|MadSci Network: Zoology|
What triggers bird migration & what keeps them on course?
There are two primary theories to explain why birds migrate based on physical change.
Both theories assume that each annual cycle is divided into seasonal halves
· The winter feeding phase
· The reproductive phase
The Wallace theory postulates that they were originally coincident but gradually diverged with geological and climatic changes. The breeding grounds representing their original ranges while the winter grounds representing the diversification to areas with milder conditions.
The Brooks theory, which is a biological theory, postulates that the winter grounds represent the original range. The breeding grounds, representing a range expansion in search of safer and more suitable locations.
In either condition an impulse to migrate would be the beginning of one part of the circannual cycle. The recognition of which maybe
· Hormonal changes.
· Changes in day night length.
Navigation is the part of migration that has puzzled biologists the most. For some species, biologists still do not know how they navigate thousands of miles to the exactly same place every year. Nevertheless, there are four theories that are accepted the most by biologists, and birds find their using a combination of these theories.
1. Use of the sun: Birds, like humans, possess an internal circadian clock that allows them to track the daily light-dark cycle. Along with this internal clock, birds use the sun's shadows to gain a sense of location. Through the use of these two devices, birds are able to use the sun as a compass. This theory is supported by an experiment that was conducted with homing pigeons. The pigeons were kept in an artificial cycle of light and darkness that was out of step with the natural 24 hour cycle. As a result, the birds misintrepreted the position of the sun. Upon being released in an umfamiliar location, instead of orientating themselves toward their home range, they orientated in an incorrect direction. One of the main problems with using the sun as a compass is that birds cannot compensate for the fast time shiftings that occur when making a long transfer in an easterly of westerly direction in a relatively short period of time. This problem would increase as the birds approach the poles, where longitude lines become closer together.
2. Use of the stars: Because many birds migrate at night, the sun is not available as a guide. These nocturnal migrants have learned to use the stars for navigation. In the northern hemisphere, the constellations appear to rotate around the North Star. Birds can orient themsleves in relation to the North Star, and unlike the sun-compass, this "star-compass" is not time dependant. Young birds learn this pattern of rotation and use this to tell north from south. This theory is supported by an experiment that was conducted with indigo buntings. Two groups of young buntings were kept in a planetarium. One of the groups experienced the planetarium's "sky" rotating around the north star. The other group experienced the constellations rotating around Betelgeuse, a star in the constellation Orion. Normally indigo buntings migrate south in the autumn, and when it came time to migrate, the birds which had seen the constellations rotate correctly orientated themselves to the south, away form the north star. The other birds, by contrast orientated themselves away from Betelgeuse. Some birds have also learned to use patterns of stars, small clusters of stars, or the moon to determine what direction they need to fly. A disadvantage of using the stars to navigate is that the North Star cannot be seen in the southern hemisphere. Another problem arises on cloudy nights, when the stars cannot be seen.
3. Use of the earth's magnetic field: Biologists have two different theories on how birds can use the earth's magnetic field to navigate. One theory is that birds have certain pigments in their eyes that become weakly magnetic when they absorb light and thus alter certain nerve signals which the eyes send to the brain. A second, and more popular theory, comes from the fact that scientists have detected tiny crystals of magnetite along the olfactory tract in the brains of some birds. A problem arises with magnetic navigation because the magnetic poles and the magnetic equator do not coincide with the geographic poles and the geographic equator. Some biologists think that the magnietic compass is not based on polarity, but the inclination of the magnetic field. Others say that the magnetic compass is recalibrated against the sun and star compasses during rest stops along the migration route, and that if the birds do not have enough time at rests, they may get lost. Biologists still do not know how the birds can sense the position of the magnetite crystals in their heads, and there is little experimental data on the subject. Interestingly enough, some researchers say that humans have the ability to sense the magnetic field as well. This came about after researchers at the California Institute of Technology identified crystals of magnetite in human brains. They did not know if these crystals were sufficient enough to give humans an unconscious magnetic sense, but a later experiment showed that may be true. Robin Baker and his colleagues at the University of Manchester blindfolded volunteers and drove them to unfamiliar locations. Still wearing the blindfolds, some of the voluteers could state their direction relative to "home". But those wearing bar magnets on their heads lost the ability.
4. Use of visual landmarks: The idea of birds using landmarks to navigate had long been a popular theory. Many birds have been known to follow visual clues such as rivers, coastlines, and mountain ranges in order to arrive at the correct destination. However, this idea does not explain how birds find their way during their very first migration without getting lost.
Other bird navigation theories: There are several other, less accepted theories dealing with bird migration as well. One of these deals with birds using polarized light from the sun as a compass. Normal sunlight oscillates in random orientations, but sunlight coming from distant parts of the sky is usually polarized. This results from particles in the atmosphere scattering some orientations of light more than others. Some biologists say that it is likely that birds use polarized light as a source of compass directions because it would work even if the sun was obscured by clouds. Another theory is that some birds use smell to navigate. Researchers found that they can disorientate pigeons by exposing the birds to air collected at one site and then temporarily destroying their sense of smell with a local anaesthetic before releasing the birds at a different site.
Despite all the theories and experiments dealing with navigation, there is much that is still not understood about how birds determine their position in relation to a fixed goal. For example, a Manx Shearwater bird was transported from its burrow in Wales to Boston, several thousand miles away. Yet within 12 days, the same bird was back in its burrow in Wales. This is just one example of birds' amazing navigational and orientational ability that still mystifies biologists.
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