Finding the way

on their journeys, the migrants not only travel vast distances overland, but also cross pathless seas and oceans. The question is - how do they find their way? How are they guided? Here we are face to face with one of the greatest mysteries to be found in the animal kingdom. (William Eagle Clark 1912.)

Many birds are capable of migrating, year after year, from their customary nesting sites to exactly the same winter quarters, sometimes using the same stopover sites on successive journeys. Young birds migrating alone can also find their way unaided by experienced adults to the usual wintering areas for their species, and back to their natal areas the following spring. This holds true even though breeding and wintering areas may lay half a world apart on different continents. Some pelagic seabirds cover enormous distances during their foraging flights and during their migrations over what appears to us as featureless ocean, yet they can return repeatedly to the tiny islands where they breed. How do birds achieve these remarkable feats of orientation and navigation over such huge distances?

It is not just a question of finding the way. Birds must know where in their journeys they need to do particular things, such as change direction or accumulate extra fuel reserves in preparation for a long non-stop flight. The fact that they can respond appropriately at specific places on their route again implies that they possess some geographical sense - an ability to detect and respond in an appropriate manner to conditions at particular locations.

To migrate effectively, birds need a sense of where they are, or need to be, a sense of direction, an ability to navigate from one place to another, and a sense of time, both seasonal and diurnal (essential for navigation by some celestial cues, see later). In short, they need the equivalents of a map, compass, calendar and clock, together with a good memory, all packed into a brain that in some birds is no bigger than a pea. In this chapter, I can provide only a brief review of this vast subject area, concentrating on ecological aspects (for more extensive reviews of particular aspects, see Berthold 1993, Wiltschko & Wiltschko 1995, 2003, Akesson 2003). Despite much research, many unanswered questions remain.

The ability of birds to navigate depends ultimately on their sensory abilities. The eyesight of normally diurnal birds at night is probably not much better than ours: good enough to allow them to fly through the open airspace, and to recognise major topographic features, such as coastlines and mountains, and potential habitat below. Collisions with obstacles such as radio-towers occur mainly on dark misty nights, when vision is restricted. In addition, at least some bird species are able to perceive ultraviolet light and the plane of polarised light.

That birds and other animals can detect and respond to the earth's magnetic field is well established, but hard for us to appreciate because we have no obvious magnetic sense ourselves. There are two competing hypotheses for the primary process underlying the avian magnetic sense. One involves the magnetic material magnetite (Fe3O4) which is found in many organisms, including pigeons in which a magnetite structure is located between the brain and the skull (Walcott et al. 1979). In theory, this structure could function in orientation through the forces exerted by the earth's magnetic field (Kirschvink & Gould 1981). The other proposed mechanism is based on a magnetically sensitive chemical reaction. It involves electron transfer between donor and acceptor molecules, and operates only under short-wavelength light (blue-green end of the spectrum). Being light dependent, it is based in the eyes, presumably enabling the bird to visualise the magnetic field in some way. This 'radical pair' model of magneto-reception has received some support in recent experiments (Ritz et al. 2004), but like the magnetite hypothesis, is still to be properly tested. Neither proposed mechanism may make it though to established fact.

Other senses that some bird species might use in navigation include smell and hearing. Most birds seem to have a poor sense of smell, but in some species, notably petrels, the olfaction sense is well developed, although nothing is known of the range of odours that they can detect (Wenzel 1991). Birds also have a generally well-developed sense of hearing, allowing individual migrants to detect the calls of other birds at night. They also seem able to detect changes in barometric pressure (see later), and can also perceive wind direction and speed during flight, perhaps by reference to the ground below. This latter ability enables birds to select optimal flight altitudes and to correct for wind drift when on migration (Chapter 4).

Birds may possess other sensory capacities that are as yet unknown to us, but which could also play a role in route finding.

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