The erratic wanderings of migratory birds, resulting in their appearance in countries far removed from their accustomed haunts, and off the routes followed to reach them, are in many cases to be attributed to their failure, from some cause or other, to inherit unimpaired this all-important faculty of unconscious orientation. (William Eagle Clarke 1912.)
Every bird-watcher delights in seeing birds that normally live far away but periodically turn up as rarities. For some, seeing such species becomes an obsession, and bird vagrancy supports a time-consuming passion, which rewards the participants with gradually lengthening personal bird lists. The term vagrant is applied to any exotic visitor of a species which does not normally breed, overwinter or pass through the region concerned. Outside the bird-watching community, little attention has been paid to vagrancy, partly because records are collected unsys-tematically (depending largely on the numbers and distribution of skilled observers), and partly because vagrants have usually been assumed to be abnormal in some way, or chance victims of unusual weather. Some may be escaped cage birds, or free-living birds that have hitched a ride on a ship. But many vagrants belong to species that cannot be kept in captivity, and could not survive on a ship for days on end. Moreover, their occurrence often follows specific seasonal patterns, as they appear in particular weather conditions, in the same places and at about the same dates year after year, different species in mainly different places. Almost all vagrants are from migratory populations, and often in their first year of life.
Vagrancy may result from: (1) normal dispersal, but over unusually long distances; (2) population growth or expansion, with vagrants being noticed because populations are larger or breeding closer to the area concerned; (3) drift, in which migrants are blown off course by winds; (4) migration overshoots, in which individuals migrate further than usual, appearing well beyond their normal breeding or wintering range; (5) deviant directional tendencies in which individuals migrate at the right time but in the wrong direction, including (6) mirror-image migration, in which birds migrate at a mirror angle to their normal direction; and (7) reversed-direction migration, in which birds migrate in the opposite direction to usual for that time of year. Some of these mechanisms could involve natural phenomena, while others could imply some inherent defect in the normal migration control mechanism.
Many vagrants are encountered on isolated islands off the fringes of continents, some of which are visited by a remarkable diversity of birds. Such islands include Heligoland off Germany ('a spot which, from the ornithological point of view, is without rival in the world', Gatke 1895), the northern Isles of Scotland (notably Fair Isle, Clarke 1912, Dymond et al. 1989), the Scilly Isles off southwest England (Gantlet 1991), the Nova Scotia Islands (notably Sable and Seal Islands) off eastern Canada (McLaren 1981), the South Farallon Islands off California (De Sante & Ainley 1980) and the Aleutian Islands off Alaska (Gibson 1981, Obmascik 2004). On the most westerly of the Aleutian Islands (notably Attu), occasional westerly winds bring migrants from Siberia, and give North American birders abundant opportunities to view Eurasian birds on American soil, sometimes dozens at a time. One feature of all these specially situated places, however, is that they receive a great variety of birds not just from neighbouring land areas, as expected, but from up to several thousands of kilometres away. They have often provided the first and sometimes only records of certain species for their particular regions. Of all the species added to the British List during the twentieth century, about one in every five was first seen on Fair Isle, which occupies less than 0.01% of the total land area of the British Isles.
During the nineteenth and early twentieth centuries, virtually all recorded vagrants were shot and preserved as skins, some of which are still available for examination. The older records may well have been biased in favour of large and conspicuous species. Subsequently, binoculars, field guides and mist nets became available, many of the extreme rarities were trapped, examined and photographed in the hand, and nowadays field photographs and written descriptions by skilled observers have often proved sufficient to identify rare species unequivocally. The recent development of 'digiscoping' (digital photography through a telescope) has further helped, as has the increasing ability to record calls and produce sonograms for subsequent inspection. The use of the web and other means to advertise fresh sightings has also increased the numbers of observers able to see and photograph individual rarities. Little wonder that the annual totals of recorded vagrants have increased over the years, and that some species once regarded as rare vagrants are now regarded as regular passage migrants (Figure 10.1, Table 10.1).
But what are the chances of any rare birds that turn up now being detected by bird-watchers? This question was addressed by Fraser (1997) using data from some of the best-watched sites around Britain. At such localities, about 40% of all passerines and waders were seen on one day only, around 15% on two days, and a decreasing proportion over longer periods (Figure 10.2). From these data, it was calculated that, even on a site as well watched as Fair Isle, with bird-watchers continually present at several per square kilometre through the migration seasons, at least 11% of all vagrants that settled on the island were probably missed. This figure rose to more than 40% for less favoured, but still well-watched coastal sites.
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