How birds find their way year after year from one part of the world to another is one of the most enduring mysteries of bird migration.
It is not just a question of finding the way, for birds must also know where on their journeys to change direction or to accumulate extra body reserves in preparation for a long flight over sea or desert. The fact that birds can respond appropriately at specific places on their route implies that they possess some geographical sense and ability to detect and react accordingly to conditions at particular locations.
Requirements for better navigation
To migrate effectively, birds need a sense of where they are or need to be migrating, an ability to navigate from one place to another, a sense of direction, and a sense of time, both seasonal and diurnal. In conclusion, they need the equivalents of a calendar, clock, map, and compass, combined with a good memory, all packed into the small brain that in some birds is no bigger than a pea. In the past 60 years, ornithologists have managed to unravel some of these mysteries, but others remain unresolved.
Eye power in migration
The ability of birds to navigate depends partly on their sensory capabilities. The eyesight of normally diurnal birds is better than ours in the daytime, but probably much the same as ours at night. It is good enough to allow diurnal birds to fly in the dark through the open airspace, and to recognize the stars above and major topographical features, such as coastlines and mountains, below. Collisions with obstacles such as radio masts occur mainly on dark misty nights when vision is restricted. In addition, at least some bird species are able to perceive ultraviolet and plane-polarised light facilities well beyond human capability.
Magnetic sense and Earth’s magnetic field
Like many other animals, birds can also detect and respond to the Earth’s magnetic field. This is hard for us to understand because we have no obvious magnetic sense ourselves.
But a study suggests that a bird’s magnetic sense is composed of two components
one near the nostrils, and the other based on the right eye.
Other senses
Other senses which some bird species might use in navigation include smell and hearing. Most birds seem to have a poor sense of smell, but there are exceptions, including petrels, in which the olfaction sense is well developed. In contrast, birds have a generally good sense of hearing, allowing individual migrants to detect the calls of other migrating birds at night. They seem also to be able to detect ultrasound, the low-frequency noises which we cannot hear but that travel over long distances.
Birds also seem able to detect changes in barometric pressure, and to perceive wind direction and speed during flight, perhaps by reference to the ground below.
COMPASS ORIENTATION AND BI-COORDINATE NAVIGATION
Some of the earliest studies involved trapping large numbers of wild birds on autumn migration, transporting and releasing them in a distant location, and using the resulting ring recoveries to find where they went. Such early experiments revealed that naive young birds behaved differently from experienced adults.
Young birds on their first autumn migration proved unable to correct for their displacement and failed to reach the normal wintering areas for their population. Instead, they continued on their usual migratory route for the same distance. The inference was that inexperienced birds migrated on the basis of innate information expressed as a direction and distance from the starting point, the distance being controlled by the duration of migratory activity.
The innate instruction would be something like:
‘travel towards the southwest for six weeks’, or, in cases of non-straight routes: ‘travel towards the southwest for six weeks and then toward the south-southeast for five weeks’. This system is known variously as clock-and-compass, bearing-and-distance, or vector migration.
In contrast to young birds used in displacement experiments, experienced adults that had traveled the route before were able to make the necessary corrections, and reach their former wintering areas. Homing towards a known site is more complicated than the ability to head only in a particular compass direction because it involves true bi-coordinate navigation, requiring a map sense an ability to head towards a specific point on the earth’s surface from some distant location.
CUES USED IN DIRECTION-FINDING
Basics of finding direction
The most obvious way in which birds and other animals find their way around on a day-to-day basis is by the use of landmarks or other consistent features of their home areas. This could explain how some migrating birds manage to return to exactly the same nesting places year after year. But such features are useful only in familiar areas, and when moving over longer distances into unknown terrain, a reliable geographical reference system is needed for navigation. At least two types of factors can act as directional aids celestial and geomagnetic.
In migratory birds, the use of compasses based on the sun and stars and on magnetic information has been confirmed by experiments.
But a prior requirement for using any compass is that the bird should ‘know’ beforehand either by experience or inheritance what direction it has to take. Also, effective use of any of these compasses requires a period of learning, as well as frequent revision as the bird continually changes location while on migration. In other words, they all entail a combination of innate and learned components.
