Predictions of Future Nearctic Landbird Vagrants to Europe Chandler S

Predictions of future Nearctic landbird vagrants to Europe Chandler S. Robbins arities generate more than their share of interest among the birdwatch- Ring public. The avid field observer thrills at breaking an arrival or departure date. The 'twitcher' travels long distances to add rare species to his list. Ringers are attracted to coastal concentration points or offshore islands hoping to handle, photograph and describe some foreign waif that has had the misfortune to be carried thousands of kilometres off its normal migratory course by vagaries of the weather. An American's interest in the landfall" of vagrant Nearctic landbirds in Europe is understandably different from that of you who are on the receiving end. Our first questions are: (1) are the small numbers of American vagrants that reach European shores indicative of tremendous losses at sea of other individuals of the same species, and (2) do these losses involve such a large proportion of the population of some species that the loss could be detected by the North American Breeding Bird Survey in the following summer? Along the western shore of the North Atlantic, one of the chief sources of vagrant birds is the occasional passage of a tropical hurricane north eastward along or parallel to the coast. Depending to some degree on the date that a hurricane affects the east coast of the United States or Canada, one may predict some of the species that will be deposited along its course. The primary concern of ornithologists who have reported on the effects of such storms, however, has been the recurrence of species such as White- tailed Tropicbirds Phaethon lepturus, Sooty Terns Sterna Juscata, Black Skimmers Rynchops nigra, or Yellow-billed Cuckoos* far outside their normal ranges. The. more difficult-to-assess storm-related mortality of other species has received less attention. Ringers along the Atlantic coast of the United States and Canada know that stiff westerly winds frequently cause a great accumulation of grounded migrants along the coast. On certain days, a large proportion of these birds arrives in emaciated condition, having been blown far offshore and then having struggled back, flying into the wind. The terrestrial birds that make a successful landfall, and become European vagrants, must often be only a tiny fraction of those that are lost at sea. Thus, my primary interest in North American vagrants in Europe is not so much to learn or to predict which species have successfully made the passage, but to use this information to gain a better understanding of the potential impact on Nearctic bird populations of the tremendous losses that may take place at sea. *See tables 1 and 2 for scientific names. 448 [Bril. Birds 73: 448-457, October 1980] Future Nearciic landbird vagrants 449 Meteorological context Paramount to any understanding of storm-blown migrants is basic in formation on both normal and abnormal atmospheric circulation across the North Atlantic. Norman Elkins (1979) has done a masterful job of addressing this subject. Before seeing his paper, I had laboured in vain trying to relate some of the British landfalls to passage of American hurricanes. As Mr Elkins has convincingly shown, however, fast eastward- moving waves associated with certain strong warm sectors provide not only a more logical explanation than that offered by western Atlantic hurricanes, but are supported by the arrival dates of 75% of the vagrants. Methods Accepted records, 1947-76 I have based this study on accepted records of North American passerines and near-passerines (cuckoos, nightjars, and woodpeckers) recorded in Britain and Ireland during the same 30-year period (1947-76) summarised in Elkins's table 1. Older records, dating from before the large network of well-manned coastal and island bird observatories, may be biased by disproportionately large numbers of the bigger and more conspicuous species (eg Yellow-billed Cuckoo); the records for 1977 and 1978 were not complete and were still under consideration by the Rarities Committee when this study was undertaken. Candidate species If one were subjectively to compile a list of candidate species composed of those most likely to make a successful transatlantic crossing, it would reasonably be based on those species already proved successful. If we seek greater objectivity, however, we can employ modern statistical techniques and computer technology to examine a large array of data and make logical predictions based on information provided to this end. It will be necessary to define the 'universe' of species from which the best candidates will be selected. Many North American species have some chance of appearing in the British Isles one or more times during the next few centuries. Long-distance migrants that travel along the east coast of Canada and the United States are more likely to make transatlantic migrations than are rare, western, or non-migratory species. Thus, a logical starting point