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Detours in bird migration
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• Between 8000 BP and the present, the flux of clastic sediment varied, charcoal became more common and pollen from disturbance-tolerant plants, such as Piper spp., became more abundant. These changes represent a more variable climate characterized by alternating wet–dry intervals. This study is not the last word on tropical climate. However, it does make three important points: First, forest-destroying
TRENDS in Ecology & Evolution Vol.16 No.9 September 2001
drought occurred during the last glacial maximum in some regions of tropical South America. Second, the degree of drying was not equal everywhere and, therefore, drawing global conclusions from local research might be particularly dangerous in the low latitudes. And third, the convergence of multiple lines of evidence in paleoenvironmental reconstruction (pollen, sedimentology and biogeochemistry) provides the best
485
possible evidence to our growing understanding of past tropical climates. 1 Sifeddine, A. et al. (2001) Variations of the Amazonian rainforest environment: a sedimentological record covering 30 000 years. Palaeogeogr. Palaeoclimatol. Palaeoecol. 168, 221–235
Nan Crystal Arens [email protected]
Detours in bird migration Many migratory birds make large detours to avoid crossing so-called ‘ecological barriers’ (usually vast expanses of sea, sand or ice). They do this in spite of being capable of storing enough fat as fuel to cross such areas in a single nonstop flight. Previous explanations for the adoption of circuitous migration routes by so many species involved factors such as wind and weather conditions and the risks of starvation or predation. New, surprisingly simple models by Thomas Alerstam1, based on the massdependent costs of flight derived from flight mechanical theory, now provide a general explanation for such detours and make quantitative predictions about their possible benefits.
The explanation is based on the fact that travelling light and stopping often to refuel uses less energy than does a single longhaul flight with a heavy fuel load. Large fuel loads increase the cost of flapping flight because a fat bird weighs more and has a larger frontal area, incurring greater induced and parasitic drag. Thus, the added benefit, in terms of the additional flight range, of a given amount of fuel decreases with the fuel load that is already stored. As a result, birds should avoid routes that require nonstop flight across barriers in favour of those that can be covered by a series of short flights with a negligible fuel load and frequent refuelling, even if these journeys are longer. http://tree.trends.com
How big a detour can a bird make and still save energy? This depends on the barrier width and the fuel load necessary to cross it nonstop versus the number of steps, and hence fuel load carried, along the alternative route. For example, a bird requiring an 80% increase in mass above lean weight to cross a barrier nonstop would save energy by migrating along a detour that is up to 22% longer if the journey comprises two equal stages, or up to 57% longer if completed by numerous short flights. For many species with very different migration routes, the observed detours do conform to the model predictions. Redbacked shrikes Lanius collurio migrating from southeastern France to Tanzania reduce their barrier crossing by half by detouring round the eastern Mediterranean – a reduction that more than compensates for the extra energy costs associated with the additional distance of the detour. The model predictions hold for wheatears Oenanthe oenanthe crossing the East
Atlantic between Greenland and Senegal and brent geese Branta bernicla migrating from the Wadden Sea to northern Russia. It is, however, not a universal explanation: American warblers, which skirt the Gulf of Mexico rather than crossing it, are following a detour that is clearly unfavourable in terms of fuel economy and which might have evolved in response to risk factors. Alerstam’s models suggest that, for many birds, the aerodynamic cost of heavy fat loads has been an important factor in the evolution of detours to avoid ecological barriers. His work now offers a plausible null model against which other explanations should be tested. 1 Alerstam, T. (2001) Detours in Bird Migration. J. Theor. Biol. 209, 319–331
Juliet Vickery [email protected] Peter Jones [email protected]
Solving seasonal puzzles My working hypothesis for why Southern California seems to attract theoretical ecologists like moths to a light is that this stretch of Pacific coast is nearly as unseasonal as the spherical cow is legless. ‘What?’ you might say if you’re a field ecologist. However, if you are a fellow theoretician, you will perhaps nod in recognition of the fact that, in spite of a century of the modeling of predator–prey, parasitoid–host, or competition systems, we barely know the outline of general theories of trophic or competitive dynamics in seasonal environments. Such theories still elude us
because it is technically very difficult to obtain useful theoretical results of any generality. It is easy to provide a bifurcation diagram showing ‘cycles and chaos’. However, because seasonally forced chaos is often quasi-periodic in nature, it is quantitative (or ‘statistical’) properties that are crucial, and those are hard to get at. King and Schaffer1,2 have recently made a big step forward, both technically and theoretically, towards this goal in their study of trophic interactions in a seasonal environment. Theoreticians have wanted to understand predator–prey dynamics for a