Finding celestial clue
The main feature of celestial cues, such as the sun and stars, is that they appear to change position during each 24-hour cycle as the Earth spins on its axis.
In the northern hemisphere, the sun lies in the south and during the day moves through the sky on an arc from east to west, while at night, the stars rotate anti-clockwise around the geographical north, marked by the polar star.
In the southern hemisphere, the sun lies in the north and moves on an arc from east to west, while the stars rotate clockwise around the geographical south unmarked by a specific star.
In using the sun and related factors in direction-finding, therefore, birds in both hemispheres must allow for a time of day. But the same is not necessarily true for star patterns if they are used solely to indicate geographical north or south, as these directions are given as the center of rotation of the night sky, which is fixed. Timekeeping depends on the bird’s ‘internal clock’, kept to time by the regular day–night cycle.
Sun as compass
The height of the sun’s arc through the sky varies with latitude and season, but it is always symmetrical with respect to true north or south. Its highest point in the sky at midday indicates due south in the northern hemisphere and due north in the southern hemisphere. That birds use the sun as a compass has been known for more than 50 years, beginning with some crucial experiments conducted by a German scientist.
Under a sunny sky, Common Starlings kept in circular wire cages during the migration period orientated in the same direction as free-living starlings. The angle they adopted towards the sun varied according to the time of day. If the sky became overcast, obscuring the sun, their directional preference disappeared. When their view of the sun’s direction was changed using mirrors, the birds orientated at the same angle to the apparent sun as they would to the real sun. These experiments proved that starlings use a sun compass, which gives accurate data only if regulated by an internal clock, allowing adjustment of directional preference as the sun moved through the sky.
Learning sun movements specific to the location
The use of a sun-azimuth compass has now been confirmed experimentally in several bird species and may be common among diurnal migrants. However, simply seeing the sun provides a naive bird with little useful information. The bird must live at a site for some time and learn the way in which the sun moves across the sky, in order to use it subsequently as an effective directional aid. If experienced northern hemisphere birds are transported to the southern hemisphere, they orientate themselves incorrectly, interpreting the sun as if they were in the northern hemisphere (indicating south rather than north). Regular migrants were able to make the necessary adjustment.
Response to polarisation
Sky polarisation patterns change with respect to the sun’s position, and are particularly striking around the time of sunset. The ability of birds to detect these patterns has been demonstrated in experiments in which at least seven species of normally nocturnal migrants responded to manipulations of polarised light from the sky. The birds were tested individually outdoors in otherwise normal conditions in cages covered by sheet polaroids. In every case, the birds changed their orientation as forecasted by changes in the alignment of the polaroids. However, the visual stimulus created by this procedure is quite unnatural, and birds sometimes orientated differently under artificial polarised light than they did under the naturally polarised skylight. Nevertheless, the experimental birds were clearly responding to polarised light as an orientation cue, rather than to the position of sunset.
The star compass
Providing that enough of the night sky was visible, nocturnal migrants proved able to use the stars as a guide. When tested in orientation cages, birds orientated correctly on clear starry nights but became inactive or disorientated under overcast skies. They also confused with varied star patterns in a planetarium for experiment. For example, when North American Indigo Buntings were tested in orientation cages under a natural starry sky during autumn migration, they preferred southerly directions. They maintained directional preference based on immitated star pattern in a planetarium. But when the artificial star pattern was changed by 180°, the birds changed their directional preference to the north. Under a static night sky, no obvious migratory restlessness occurred. Effective use of a star compass evidently involved learning. As with the sun compass, the ability and tendency to acquire the necessary knowledge was apparently innate.
With the star compass, the key factor was the rotation of the night sky about the Pole Star, and in experimental conditions, birds learned to respond to an extremely simplified and reduced star pattern, providing that it rotated about a single conspicuous star.
