Probability Of Arrival

At the time of writing, more than half of the 560 or so species on the British list are classed as vagrants. Some such species occur in numbers every year, while others are seen less often, in extreme cases perhaps only once in several decades. The same has been noted in every well-watched region of the world, leaving no doubt that many species continually reach new areas. Even a land mass as isolated from the rest of the world as New Zealand receives a continual supply of avian vagrants, about 29% of all species recorded there being classed as 'irregular visitors' (Bell 1991). Small oceanic islands similarly receive a steady trickle of exotic birds from distant lands. Almost all these birds turn up in autumn or spring, during the normal migration seasons, and derive from known migratory populations.

The importance of distance from a source area in influencing the frequency of arrivals by non-native species is shown by three types of observation. First, as expected, islands close to continents (such as Heligoland off Germany) receive many more vagrant birds than remote islands (such as Ascension in mid-Atlantic). Second, on continents, the numbers of vagrant species which appear in particular localities each year, and their proportion of the overall species numbers, decline

Northern British Columbia

Southern British Columbia

Washington

Oregon

Northern California

Southern California

0 200 400 600 Number of species

Figure 10.3 Relative contributions of resident, migrant and vagrant species to the regional bird lists of western North America. The percentage of vagrants in the total avifauna of southern California is 27%, northern California 29%, Oregon 20%, Washington 21%, southern British Columbia 20%, and northern British Columbia 9% (r = -0.84, testing percentage against latitude, P < 0.05). Redrawn from Stevens (1992).

with increasing latitude, as shown for western North America in Figure 10.3. This matches the latitudinal decline in overall species numbers. Third, the chance of any one species appearing in a given area declines, on average, with increasing distance from its regular range, as shown for various warblers in California in Figure 10.4.

Figure 10.4 Total numbers of sightings of different species of eastern North American warblers in • California in relation to distance between California and the

• western edge of . the breeding range .. . (r = -0.55, P < 0.01). . . . * . Redrawn from ^-'-' * 1 * * '-' Stevens (1992).

500 1000 1500 2000 2500 Distance to geographical range (km)

Northern British Columbia

Southern British Columbia

Washington

Oregon

Northern California

Southern California

0 200 400 600 Number of species

Figure 10.3 Relative contributions of resident, migrant and vagrant species to the regional bird lists of western North America. The percentage of vagrants in the total avifauna of southern California is 27%, northern California 29%, Oregon 20%, Washington 21%, southern British Columbia 20%, and northern British Columbia 9% (r = -0.84, testing percentage against latitude, P < 0.05). Redrawn from Stevens (1992).

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