is to define the universe of interest in terms of numerical abundance along the North American east coast in autumn. The best data sources for this purpose are banding (ringing) records from coastal stations at Manomet Bird Observatory in Massachusetts, Island Beach State Park in New Jersey, and Kiptopeke Beach in Virginia. These three stations are about evenly spaced from Cape Cod Bay to the mouth of Chesapeake Bay and each has a large sample of recent banding records. The years selected for this study varied among stations, depending on consistency of coverage and availability of summarised data. The Manomet figures were from 52,081 birds banded during August to November 1969-76; the Island Beach data were from 207,253 birds banded in autumn 1956-68; the Kiptopeke data were from 450 Future Nearctic landbird vagrants 94,865 birds banded during late August to late October 1967-75. Each passerine and near-passerine species that averaged two or more captures per year at one or more of the three stations was included in the analysis. Excluded, however, were those species that breed on both sides of the Atlantic: Sand Martin (Bank Swallow) Riparia riparia, Swallow (Barn Swallow) Hirundo rustica, Wren (Winter Wren) Troglodytes troglodytes, Treecreeper (Brown Creeper) Certhia jamiliaris, Starling (European Starling) Stumus vulgaris and House Sparrow Passer domesticus. Unfortunately, two vagrants already on the British and Irish list did not qualify for inclusion in this study. These were the Common Nighthawk, an aerial feeder that is rarely captured at the coastal banding stations, and the Summer Tanager, which nests primarily southwest of the banding stations and was too infrequently caught to qualify for inclusion. These two species were omitted from the analysis to be consistent. The analysis, therefore, is based on 31 species that occurred in Britain and Ireland during the period 1947-76, and 73 candidate species for which there were no accepted records up to and including 1976. Definition oj migration characteristics To be a likely candidate for transatlantic" migration, a passerine species must be present in good numbers at the right place at the right time, and must be physically capable of surviving an extended trip with little or no food intake along the way. Thus, an index of these characteristics, or of associated factors, should be correlated with the probability (success or failure) of the species crossing to Europe. If numerical values are assigned to various migration characteristics, a computer algorithm (calculation in a number of steps) could select those variables that are most closely cor related with successful crossings and derive a formula from which candi date species could be ranked according to their likelihood of completing the trip. The success of the exercise depends largely on the ability of the investigator to select a powerful battery of predictor variables for the computer to select from, and to assign the most appropriate numerical data to each variable. I will specify in detail the data I used so that future researchers can profit from my inadequacies and generate better predictions. ABUNDANCE ON EAST COAST OF NORTH AMERICA A glance at the list of North American vagrants that have been recorded in Britain and Ireland supports the hypothesis that abundance during autumnal migration on the western shore of the North Atlantic is a prime factor for inclusion. For each of the three banding stations, the total number of new banded birds of each species within a sampling period was divided by the total bandings during the same period to obtain the relative percentage of each species. Percentages were individually summed across the three stations to obtain a pooled index of abundance for each of the 104 species. BODY WEIGHT The North American passerines and near-passerines recorded most frequently in the British Isles tend to be medium to large birds. Even among the wood-warblers, the large species are primarily the ones with the most records. For this reason, body weight was selected as one of the input characteristics. The variable was defined as mean autumn weight of primarily east coast birds from the Operation Recovery banding program for the years 1966-68. STANDARD DEVIATION OF BODY WEIGHT This statistic, which measures variability within a sample, is high for species whose individuals differ extensively in amount of body fat, and is low for species in which all individuals weigh about the same. It is one way, therefore, of detecting Future Nearctic landbird vagrants 451 species in which some individuals are prepared for long nonstop flights. The Blackpoll Warbler, for example, stores much fat for useduring migration (SD=3.63, n= 1,635), whereas the Eastern Pewee does not (SD= 1.45, n=229); both species have a mean weight of 14g. The standard deviations were taken from the same source as the body weights and are based on samples of at least several hundred individuals for most species.

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