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Similar relationships between migration and latitude have been shown among the birds of particular habitats from grassland to forests (Wilson 1976, Herrera 1978), and among the birds of particular families, such as flycatchers in South America (Chesser 1998). They presumably hold worldwide in both hemispheres.

The decline in proportion of migrants with decreasing latitude, established above for Europe and North America, continues southwards towards the equator. Thus, by 8° latitude in Panama, only five (0.6%) out of 807 breeding species are wholly summer migrants (Ridgely & Gwynne 1989). This is consistent with the regression line between proportion of migrants and latitude derived from the data for North America in Figure 13.3. The five migratory species found at Panama are all insectivores which leave for the winter dry season and head further south, namely Swallow-tailed Kite Elanoides forficatus, Plumbeous Kite Ictinia plumbea, Common Nighthawk Chordeiles minor, Piratic Flycatcher Legatus leu-cophaius, and Yellow-eyed (Yellow-green) Vireo Vireoflavoviridis.

The converse of these relationships for western Europe and eastern North America is shown in Figure 13.3 as the proportions of the birds wintering at different latitudes that move north for the summer. As expected, this proportion is greatest in the south, in western Europe affecting 36% of species wintering at 35°N, and declining northward to 8% of species wintering at 70°N (mostly sea-birds) and none at 80°N. In eastern North America, the equivalent figures are 52% at 25°N decreasing to none at 70°N. Again, the slopes of the regression lines do not differ between continents but the intercepts do (F116 = 9.9, P < 0.01). Throughout the latitudinal range, the proportion of wintering species that leaves northward for the summer averages around 10% greater in eastern North America than in western Europe, again reflecting the climatic difference between the two regions. The precise regression relationship varies somewhat between birds of different habitats. In addition, 23% of all species breeding in western Europe, and 24% of those in eastern North America, leave these areas completely in autumn for the tropics, returning in spring (Newton & Dale 1996b).

The proportions of all bird species that are migratory are correlated not only with latitude, but also with various climatic factors that vary with latitude, such as the temperatures of the hottest or coldest months or the temperature difference between the hottest and coldest months (Appendix 13.1). These various measures are of course interrelated, but what really matters is the degree of climatic difference between summer and winter. It is this difference that, for many birds, governs the difference in food supply between summer and winter at particular latitudes, and hence the difference in environmental carrying capacity between the two seasons.

The seasonal difference in carrying capacity may also vary from west to east, according to changes in climate (as between west and east sides of the Atlantic). From west to east across Europe, summer climates become warmer and drier, and winter climates become colder. In consequence, progressing eastward through Europe into Asia, increasing proportions of the local breeding species become migratory. This is especially obvious in comparing populations of coastal areas that live under mild oceanic climates with those further inland that live under more extreme continental climates. For example, Common Starlings Sturnus vulgaris live year-round on the Shetland Islands at 60°N, while at the same latitude in Russia (and for 10-15° south of it) they are wholly migratory.

At some mid-latitude areas, similar numbers of species may be present in summer and winter, but species composition changes somewhat between seasons, as some species from lower latitudes are present only in summer and other species from higher latitudes only in winter (Gauthreaux 1982a, Newton & Dale 1996a, 1996b). In southern England, for example, insectivorous swallows and warblers arrive from the tropics for the summer, whereas fruit-eating thrushes from further north arrive for the winter. Seasonal changes in bird communities in particular regions are thus tied to seasonal changes in the types of food available. This emphasises the point that migrants often exploit seasonal abundances in both their breeding and non-breeding areas. It is a strategy that, for obvious reasons, is much more developed in birds than in most other animals.

Although seasonality in the movements of birds is evident worldwide, in warmer regions, rainfall becomes more important than temperature. In tropical regions, away from the equator, overall rainfall declines and becomes increasingly unpredictable and localised. So superimposed on their north-south pattern, many bird species tend to concentrate wherever rain has fallen, and food is most available at the time, varying in distribution from year to year. In the most arid regions, some species are truly nomadic, with no regular directional movements. This is apparent in parts of Africa and South America, but particularly in Australia (Chapter 16). In addition, at any given latitude, islands tend to have lower percentages of migrants than comparable mainland localities, presumably because the surrounding seas buffer islands from the extremes of seasonal climatic fluctuation.

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