A new, all-singing, all-dancing app for bird-spotting had captivated one member of a science discussion group. Apparently, it was able to identify a species by its song as well as its plumage. The very thought promoted a host of questions.
Do all birds of a kind sing the same song? Is each species’ song unique? Why do birds sing in the first place? Do all birds sing?
Curiosity aroused, further thoughts about the everyday behaviour of birds followed: courtship, flocking, flight, migration – ordinary behaviour, for all to see. This blog looks into some of these.
Not all species of bird sing, but a large number do. The order known colloquially as songbirds (the “passerines”) accounts for 60% of all birds and includes 6,500 different species. Birds are able to make their various utterances thanks to one or two membranes that vibrate in a part of their throat known delightfully as the “syrinx’ – the name of a nymph in Greek mythology who was transformed into a reed when chased by the god Pan. The reeds made a haunting sound – the basis of panpipes,
But, of course, not all birds sounds are melodious. Ornithologists break them into two broad groups: calls, which are brief (“cheep cheep”) and songs, which are longer and more melodious. As you might expect these serve different purposes – all critical for survival and reproduction. Calls are commonly associated with alarms: warning of nearby predators, instructing the young or guiding a flock, for example. Small birds may create a chorus of calls around a predator to identify it and with luck scare it away. Calls to chick may command them to stay still when a predator is around or to swim or peck at food. They, in turn, may use calls to encourage their parents to feed them.
Songs, on the other hand, serve different purposes. Primary among these is finding a mate. Singing may attract a mate and can kick off courtship behaviour by signalling readiness to breed. It may also be used between an established pair to help maintain their bond. Research suggests that the quality of singing and the repertoire are associated with the health of the individual. In this way genes favouring good singing would be more likely to be passed on to the next generation. This is how birdsong would have been favoured by evolution.
Another important function of birdsong also favours survival of the individual and his genes. And, yes, it’s usually a “he”- males do most of the singing. By getting to the breeding area first and chanting loudly and clearly an individual marks out and maintains his territory. Sometimes a kind of dialogue with a rival develops in which boundaries are in effect negotiated. Superior vocal quality can mean that one individual gets established as the dominant one without the need for physical conflict.
Each species has its own identifiable song (though, just to confuse us, some species go out of their way to mimic others). The precise pattern can vary according to the age of the individual and the time of year. More surprising, perhaps is the evidence of distinct regional dialects. Different populations of the same species, isolated from one another, may develop distinctive variations on a common theme. Song types have also developed in response to the local habitat. Those that live in dense woodland or reed beds, for example tend to make louder sounds. This audio example is of a reed-loving bittern. Other types choose high wires or posts to ensure their chants are heard far and wide. This audio example is of a European robin.
It’s in early morning and evening that most birds choose to sing. This audio link recorded by Bob Harvey in a wood in Lincolnshire is a reminder of that glorious sound. The light level guides them, so a cloudy morning will affect their timing. Because each species reacts to a different level of light, the order in which singers join the dawn chorus is always the same. Springtime marks the climax of the singing year as territories and courting are being negotiated. The Royal Society for the Protection of Birds (RSPB) suggests that the low levels of light at dawn and evening are favoured as it’s too dim to go out hunting for food yet a good time to risk singing out for a mate, while remaining obscure to predators. Things quieten down after eggs have been laid, to avoid signalling their location.
Research has focussed on whether songs are learned or inherited. An experiment with young chaffinches reared away from all chaffinch song resulted in a very limited kind of singing. This suggests that the ability is partly genetic and partly learned from parents – like so many of our human traits. Calls, on the other hand, with their much simpler patterns, are simply inherited through the genes.
The way birds produce their sounds is not quite the same as us humans and other mammals. They don’t produce their sound in the larynx part of the throat but lower down, at the point where the two bronchi branch off into the two lungs. Here the walls are made of flexible membranes for a short distance, and these vibrate as air passes over them. In this area, known as the syrinx, muscles can tense-up or relax the membranes enabling a variety of sounds to be produced. With the advantage of two sets of vibrating membranes, one in each bronchial tube, it is possible for a bird to produce two sounds simultaneously – they have the edge on us there.
The arrival of Spring, with its warmer weather and longer days, brings out the best of birdsong – whether to declare ownership of a territorial patch or to proclaim readiness for mating. But how did the migrating birds manage to find their way back to their breeding grounds after wintering in warmer regions. And how do they manage to stay aloft for so long while doing so?
Evolution has ensured that bird anatomy is well adapted for flight. Wings are shaped like aerofoils to maximise lift as air streams over them and bones are hollow to minimise their weight. Flapping the wings produces both an upward force that counters gravity and a forward thrust to move them ahead. When conditions are right air currents move upwards; warm air at the Earth’s surface is lighter than cooler air above, so rises, taking the lonely hunter with it. These thermal currents enable birds to soar and glide, much as paragliders do near cliff edges.
A study in Switzerland followed swifts in their migration and to the astonishment of the researchers showed that they remain aloft throughout their entire stay in Africa. They didn’t touch down for a full 200 days. The reason for this and the means of achieving it are not fully understood, but a suggestion is that by staying aloft they avoid predators and perhaps disease too. The energy to remain airborne comes from their diet of airborne insects. It’s not certain whether they actually sleep on the wing but they must at least be able to rest in flight if they spend all day and night on the move.
The method of navigation has been studied in many clever experiments and it is clear that several distinct mechanisms are used, sometimes by the same bird.
In one experiment, homing pigeons flew straight back home without a problem after having been transported thousands of miles and placed in closed, airtight cylinders surrounded by magnetic coils on a tilting turntable! Despite being blasted with random noise and randomly intermittent light, the experiment proved they must have both a compass and a map built in.
