Table 101 Vagrants that greatly increased or decreased in their occurrence in Britain during the twentieth century in association with changes in population sizes or distributions

Increasing populations and increasing frequency of occurrence

Decreasing populations and decreasing frequency of occurrence

Squacco Heron Ardeola ralloides, Cattle Egret Bubulcus ibis, Black Stork Ciconia nigra, Snow Goose Chen caerulescens, Red-breasted Goose Branta ruficollis, Ring-necked Duck Aythya collaris, Black Kite Milvus migrans, Common Crane Grus grus, Terek Sandpiper Tringa cinerea, Forster's Tern Sterna forsteri, Ring-billed Gull Larus delawarensis, Red-rumped Swallow Hirundo daurica, Greenish Warbler Phylloscopus trochiloides, Melodious Warbler Hippolais polyglotta, Eurasian Penduline Tit Remiz pendulinus, European Serin Serinus serinus, Common Rosefinch Carpodacus erythrinus, Rustic Bunting Emberiza rustica, Thrush Nightingale Luscinia luscinia

American Bittern Botaurus lentiginosus, Little Crake Porzana parva, Little Bustard Tetrax tetrax, Pallas's Sandgrouse Syrrhaptes paradoxus

Although islands and coastal localities are most likely to attract incoming migrants, in most inland areas, where bird-watching is more desultory and sporadic, the chance of detecting any rare passerines that turn up is presumably extremely low.

In addition, more than 40% of all rarities recorded in Britain in recent decades were seen at weekends, significantly more than the 28% expected if coverage was uniform through the week (Fraser 1997). Weekend bias was hardly noticeable at the best-watched sites, such as Fair Isle and Scilly Isles, and became much greater at other less-watched sites. It did not affect all birds in the same way, being much more marked among ducks than among passerines and waders. This was probably because finding rare ducks frequently involves sifting through large flocks of commoner species for the elusive rarity, a practice most commensurate with leisured weekend birding. As a general conclusion, we can assume that, even in a country with as many bird-watchers as Britain, only tiny (and unknown) proportions of vagrants that turn up away from migration hotspots are likely to be detected, with some species being more often missed than others.

45 "I

Fair Isle Passerines Scilly Isles Passerines Scilly Isles Waders

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