Birds and constellation
The use of a star compass has now been demonstrated experimentally in at least six different bird species and may be widespread in nocturnal migrants. If the birds use the rotating star pattern only to define the positions of the poles, then as mentioned above, no correction for the time of day is necessary. They may, however, gain further information from star patterns. As birds proceed on their journeys, lasting up to several weeks, some stars disappear below the horizon behind them, while others appear above the horizon in front, another indication that birds are unlikely to rely throughout on particular star patterns.
Also when shown northerly skies in a planetarium during the autumn migration season, individuals of this species headed southeast, but when shown skies characteristic of more southerly latitudes, they headed southwest. This finding revealed that birds could respond to skies encountered at different locations in their journey.
Integrated use of celestial cues
Nocturnal migrants usually set off around dusk, when celestial cues related to the sun (such as sunset position, horizon glow and skylight polarisation pattern) are clearly visible and the star pattern is gradually emerging. They could use all these celestial cues within a short period. Radar observations reveal that migratory birds keep flying in the same direction during the transition from day to night or night to day. This finding implies that birds can switch between the sun and stars for navigation, or that they rely on some other cue, such as the earth’s magnetic field, to maintain their course. In addition, some arctic species which prefer to migrate by night at lower latitudes necessarily migrate in daylight at high latitudes in summer when the sun never sets. Bright moonlight can make ground-based features more visible, but can apparently hinder the use of star patterns and produce the same disturbing effects as the cloud.
The magnetic compass
A second major system of bird orientation makes use of the earth’s magnetic field. Imagine the earth as a hugely powerful magnet, whose magnetic north pole is situated fairly close to the geographic North Pole, and whose magnetic south pole is similarly close to the geographic South Pole.
Running through the atmosphere between the two magnetic poles like segments of orange lines of magnetic force circle the globe. The field lines leave the earth vertically at the Antarctic pole, then curve around the earth and re-enter its surface vertically at the Arctic pole. The magnetic vectors point downwards in the northern hemisphere and upwards in the southern and lie parallel to the earth’s surface at the equator.
Hence, for any animal that can measure the inclination of the force lines, the earth’s magnetic field can indicate latitude and direction (toward the equator or pole) within each hemisphere (except near the equator itself, where the force lines are horizontal and near the poles where they are vertical).
The intensity of the magnetic field can also give positional information, for it declines progressively from pole to equator in each hemisphere.
The role of the geomagnetic field as a reference for migratory direction was later shown in juveniles of other species. Passerines were hand-raised without access to celestial cues and, when tested during autumn migration, they headed in their population-specific migratory direction with the magnetic field as the only cue. It seemed that the geomagnetic field alone was sufficient to establish a migratory bearing, at least in temperate latitudes. Birds tested at higher latitudes, where the angle of magnetic inclination was steeper, needed to have observed celestial rotation (to fix north) in order subsequently to adopt the correct heading in relation to magnetic cues alone. Overall, the use of a magnetic compass has now been demonstrated experimentally in more than 20 bird species, including diurnal and nocturnal migrants in both hemispheres. Its use thus seems widespread, but partly in association with celestial information.
Other potential cues
From time to time, birds could make use of yet other directional cues, including auditory and olfactory ones.
For example, infrasound emanating from wind against mountain ridges or from waves breaking on shorelines can travel long distances and could give strong directional cues to birds able to detect them.
In addition, homing pigeons have proved capable of finding their way by smell, at least over short distances. However, it is not known how much birds make use of such potential cues, or of the distances over which they could do so. For most birds, it is unlikely that olfaction could work over more than a few tens of kilometers in a favorable wind.
Birds often behave in a way that suggests they can predict impending weather, for example by leaving before a storm arrives. One way in which they might do this is by monitoring barometric pressure, and experiments have suggested that pigeons have this ability. Such a pressure sense could assist migrants in maintaining their flight altitude, or in moving from areas of high to low pressure, or vice versa. By this means, in arid areas, birds could move towards areas where rain is falling, a frequently observed behavior that has so far defied explanation.
To summarise so far, birds can use a number of different compasses for orientation during long-distance migrations, based on information from the sun and related patterns of skylight polarisation, star patterns, and the earth’s magnetic field also their senses. After calibration, these different cues used by the same individual would normally give the same directional message.