Lesser Goldfinches are the only goldfinches that stay in central Texas year-round. Unlike the American Goldfinch, a male Lesser keeps his flashy yellow and black plumage year-round. Females look very much like winter American Goldfinches; however, the yellow wash on the underparts extends under the tail, unlike on most Americans.
The variation in plumage of male Lesser Goldfinches is quite interesting. Young males in central Texas have green backs and black caps. The amount of black on the upper parts varies somewhat by individual. They acquire the jet-black back plumage after they are a year old. However, from Arizona westward, all male Lesser Goldfinches have green backs. Between central Texas and Arizona, the proportion of black-backed males decreases. Males resident elsewhere, outside the United States, also have black backs.
Lesser Goldfinches are also interesting in that they mimic the calls and songs of other birds. They are not as obvious about it as mockingbirds, who repeat a borrowed phrase a couple of times before moving on. Rather, Lesser Goldfinches incorporate a snatch of song or a call into a rapid, rambling song. Researchers in California documented imitations of at least 31 species. Of course, the source material will vary depending on the birds where the individual Lesser Goldfinch resides.
To say that any finches are “resident,” though, is something of a stretch of the word. All finches are somewhat nomadic, wandering where food resources are abundant, often recognizable by their undulating flight and distinctive calls as the flock moves from place to place.
The small sizes of the goldfinches allow them to exploit food resources inaccessible to other animals. Pine Siskins, the heaviest of the lot, weigh in at only about 15 grams – much lighter than a titmouse. They can easily reach the pendant clusters of sycamore seeds, that hang from the tips of twigs like old Christmas tree ornaments. They then extract the seeds with their tweezer-like bills. American Goldfinches are still lighter, at 13 grams about the same as a Field Sparrow.
As the name implies, Lesser Goldfinches are the lightest of the bunch: weighing less than 10 grams, lighter than a chickadee. At that weight, even the stalk of a Zexmenia seedhead can support a Lesser Goldfinch.
Most other seed-eating birds switch to an animal diet (such as insects and spiders) during the breeding season. The high-protein diet helps chicks to grow more quickly. The goldfinches, however, stay with seeds, the male regurgitating seeds to feed his nest-bound mate, then both parents doing the same for their offspring.
Sticking to a vegetarian diet does have one side benefit: goldfinches and siskins are poor cowbird hosts. In a Canadian study of 47 American Goldfinch nests in which a cowbird had laid an egg, no cowbird chicks survived. Malnutrition appeared to be the cause of mortality.
– Bill Reiner