First, it helps to understand why birds make sounds. The great eruption of song that we hear in spring is mostly the sound of males trying to impress females. For most species, it is the male who “sings” and the non-singing female who chooses a mate based, in large part, on his singing prowess. A bachelor male will sing all day long, but he sings far less often after he attracts a female to pair with him. Even a paired male continues to sing throughout the spring and summer, however, because females often make additional mating choices after pairing. Although she initially chooses and pairs with a social partner to help raise her offspring, there’s no guarantee that he will be the genetic father of all of her young. It’s not atypical for a male to father only half of the young in his nest (as revealed by DNA fingerprinting), the other young being fathered by male neighbors who for some reason impress his mate. Thanks largely to these kinds of mating choices made by females, males continue to sing long after pairing, so throughout the spring and summer, we are eavesdropping on some of these most fundamental and complex rituals in the lives of birds.
How females make their choices remains one of the great mysteries of birdsong and is the focus of much current research. She probably listens for clues that tell something of his history: Has he been in this neighborhood long enough to learn the local dialect? She’d also do well to know his age, as older birds are more experienced and have proven that they can survive from year to year: Does there remain any hint of his babbling stage, suggesting he’s just a yearling, or are his songs steady and perfect, like those of an older male? She must listen for clues to his overall health: How many different songs has he mastered, how intricate are his songs, how consistently can he sing a precise tune, or how much singing stamina does he have? She’d want to know what other males think of him, too, and how he fits into the local hierarchy of singing males: Do other males “trash” him by singing over him and overlapping his songs?
Whatever her standards, it is the non-singing female songbird we can thank for all of the song that we hear. Over evolutionary time, females have chosen males who sing in certain ways; those males father the offspring, who then inherit the genes to sing like their fathers or be as choosy as their mothers. She is the silent composer of the male orchestra that we hear and enjoy throughout the spring and summer.
While a primary function of birdsong is to impress females, it can serve other purposes as well. As a male sings, he announces his presence on a territory and warns other males that he will defend his space. Singing isn’t necessary for territorial defense, however, because territories in many species are defended by both males and females without song during the non-breeding season. It’s tempting for birdsong lovers to conclude that male birds sing “for joy,” but that is scientifically questionable for several reasons. Scientists dismiss such an anthropomorphic thought outright, for example, because it attributes human emotions to birds. The “for joy” explanation is also male-centered, as if he sings for himself, and I view it as an insult to every female songbird and all her female ancestors who have, by their mating choices, designed his songs and how he delivers them. In addition, if a male bird sings largely for joy, we’d then have to explain why females of most species are such joyless creatures, or why males are joyful for such a short time each year. It is possible, of course, that birds experience some pleasure in singing; perhaps endorphins surge through the body of a singing bird so that it experiences something comparable to a “runner’s high.” In the end, though, it’s all rather simple: He sings “for her.”
In the end, though, it’s all rather simple: He sings “for her.”