Projecting the rounded surface of the earth onto a flat, two-dimensional map causes inevitable distortions which are important to bear in mind when charting and analysing migration routes, or when plotting the geographical ranges of birds.
The familiar Mercator projection (a cylindrical projection) is useful because it represents routes with constant geographical courses (rhumblines or loxo-dromes) as straight lines (Figure 9.7). This type of projection has been traditionally used for nautical charts. However, because the longitude lines are drawn parallel to one another rather than converging towards the poles, the scale varies with latitude, so that polar distances and areas are exaggerated in comparison with equatorial ones.
The gnomonic projection (a central azimuth projection) is useful because it depicts great circle routes as straight lines (Figure 9.7). Different tangent points must be selected on the earth's surface for gnomonic maps covering different parts of the globe. Central polar projections give a satisfactory correspondence with true global geography around the poles, and at the same time show great circles as straight lines. Gnomonic maps are not equidistant, and do not reflect the true area proportions.
In considering bird geographical ranges, maps of equal area projection are desirable, because on such a projection all areas are drawn to precisely the same scale, so that they appear on a flat map in the same relative proportions as they occur on the earth's spherical surface.
changed their orientation to the right during these flights, they would travel towards Alaska and neighbouring parts of Canada along the shortest possible great circle route to South America. The commonest species involved were the Pectoral Sandpiper Calidris melanotos and Grey Phalarope Phalaropus fulicaria, which winter on and near South American coastlines, respectively. This is one piece of evidence indicating that some long-distance migrants might travel along approximate great circle routes, but it will remain inconclusive until birds have been followed along more of the route. Ring recoveries from these or other candidate species are also insufficient to confirm travel by great circle routes.
Different compass systems allow different possibilities. The sun compass allows birds to identify the azimuth of the sun during the day in association with local time measured by their internal clock (Schmidt-Koenig et al. 1991). As long as the birds do not compensate for the change in local time when travelling across longitudes, a sun compass would direct birds along migration routes that are similar to great circle routes at high latitudes (Alerstam & Pettersson 1991, Akesson et al. 2001). In contrast, if birds were to compensate for the longitudinal shift in time and reset their internal clocks regularly as they crossed longitudes, they would follow a constant geographic rhumbline route when using the sun compass (Alerstam & Pettersson 1991). Ground-based radio-tracking suggested that a Catharus thrush oriented at a constant angle in relation to the sunset azimuth during six successive nocturnal flights over a total distance of 1500 km, again suggesting a rhumbline route (Cochran et al. 2004).
The star compass provides birds with geographic north-south information based on the rotation centre of the starry sky (see above). A migration route following a star compass will therefore lead the birds along a constant geographic rhumbline route. On geomagnetic information, birds could use the inclination angle of the field lines to gain information about latitude as well as the direction towards the magnetic pole or equator. A migratory route based on geomagnetic orientation alone will lead birds along a constant magnetic course, a magnetic loxodrome (Alerstam & Gudmundsson 1999), or possibly along so-called magneto-clinic routes, assuming the birds follow an apparent angle of inclination, a constant angle between the direction of the field lines and the heading of the bird. Hence, among the known mechanisms, only a sun compass with no compensation for changes in local time could lead birds along a track similar to a great circle route. In addition, however, birds might achieve an approximate great circle route by using one or more appropriately positioned stopover sites, flying straight from one to another, but making a directional change at each one. Many landbirds take roundabout routes in order to avoid long water crossings or high mountains, or to make use of refuelling sites that are off the most direct route. The journey is thus divided into successive legs with different main orientations.
Despite the advantage of a great circle route, a straight rhumbline route based on a constant compass heading appears more likely in many birds, and is consistent with the routes frequently recorded by ringing and radio-tracking. It also fits the experimental evidence (based mainly on passerines) of a genetically fixed directional preference that steers inexperienced juveniles towards their wintering areas (although they may change directions at specific points on their journeys). Clearly, more research is needed on the precise routes taken by long-distance migrants before their navigation systems can be more thoroughly assessed.
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