Robins sing at night so mates will hear call

THE ROBINS of Sheffield have had enough. The noise of England’s fourth largest city is just too much for them and they have given up trying to compete with the din.

Many are rescheduling their song recitals to the middle of the night, when motorists are tucked up in their beds and cars are off the road. A singer on the nightshift can get his message out without having to strain his syrinx, the avian equivalent of vocal chords.

Hearing birdsong at night is no longer front-page news. Nocturnal singing is now heard, not only in Sheffield, but widely throughout Britain and Ireland. Robins are the commonest performers, but blackbirds and starlings occasionally sing in the dark. Artificial light, it was thought, was responsible for the strange behaviour; streetlights were fooling the birds into thinking that dawn had arrived and that it was time to get up and welcome the new day.

Now, new research by Dr Richard Fuller at Sheffield University has knocked that theory on the head.

Dr Fuller had a hunch that the strange robin behaviour had less to do with light levels than with man-made noise. Robins sing to warn rivals to keep out of their territories and to entice females into becoming their mates. When traffic noise is severe, these proclamations, as it were, fall on deaf ears. Some recent work on great tits showed that they changed the pitch of their songs in response to the urban din. In a noisy environment, tits become sopranos because high notes can be heard more easily than normal, moderately pitched, ones. Fuller thought that, likewise, the Sheffield robins were trying to beat the traffic, but how could he prove that his theory was correct?

Armed with noise meters, Fuller’s team visited 121 sites in Sheffield and measured the noise levels at each one. The team returned to the sites at night to record the levels of artificial light and to listen for singing robins. They found that the locations where birds sang at night had been the noisiest ones during the day. The day-time sound levels at these sites were 10 decibels above those recorded at locations were robins remained silent at night. The levels of artificial light, on the other hand, did not seem to matter, and didn’t affect singing behaviour.

Does this mean that a robin which has become exasperated by the noise, makes a conscious decision to postpone singing until things quieten down at the dead of night? This seems a bit unlikely; such behaviour would display extraordinary foresight and planning on the part of a bird. There is another possibility, however; a kind of artificial natural selection may be operating. According to this view, a tiny minority of robins have an inherited tendency towards night singing. In an environment where singing by day is ineffective, these nocturnally disposed vocalists have an advantage over their peers. By singing in the dark they are able to get their messages out more effectively than the day-time singers can and so, the theory suggests, they obtain the best territories and are able to hold on them. When it comes to securing a mate they also fare better; a singer who is heard clearly is likely to have the pick of the females. With these advantages such birds would soon outbreed the normal robins, and the genes for night-singing would become widespread in the population. Interviewed on the Mooney Show, Dr Fuller would not be drawn on the issue. More research, he felt, is needed before we can answer this intriguing question.

Nor do we know what effect working the nightshift has on the health and well-being of robins. A bird’s song is, in relation to its size, extraordinarily loud, and a huge proportion of a bird’s energy is expended on singing. All birds need to recharge their batteries, but when do our nocturnal robins sleep and do they get enough rest? Working by night has health implications for humans and why should birds be any different?

The sleep behaviour of birds has been investigated and, as far as we can tell, bird sleep is remarkably similar to our own. Electrodes can be attached to the head of a sleeper, and the electrical fluctuations in the brain recorded. When people sleep two distinct electrical waves are observed. These correspond to ‘active’ and ‘calm’ sleep periods. The active periods are those in which we dream. When we do so, our eyes move about rapidly and this state is known as ‘Rapid Eye Movement’, or ‘REM’, sleep. In the calm periods of the night our eyes don’t move and we are completely unconscious.

The brain waves recorded from captive birds are similar to those of humans. Birds’ eyes are fixed in their heads and can’t move about in the way that ours can, so it’s no use looking for rapid eye movements. Birds do, however, blink. In their equivalent of ‘active’ sleep, birds open and close their eyes frequently. There is thus some circumstantial evidence that birds dream, although, if they do, we have no idea what they dream about.

But there is an alternative explanation. Even in calm sleep, birds open their eyes every few minutes or. This seems to be a form of semi-conscious vigilance. Birds may not be dreaming when they blink in active sleep, but are keeping an eye out for danger.