Why do birds sing as dawn breaks? Indeed, why do some sing at dusk? And for what reasons do they eventually stop?
• The main function of most birdsong is long-distance communication, either to mark territory or be sociable. As such it is largely ; blackbirds sing to impress blackbirds, not buntings. In contrast, social vocalisation, such as coordinating group activity, largely occurs at short range during active flight or foraging, or when settling down for the night or preparing to take flight as a flock.
Like any form of communication, birdsong bears an energy cost and requires channel capacity, which is limited largely by background noise and the quality of the medium, in this case air. In the mornings and evenings the air tends to be still, which reduces competing noise. It is also cooler at low altitudes, which favours transmission of clear sounds. Also, few birds forage at dusk, so in terms of energy use that is an economical time to perform.
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Because much birdsong is territorial, it is practical for each species to sing at fixed times to avoid wasting energy on talking when no one is listening or when other species are competing for air time. Ideally that male blackbird would be saying: “If you are a male, keep off, or else! But if you are a female, let’s get together.” Later, when other species are singing, he can go off to catch the early, deaf, worm.
Antony David, London, UK
• Birds sing more at dawn and dusk than at other times because that is when they can hear more birds singing. Frequently at these times the wind drops and a temperature inversion forms – this is a layer of warmer air above cooler air. This changes the way in which sound is carried through the air, refracting sound waves back towards the ground that otherwise would have dissipated in the air (Last Word, 18 April 2009). The upshot is that sound is carried further at dawn and dusk.
Thus, if a bird devotes most of the energy it spends on singing to those times, it is heard by the widest possible audience. Of course, there are birds that can be heard singing at any time of day, but even these will tend to sing more at dawn and dusk when there is a temperature inversion.
Nigel Depledge, Spennymoor, Durham, UK