Population size is likely to influence the number of individuals that appear as vagrants outside their regular range. For example, several species which have increased in eastern North America in recent decades have appeared with increasing frequency as vagrants in western Europe. Examples include the Red-eyed Vireo Vireo olivaceus and Ring-billed Gull Larus delawarensis.
Comparing species, the size of the source population accounted for nearly 60% of the variance in the occurrence of vagrant warblers and vireos in California (DeBenedictis 1971), and for about 30% of the variance in occurrence of various birds in the South Farallon Islands (De Sante 1983a). Similarly, changes in the numbers of vagrant warblers in California over the years have been attributed to known changes in regional population trends (Patten & Marantz 1996). In particular, Bay-breasted Warblers Dendoica castanea and Cape May Warblers Dendoica tigrina appeared in greatest numbers in California in years when their breeding populations further north were swollen in association with outbreaks of
Spruce Budworm Choristoneura fumiferana which forms their food (Robertson 1980). In addition, the relationship between annual variations of vagrant numbers and population size was studied in the Yellow-headed Blackbird Xanthocephalus xanthocephalus which appears on the Eastern Seaboard of North America every autumn from breeding areas far to the west. The numbers in different autumns in Massachusetts were correlated with annual reproductive output at the northern and eastern periphery of the breeding range (Veit 1997). The greater the estimated post-breeding population, the greater the numbers of vagrants reported. Such annual variations had previously been attributed to weather and other conditions at the time. But if the tendency of individuals to disperse in given directions and distances is constant from year to year, more individuals would be expected to turn up at distant localities in years when overall numbers are high.
Range changes are also likely to influence the frequency with which vagrants occur in particular areas. As several species spread westward across Eurasia during the twentieth century, they appeared with increasing frequency in Britain. Examples included the Red-flanked Bluetail Tarsiger cyanurus, River Warbler Locustella fluviatilis, Greenish Warbler Phylloscopus trochiloides, Penduline Tit Remiz pendulinus, Common Rosefinch Carpodacus erythrinus, Rustic Bunting Emberiza rustica and Little Bunting E. pusilla. Likewise, the first records of the Shiny Cowbird Molothus bonariensis in Florida followed the numerical increase and expansion of the species northwards through the West Indies (Post et al. 1993).
It is of course hard to separate the effects of increased population size over periods of decades from increased numbers of observers, but records have increased much more rapidly for some species than for others, while records of other species have fallen. Examples of the latter in Britain include Little Bustard Tetrax tetrax and Pallas's Sandgrouse Syrrhaptes paradoxus, whose numbers are known to have declined in their regular range further east (Table 10.1). Moreover, sustained observations for more than a hundred years at such well-watched places as Fair Isle surely indicate that some of the increases are real and not just a result of greater observer coverage and awareness.
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