent, adults go through a complete molt in late summer and fall, growing the dull olive-brown plumage they wear in winter. In spring they molt again into breeding colors, as explained in why goldfinches change color.

This tight schedule means most pairs have time for only one brood a year. Where summers are long, some females start a second nest, sometimes leaving the male to finish raising the first family.

How to help nesting goldfinches

Gardens can support goldfinches through their long breeding season by:

• Leaving native coneflowers, sunflowers, asters and other plants to go to seed rather than cutting them back.
• Allowing native thistles and milkweed to grow in wild corners, which provide both seeds and nest down.
• Avoiding herbicides that remove the seed plants goldfinches depend on.

A summer garden full of seed heads may reward you with goldfinches gathering down in July and family groups visiting in August, with brown young begging noisily beside their parents.