Birdsong - more than magical

There's nothing more uplifting than the voices of songbirds gladdening the morning chill. Some, like chickadees and goldfinches, have been here all winter long, twittering and calling back and forth in their flocks. On Poor Farm Road a northern parula or two were heard as early as mid-March and newly-returned song sparrows and phebes are starting to proclaim their territories. 

All birds make vocalizations, but not all can sing, for example turkey buzzards can only grunt, hiss, and rattle their beaks. That's because they lack a syrinx, the paired organ that is unique  to birds and contains the vocal cords. It is situated deep in the chest where the bronchus divides in two to supply each lung. Because each side of the syrinx can make sound independently, a bird can sing in harmony with itself.

Bird vocalizations can be divided into songs and calls. Almost all birds make calls, which are short and simpler than songs, but are nevertheless numerous and varied even in one species. They may be used for very specific situations, for instance the male cardinal has a call used when driving away his offspring from a previous brood . This is one of about 16 calls in the cardinal’s repertoire. Other calls are made in response to invasion of territory, approach of a predator, or when one cardinal brings food to another, etc. In other words, calls are used to accompany behaviors of different kinds.

Songs are different from calls. They are typically longer and more complex and made primarily in the breeding season. Singing is a behavior unto itself with two purposes - to defend a breeding territory and to attract a mate. We think of songsters as being males, but there's growing evidence that female birdsong is more widespread than previously thought.

The male's song determines his territory - literally. The song announces his ownership and the further the song can carry the larger the territory. When songs can't carry very far, for instance in noisy urban environments , birds have smaller territories compared with the same species in a rural environment.  That's in spite of the fact that urban birds sing more loudly and at a higher pitch, to avoid being drowned out by low-pitched traffic noise. Urbanization of habitat is even changing the songs themselves. Thus white-crowned sparrows in a San Francisco park used three different "dialects" of songs 50 years ago. That has changed, with the song in the highest pitch becoming dominant and another song disappearing completely.

The song of the male attracts females and also motivates females to engage in courtship and breeding. On their part, the females use the songs to compare different males and decide which one will make the most desirable breeding partner. In many species the males have more than one song. For instance song sparrows have between 5 and 15 different songs that the young bird develops in the first year of his life. What's the point of this? It turns out that females are attracted to the loudest males with the most extensive song repertoire. And it is not just a fascination with variety. Male song sparrows with a wide repertoire rear more offspring and live longer, a measure of their superior genetic fitness. This is corroborated by studies on inbreeding in song sparrows. The more inbred the males were, the smaller their song repertoire.

As far as loudness goes, there's no correlation between a bird's size and the volume of its song. The loudest bird is not your neighbor's rooster, it is the white bellbird, a rainforest native of Venezuela, Guyana, Suriname, French Guiana, and northern Brazil, (collectively known as the Guyiana Shield) - a bird about the size of a pigeon. Its metallic call packs up to 125.4 decibels, which is louder than a rock concert or a chainsaw, and can be heard a mile away. The Carolina Wren isn't far behind, with a song of up to 100 decibels. That’s about as loud as a healthy rooster at close range. Note that the decibel scale is logarithmic, so an increase of 10 decibels means a 10-fold increase in sound energy, while a 20 decibel increase means a 100-fold increase in sound energy. Thus a white bellbird, at 125 decibels is over 100 times louder than a rooster at 100 decibels.

A hundred decibels is astonishing for a Carolina wren that weighs about three quarters of an ounce. There's a hypothesis that loud voices developed to carry further in densely-vegetated forests, but there are many exceptions to this assertion. 

Interestingly, the energy expended in singing is quite small, even for very loud songs. Studies on a few species found that singing raised their metabolic rate 1.02- to 1.36-fold compared to the non-singing state. The real cost of singing is that it can reveal a bird to predators while time spent singing is time that could have been spent foraging for food.

One may wonder why birds aren't deafened by their own voices. Roosters, that are more amenable to study than wild birds, turn out to have built-in ear plugs. A knob of tissue moves to cover the rooster's ear canal when it opens its beak and throws its head back to crow. Other birds may have similar muscular mechanisms. Birds also have a lifelong ability to regenerate damaged inner ear sensory hair cells, something that humans wish they could do, but cannot. 

Migratory birds exhibit remarkable memory, up to 60% return to the same nesting location every year, as long as they survive their shocking annual mortality, revealed by the Audubon Society's research to be over 50% or 2.6 BILLION birds. (Please keep your cats indoors, outdoor cats are a leading cause of human-related bird deaths.) 

We humans reap a benefit from birdsong. There is evidence that it relaxes people physically while stimulating them cognitively. For the birds themselves, songs reduce competition for food by spacing breeding pairs apart, thus increasing the odds of a successful nesting season.