Birds’ songbook spectrum features many variations

The more I learn about birds, the more they fascinate me. Their complexities are endlessly intriguing.

An acquaintance of mine recently asked if birds are born knowing their respective songs, or if they had to learn them. The answer may surprise you.

Take the brown-headed cowbird, for instance. This bird – similar in size to the red-winged blackbird, but chunkier – is known as a brood parasite. The female has the audacity to dump her eggs into other birds’ nests. She compounds this offense by often choosing birds that are about a quarter of her size, such as the diminutive wood warblers. She often will push the host bird’s eggs right out of the nest. If any remain to hatch, the young often are not able to compete with a chick four times their size and they die off. The parent bird is left with this monster of a nestling and its prodigious demands for food.

The incredible thing is the young cowbird does not become “imprinted” upon its foster parent: Among other things, it does not adopt its song, but instead sings the song of its biological parents. When it is grown it seeks out its own kind to mate with and produce young.

All in all, it is a brilliant and successful reproduction strategy, but one dependent on innate or “hardwired” behaviors.

At the other end of the spectrum is the northern mockingbird. In his book, “Ornithology,” Frank Gill, currently the National Audubon Society’s senior vice president for science, relates that male mockingbirds have been known to sing more than 150 different songs; most are imitations of other birds but some are reproductions of mechanical sounds, as well as frog and insect noises. In addition, he writes, the songs change from year to year and increase over time.

This strategy clearly relies on the birds’ learning ability; it is not born knowing how to imitate all of those sounds. In addition, repertoire size has been found to reflect the overall quality of a male and his territory, and there is apparently no limit to the bird’s window of song learning.

Somewhere in the middle of this spectrum are birds with fixed repertoires whose song learning is restricted to a certain stage of growth, which, Gill writes, is at an early age. He describes the four stages of song learning: the critical learning period, the silent period, the subsong period (which Gill describes as analogous to infant babbling), and the final song crystallization period, when the bird perfects its ability.

It doesn’t stop here. Each bird adds variations to its own songs, and there are actually different dialects among populations – just as there are in human societies.

Surprised? Intrigued? I know I am, and will always continue to be.

Chris Corio, a volunteer at Fields Pond Nature Center in Holden, can be reached at fieldspond@juno.com.