By Benjamin P. Burtt
The Time Schedule for the Spring Migration During the Next two Weeks
Why do birds sing and Why is so much of this singing done at dawn?
This week a number of new birds appear from the south. These include the brown thrasher, the chipping sparrow, the ruby-crowned kinglet and the white-throated sparrow.
This is the chipping sparrow that is due back from the south now. In breeding plumage it has a reddish cap, a black line through the eye and a distinct white line above it. The breast is white or grayish and without a central breast spot. (This chipping sparrow photograph was provided through the courtesy of Kevin and Jay McGowan of Dryden, NY )
In the winter plumage of the chipping sparrow, the red is not there, but the crown is brown with darker lines running front to back . Keep this in mind, and if you spot a red-capped sparrow in winter, it is most likely a tree sparrow down from the north.
Very often observers report chipping sparrows to me in the winter, but here in Central New York almost every such bird turns out to be a tree sparrow whose central breast spot was not noticed.
On or about April 25 we will see the first warbler. Usually it is the yellow-rumped warbler. Others species due then are the green heron, the spotted sandpiper the house wren and the common tern.
Why do birds sing?
In the breeding season, male birds select a territory , an area where they will nest. Since only about one egg in four successfully hatches and produces a youngster that survives, they must take elaborate steps to lessen competition and to find food for their offspring.
They try to attract a mate to their territory and keep other males from nesting nearby. That is, they are more likely to produce young and to find food for them if other nests of their species and not packed in too closely.
These purposes are served by the song which is a proclamation that a female would be welcome and that other pairs are to stay away.
Once a mate has been selected, the male still keeps singing to identify himself to other males and to keep them away. The song also tells his mate that he is nearby.
For a given species, the song is almost the same for all individuals, but not exactly. For example, we can recognize a song as that of a cardinal, but the song of each male differs just a little in tone or pitch from all others of his kind. You and I are not aware of this difference unless we use some electronic equipment, but other male cardinals can identify the singer. It is almost as if, as the bird sings, he gives his name or home address over and over again.
For the song to be effective, it is very important that it be clearly heard and understood by the other males. They will then know that the territory has been established and just whose it is. If another noise interferes or if the song is somehow garbled, then the message will not be understood and will not serve the purpose for which it is intended.
Why is there more singing at dawn than at other times of the day?
This brings us to the question, why is there a dawn chorus? Why is there so much bird singing just after sunrise and so much less later in the day? Are songs more clearly heard at dawn? Can they be better understood at a distance at dawn?
At that time of day sound travels better and all the special notes and tones that identify a particular bird can be heard. There usually is not much wind at dawn. Gusts would cause a sound to fade in and out. The heating of the air by the sun at mid day affects the sound in a similar way.
Have you ever been at a lake and noticed how the sound of your neighbor's radio comes and goes as the wind changes? Whether a note is loud or soft may help to identify a particular bird and all such tones must be heard.
At dawn there is less wind and heat and the identitiy of the singer can be determined better at that time than at mid day when other noises spoil the song.
Prof. Paul Handford of the University of Western Ontario has recently done some experiments on this in which he played recorded songs of the swamp sparrow and the white-throated sparrow at different times of the day and measured the strength and quality of the sound at different distances. He tried it in a wooded area and in an open marshy habitat.
Clearly, from his experiments the sounds at a distance were more often garbled at mid day than early in the morning. In the woods, the tree trunks and limbs also reflect the sounds and little echoes are produced.
At a distance such songs heard by the ear include the original song plus all the echoes. Thus it does not sound the same as the original song made by the bird. It is similar to the difficulties we have in following a conversation when several people are talking at once.
Birds have developed the habit of doing much of their singing early in the morning when the songs can be heard more clearly and at greater distances. Those that sing at dawn do a better job of keeping competitors away and are more likely to successfully raise their young.