Why Birds Sing at Dawm

Forest and Bird, Issue 303, 1 February 2002, Page 18

Why Birds Sing at Dawm

CRAIG BARNETT

finds ‘that | bigger is

better when it comes to singing:

peak of songbirds singing around sunrise signifies what is termed the dawn chorus. Often starting before sunrise, a bird may sing non-stop for up to an hour. Birdsong has aroused human interest for millennia. Our Palaeolithic ancestors made bone whistles with which they imitated the songs of birds. Popular folk wisdom saw birds as singing because they were in love, or because of their love of spring. These romantic notions surrounding birdsong are evident in the works of poets such as Keats and persist to this day. Despite centuries of human fascination with the dawn chorus, the reasons why birds sing at dawn remain mysterious, however. Why should birds sing more at dawn rather than later in the day? It is the human equivalent of waking at first light, and singing an aria over and over at the top of one’s lungs before even a cup of coffee!

The question of why birds sing at dawn is not the same as asking why birds sing at all. This has been extensively studied and there is clear evidence that birds sing for two reasons: to attract a mate and to protect resources, such as territories. The real question is what benefit a male gets by singing at dawn rather than later in the day. Although the matter is still the subject of debate, there are a number of possible explanations for why males sing at dawn. Firstly, dawn may be a particularly suitable time for sound transmission. It has been estimated that singing at dawn may be 20 times more effective than singing at midday. This is because there is less wind at dawn and so less interfering wind noise. Another reason males may sing at dawn is in response to overnight mortality of rival males, and so dawn is the time that vacancies in a neighbour’s territory are first noticed. By singing, males are able to assess which of their neighbours are not present and so invade their territories. Also, female birds are at their most fertile at dawn, and so copulation at this time is more likely to be successful in fertilising an egg. By singing at dawn, a male may encourage his mate to copulate with him, or prevent rivals from entering his territory and courting his mate. Again, the conditions at dawn may be particularly unsuitable for other activities such as foraging, making singing the best alternative. Singing requires energy and dawn is the time when a bird’s energy levels are at their daily lowest. Perhaps by singing at this time, it tells a listener something about the worth of the singer.

Finally, recent models of foraging behaviour suggest birds aim to gain a certain amount of fat by the end of the day, which is sufficient to guarantee their survival, even on the coldest nights. Because birds rarely experience the worst conditions, on most mornings they will have surplus energy, which is used to sing at dawn and throughout the rest of the day. This theory is supported by the fact that birds sing more when they receive supplementary food. However, there have been no studies showing that birds increase their fat reserves along with song output when they receive supplementary food. To test this theory, I measured the daily weight gain (as fat reserves increase with weight) and song output of two native bird species (silvereyes and New Zealand robins) over three days. On one of these days, I provided the birds with supplementary food and found that an individual’s weight increased by several grams after feeding. The birds also sang

more when they were fed. Both New Zealand robins and silvereyes had higher fat reserves when they were fed and were able to sing more at dawn chorus and throughout the day, compared with when they received no supplementary food. These results suggest that the dawn chorus and singing in general, is dependent on the bird’s energy levels. This has important implications for our understanding of the meaning of song. Singing to convey information about the singer has an an energy cost. Males that sing at higher rates under natural conditions may be demonstrating their ability to reach higher fat reserves by the end of the day. Birds obtain larger reserves by being on better territories or through excluding rivals from food. Through this mechanism, birdsong could be conveying information on the competitive quality of the singer. This means that males may be singing at dawn to demonstrate their high quality to mates and rivals. This adds a new dimension to understanding the dawn chorus. Although the conventional explanations probably play some role in explaining why birds sing at dawn, my research shows that a bird’s energy levels are also important in determining the amount of time a bird can sing. This supports the theory that a bird’s song is dependent on its fat reserves. So it seems, the night isn’t over until the fat bird sings!

CRAIG BARNETT, has recently completed an M.Sc. in zoology at the University of Canterbury.