0169-5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
• Between 8000 BP and the present, the flux of clastic sediment varied, charcoal became more common and pollen from disturbance-tolerant plants, such as Piper spp., became more abundant. These changes represent a more variable climate characterized by alternating wet–dry intervals. This study is not the last word on tropical climate. However, it does make three important points: First, forest-destroying
TRENDS in Ecology & Evolution Vol.16 No.9 September 2001
drought occurred during the last glacial maximum in some regions of tropical South America. Second, the degree of drying was not equal everywhere and, therefore, drawing global conclusions from local research might be particularly dangerous in the low latitudes. And third, the convergence of multiple lines of evidence in paleoenvironmental reconstruction (pollen, sedimentology and biogeochemistry) provides the best
485
possible evidence to our growing understanding of past tropical climates. 1 Sifeddine, A. et al. (2001) Variations of the Amazonian rainforest environment: a sedimentological record covering 30 000 years. Palaeogeogr. Palaeoclimatol. Palaeoecol. 168, 221–235
Nan Crystal Arens [email protected]
Detours in bird migration Many migratory birds make large detours to avoid crossing so-called ‘ecological barriers’ (usually vast expanses of sea, sand or ice). They do this in spite of being capable of storing enough fat as fuel to cross such areas in a single nonstop flight. Previous explanations for the adoption of circuitous migration routes by so many species involved factors such as wind and weather conditions and the risks of starvation or predation. New, surprisingly simple models by Thomas Alerstam1, based on the massdependent costs of flight derived from flight mechanical theory, now provide a general explanation for such detours and make quantitative predictions about their possible benefits.
The explanation is based on the fact that travelling light and stopping often to refuel uses less energy than does a single longhaul flight with a heavy fuel load. Large fuel loads increase the cost of flapping flight because a fat bird weighs more and has a larger frontal area, incurring greater induced and parasitic drag. Thus, the added benefit, in terms of the additional flight range, of a given amount of fuel decreases with the fuel load that is already stored. As a result, birds should avoid routes that require nonstop flight across barriers in favour of those that can be covered by a series of short flights with a negligible fuel load and frequent refuelling, even if these journeys are longer. http://tree.trends.com
How big a detour can a bird make and still save energy? This depends on the barrier width and the fuel load necessary to cross it nonstop versus the number of steps, and hence fuel load carried, along the alternative route. For example, a bird requiring an 80% increase in mass above lean weight to cross a barrier nonstop would save energy by migrating along a detour that is up to 22% longer if the journey comprises two equal stages, or up to 57% longer if completed by numerous short flights. For many species with very different migration routes, the observed detours do conform to the model predictions. Redbacked shrikes Lanius collurio migrating from southeastern France to Tanzania reduce their barrier crossing by half by detouring round the eastern Mediterranean – a reduction that more than compensates for the extra energy costs associated with the additional distance of the detour. The model predictions hold for wheatears Oenanthe oenanthe crossing the East
Atlantic between Greenland and Senegal and brent geese Branta bernicla migrating from the Wadden Sea to northern Russia. It is, however, not a universal explanation: American warblers, which skirt the Gulf of Mexico rather than crossing it, are following a detour that is clearly unfavourable in terms of fuel economy and which might have evolved in response to risk factors. Alerstam’s models suggest that, for many birds, the aerodynamic cost of heavy fat loads has been an important factor in the evolution of detours to avoid ecological barriers. His work now offers a plausible null model against which other explanations should be tested. 1 Alerstam, T. (2001) Detours in Bird Migration. J. Theor. Biol. 209, 319–331
Juliet Vickery [email protected] Peter Jones [email protected]
Solving seasonal puzzles My working hypothesis for why Southern California seems to attract theoretical ecologists like moths to a light is that this stretch of Pacific coast is nearly as unseasonal as the spherical cow is legless. ‘What?’ you might say if you’re a field ecologist. However, if you are a fellow theoretician, you will perhaps nod in recognition of the fact that, in spite of a century of the modeling of predator–prey, parasitoid–host, or competition systems, we barely know the outline of general theories of trophic or competitive dynamics in seasonal environments. Such theories still elude us
because it is technically very difficult to obtain useful theoretical results of any generality. It is easy to provide a bifurcation diagram showing ‘cycles and chaos’. However, because seasonally forced chaos is often quasi-periodic in nature, it is quantitative (or ‘statistical’) properties that are crucial, and those are hard to get at. King and Schaffer1,2 have recently made a big step forward, both technically and theoretically, towards this goal in their study of trophic interactions in a seasonal environment. Theoreticians have wanted to understand predator–prey dynamics for a
0169-5347/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.