Many other experiments have been conducted in an effort to understand how birds manage these extraordinary feats of navigation. There seem to be several distinct methods used by various species. One is by detecting tiny variations in the Earth’s magnetic field. Towards the north and south poles, for example, it gets stronger, towards the equator, weaker. This offers an approximate sense of latitude. Experiments at Lund University in Sweden and Oldenburg in Germany suggest that there are two distinct pieces of apparatus in a bird that are sensitive to the Earth’s magnetic field: one is a map, indicating where they are; the other is a compass, showing where they are headed.
Tiny amounts of the magnetic material iron oxide in the upper part of their beaks line up with the Earth’s magnetic field. This causes cells in the region to send off a signal along the nerves to the brain. When researchers numb these nerves this ability is lost. Slight variations in the strength of the Earth’s magnetic field may be detected, providing a kind of mental map.
The sense of direction – the “compass” seems to come from the action of particular proteins in the retina of the eye. Studies with robins show that when blue light shines on the retina it knocks an electron out of certain atoms in the protein, which renders the latter sensitive to magnetic fields. This in turn results in a signal sent along the optic nerve to the brain. It’s almost as though a bird can “see” a magnetic field. Amazing!
This effect seems to be sensitive to the angle the magnetic field makes with the surface of the Earth – the so-called angle of dip. This angle is zero at the equator and nearly 90° at the poles – so it gives a clear indication of how far from the equator a bird is – its latitude.
Magnetic cues are just one way birds find their way across the globe. The shifting location of the Sun provides another option.
As the Sun changes position with the time of day as well as with viewing position round the globe, birds’ have to be aware of the time of day to work out their location. It appears that the bird’s “body clock” or circadian rhythms enable it to allow for changes in the Sun’s position over the course of the day. It seems from experiments with pigeons that young birds have to learn this skill – it’s not inherited for their parents. What an extraordinary feat!
A completely different source of information for a bird migrating at night time is the stars.
In experiments involving Indigo Buntings, young birds that had been raised in a lab were placed inside a planetary dome onto which an image of the night sky was projected. This revealed that the buntings did indeed navigate by starlight but did not use the pattern of stars as you might expect.
We are accustomed to the illusion of a relatively static backdrop of stars in the night sky, grouped in apparent patterns we call constellations. A first guess might be that birds somehow memorise these patterns and fly at particular angle to them. But experiments reveal this is wrong. Birds turn out to are smarter than that.
In reality the night sky does not remain static from our point of view on Earth. We are spinning around once every 24 hours, so through the course of a night the stars in the sky appear to move slowly around a circle (or part thereof, depending on where we are on the Earth’s surface). The constellations do not remain in one place.
By good fortune however, in the northern hemisphere, one star appears to lie at the very centre of these circles – the North Star or Polaris – meaning that it lies directly above the north pole. Here is a short video clip of constellations appearing to revolve around the North Star.
Birds appear to register the slow rotation of the stars during the night and from this are able to locate the still centre of rotation- the North Star. This enables them to identify the direction of North wherever they are, so they can just point the opposite way to head South for their winter migration.
An alternative method of navigation involves the landscape over which birds fly. They may pick out lengthy landmarks such as coastlines, mountains or rivers. Another more controversial theory suggest some birds may use an olfactory map sensitive to the smell of pine trees or an onion farm for example over range of a few hundred miles. Some seabirds may use this sense for local navigation.
A gaggle of geese crouching in a meadow; a mass of swifts whirling around a tower – everyday evidence of a social aspect of bird behaviour. Even more dramatic are the giant murmurations of starlings: gripping displays of aerobatic skill (see short video clip). But what’s it all about? How does the individual benefit from such group activity?
The UK Royal Society of Birds (RSPB) has brought together the results of research on the flocking habits of many species. As with us humans there are several distinct advantages to social activity. To start with hundreds or even thousands of pairs of eyes are better than one at spotting a potential predator. It’s safer for the individual when everyone is on the lookout. Together they can also move around in a dazzling display to confuse a predator. Mobbing them as a pack can be even more effective. From the predator’s point of view, it can be harder to pick out a single bird when surrounded by so many.
Another reason for working in groups is the hunt for food. Geese for example often forage together, enabling all to benefit from whichever one finds the source. Some species take it further by nesting close together. Parent birds look after their own chicks but all in the group can help protect each other against predators.
Keeping warm in winter is yet another advantage of close living. Small birds will sometimes share a small space for this very reason. Congregating in a tree is an equivalent way of sharing heat for larger birds.
In flight, flocking also offers significant energy saving advantages in much the same way as a peloton does for racing cyclists. By sticking close behind a frontrunning individual a trailing bird (or cyclist) can enjoy the calm of the slipstream left behind by their front-running fellow. Less energy is used up in overcoming the viscous drag of the rushing air, making a longer journey possible.
The V formation of a skein of geese is a special case of group benefit. In this case each bird positions itself carefully in relation to the tip of the wing of the one in front. In the right position they benefit from an updraft resulting from the vortex produced at the wing tip as air rushes over it.
The powerful effect of this is illustrated in a dramatic photograph from NASA research. Disturbance to the air at the tip of an aeroplane’s wings causes the air to rotate in a huge horizontal vortex. The coloured substance injected into the air in this experimental study helps us visualise where the air is beating down on a wing and where on the opposite side of the vortex it provides an updraft.
Once again, exploration of matters of everyday interest – the fascinating behaviour of birds – leads us into diverse branches of science: from animal behaviour and throat anatomy to magnetism and astronomy. The effort and imagination of countless scientists and nature lovers around the world help illuminate the everyday world we observe.
© Andrew Morris 21st June© 2021