Loxodromes And Orthodromes

An unresolved question concerns the type of route that long-distance migrants take on their journeys. The most straightforward procedure would be to set off in the appropriate direction and maintain the same heading throughout the journey on a rhumbline (loxodrome) route. This type of journey has simple navigational needs, and if it ran directly north-south it would also be the shortest route between two points and would not involve a time-shift. If the journey had an easterly or westerly component, so that it involved crossing lines of longitude (as most routes do), a constant heading would still be the simplest but not the shortest route. The great circle (or orthodrome) route covers the shortest distance between two longitudinally separated points, but requires continual change in direction during the journey (Figure 9.7). Such a route is thus more demanding in its navigational needs. Great circle routes can be accomplished by aeroplanes, with sophisticated navigation equipment, but whether by birds remains an open question. Moreover, on any journey that involves longitudinal displacement, whether on loxodrome or orthodrome routes, the bird is also subject to time-shifts. Both the distance and the time-shift problems are greatest at high latitudes where the longitude lines are closest together.

The tracking of individual birds on their journeys has so far done little to resolve the question of whether long-distance migrants take constant direction (rhumbline) or great circle routes. Brent Geese Branta bernicla migrating between the Wadden Sea and the Taimyr Peninsula took a rhumbline route, with constant compass direction (Figure 9.7). However, it was not clear whether they took this route because they are not capable of navigating the shorter (overwater) route or in order to stay near the coast with its feeding areas. The shortest (great circle) route was about 4300 km, compared to the rhumbline of about 4700 km. Although the birds that were tracked kept closer to a rhumbline than a great circle, they also made continual minor deviations, bringing their average flight distance to 5000 km, at least 700 km (16%) further than the shortest possible route (Green et al. 2002). Similarly, Brent Geese travelling from Iceland to the Queen Elizabeth Islands in northeast Canada tended to migrate along fairly straight rhumbline routes to their breeding areas, as did Red Knots Calidris canuta. Again these routes took the birds mostly over land, where they could come to ground in inclement weather (Gudmundsson et al. 1991). Only when not influenced by topographic features, important feeding sites or weather patterns, would migrants be expected to follow either straightforward orthodrome or loxodrome routes, and this ideal may be quite rare. Much radio-tracking has involved soaring birds, most of which migrate along dog-leg routes to avoid long sea-crossings. In these and other birds, large-scale topography seems more important than distance in shaping migration routes.

Evidence that any birds take great circle routes (other than north-south) is as yet rather slender. Use of radar on the coast of northern Siberia revealed the occurrence of an east-northeast, post-breeding migration, indicating direct flights between Siberia and North America, 1800-3000 km across the pack-ice of the Arctic Ocean (Alerstam & Gudmundsson 1999). If the migrants gradually

Elisabeth Alerstam

Figure 9.7 Great circle (orthodrome) and rhumbline (loxodrome) routes between points of departure and destination for migratory flights by certain high-arctic shorebirds and Brent Geese Branta bernicla, drawn on an azimuthal stereographic map projection (above) and on a Mercator map projection (below). Between Iceland and the Queen Elizabeth Islands, great circle (1A) distance and courses are 2535 km and 328°/265° (initial/final course). Rhumbline (1B) distance and course are 2665 km and 300° throughout. Between the Wadden Sea and Taimyr Peninsula, great circle (2A) distance and courses are 4234 km and 23°/110° (initial/final course). Rhumbline (2B) distance and course are 4634 km and 59° throughout. Spring flight routes by the high-arctic migrants are in agreement with rhumbline but not with great circle routes. The position of the magnetic North Pole is indicated by a star. For further details see Box 9.1. From Alerstam (1990b).

Figure 9.7 Great circle (orthodrome) and rhumbline (loxodrome) routes between points of departure and destination for migratory flights by certain high-arctic shorebirds and Brent Geese Branta bernicla, drawn on an azimuthal stereographic map projection (above) and on a Mercator map projection (below). Between Iceland and the Queen Elizabeth Islands, great circle (1A) distance and courses are 2535 km and 328°/265° (initial/final course). Rhumbline (1B) distance and course are 2665 km and 300° throughout. Between the Wadden Sea and Taimyr Peninsula, great circle (2A) distance and courses are 4234 km and 23°/110° (initial/final course). Rhumbline (2B) distance and course are 4634 km and 59° throughout. Spring flight routes by the high-arctic migrants are in agreement with rhumbline but not with great circle routes. The position of the magnetic North Pole is indicated by a star. For further details see Box 9.1. From Alerstam (1990b).

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