Birds from the Far East in Central Europe: a test of the reverse migration hypothesis
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Every year birdwatchers throughout Europe record the arrival of a considerable number of vagrants from the eastern Palaearctic. We analysed the occurrence of such vagrants in Central Europe of the genera Phylloscopus, Turdus and Zoothera in Central Europe. Our results revealed that the occurrence of a species depended on total population size, but not on body size. Furthermore, the occurrence of species in Central Europe increased with distance between the breeding range and wintering range, but the distance between the breeding range and Europe had no effect. These results indicate that the migratory restlessness of species determines whether an East Asian species arrives in Europe. Overall, our data support the hypothesis that the migratory program determines vagrancy.
KeywordsPhylloscopus Reverse migration hypothesis Turdus Weather hypothesis Vagrants Zoothera
Every year birdwatchers report the occurrence of species in Europe that do not breed or regularly overwinter there. For some of these vagrants, the breeding and wintering ranges are thousands of kilometres distant from Europe. An impressive array of committees has been set up over the years which verifies and evaluates these records (van den Berg and Bosman 1999). While one trend of thought views these vagrants solely as curiosities without any deep or lasting scientific implication, others lines of thought feel that the records call for a scientific evaluation of causes and consequences (Barthel and Bezzel 1990). Firstly, vagrants may be propagules for an expansion of the distributional range. A well-known example of this is the colonization of South America by the Cattle Egret Bubulcus ibis (Maddock and Geering 1994; see also Veit 1997, 2000). Secondly, a study of the causes of the occurrence of individuals far away from their breeding grounds, their regular migration route and their wintering areas may facilitate scientists in gaining an understanding of the migration system of birds (Rabøl 1969, 1976; Thorup 2004). In the present study, this latter issue will be addressed from a macroecological perspective.
The documented recordings of vagrants give an impression of relative abundance as well as seasonal occurrence in Europe. A qualitative evaluation of these records reveals three general patterns. Firstly, an important group of vagrants consists of eastern Palaearctic species (especially Leaf Warblers Phylloscopus, Thrushes Turdus and Zoothera, Buntings Emberiza and Pipits Anthus). Secondly, the relative abundance of vagrants differs considerably between species. Thirdly, most recordings of rarities occur in the autumn. Two hypotheses have been proposed to explain these observations: the “weather hypothesis” and the “reverse migration hypothesis”.
If reversed orientation or a switch from the autumn to the spring programme is the principal mechanism of reversed migration, one would expect that, given a simple compass-based migration system, the occurrence of vagrants would be independent of the migratory direction (Thorup 2004). However, Thorup (2004) showed that the relative abundance of vagrants does depend on this direction, with many more vagrants from the eastern Palaearctic reaching Europe than from the Mediterranean. This results indicates that cues along the migratory routes influence the orientation system (see also Weindler et al. 1996). As well as the orientation system, the endogenous clock to terminate the migration programme should also influence vagrancy. For warblers, migration distance and migratory restlessness (“Zugunruhe”) are correlated (Gwinner 1968; Berthold 1973). Hence, only species with a sufficiently long migratory restlessness should reach Central Europe. Otherwise, species should stop along the great circle before they reach Central Europe (see Fig. 1).
To test this hypothesis, we need to consider other factors which influence the occurrence of vagrants. (1) Abundance should affect vagrancy (Thorup 2004). If, irrespective of species, all individuals have the same probability to deviate from the regular migratory direction, we expect that the number of vagrant individuals increases with total population size of the species (see also McLaren et al. 2006). Given that abundance increases with distributional range size (e.g. Gaston 1994), we would expect a positive correlation between the numbers of vagrant individuals and the distributional range size across species. Furthermore, the probability that a species (not individual!) is recorded as a vagrant should also increase with distributional range size. (2) The probability that an individual dies increases with the distance travelled. Therefore, the number of vagrant individuals and the probability that a species occurs as a vagrant should decrease with the distance between Europe and the breeding range. Thorup (2004) failed to document the latter prediction, perhaps because he considered only species with records in Europe. Therefore, to test our hypothesis that only those species with a sufficiently long migration distance arrive in Europe, we consider not only species which have arrived in Europe but also species within the same genera for which no records of vagrancy to Central Europe are available.
Material and methods
Eastern Palaearctic species of the bird genera Phylloscopus (leaf warblers), Turdus and Zoothera (thrushes) are recorded every year as vagrants in Central Europe
Recorded in Europe
For the morphological variables we used the data presented in Baker (1997) as well as Clement and Hathway (2000). Where these two sources reported only ranges of measurements, we used the midpoint. For some species, Baker (1997) and Clement and Hathway (2000) provide no data on body mass or wing length, and in these cases we used data given in Glutz von Blotzheim and Bauer (1985, 1988, 1991) as well as Ali and Ripley (1973). For Turdus cardis and T. feae we found no data at all on body mass. To estimate body mass for these two species, we therefore constructed a regression between wing length and body mass across 20 species of thrushes occurring in the eastern Palaearctic. We used the least square regression line [body mass (g) = −167.7 (g) + 1.965 (g mm−1) wing length (mm); r² = 0.79] to estimate the expected body mass for T. cardis and T. feae from their wing length.
The variables characterizing range size, the distances between breeding and wintering ranges as well as the distance between the breeding range and Central Europe were estimated from the maps in Baker (1997) and Clement and Hathway (2000). As these maps differ in scale between species, we first used certain geographic features (e.g. estuaries, peninsulas) on these maps to estimated the scale and then measured the extent of distributional range in south–north and east–west directions as well as the distances to the centre of the wintering range and to Central Europe. These measurements were converted to kilometres. The product of the two extents provided a rough estimate of the distributional range size (in km²). Price et al. (1997) published estimates of the distributional range sizes of Phylloscopus species, and our data set on distances correlated well with that of these authors [r² = 0.87; n = 26 excluding P. cantator and P.ricketti; Price et al. 1997 combined these two species; Price et al. (1997) and Baker (1997) combined P. tenellipes and borealoides into one species, Price et al. using the name borealoides).
The records of the selected species in Central Europe were extracted from Glutz von Blotzheim and Bauer (1985, 1988, 1991; see Appendix). All records were summed up, and we used this sum as an index of the relative abundance of vagrants in Central Europe. For comparison, we used information on the occurrence and abundance of passerine species from the eastern Palaearctic recorded in The Netherlands (extracted from van den Berg and Bosmann 1999). In a second step, we recoded the relative abundance into a binary variable (0 and 1). Zero indicates that a species was not recorded, whereas one indicates a species was recorded as vagrants in Central Europe. This coding allows the probability that at least one record exists to be estimated.
The distribution of the number of records, distributional range size and the distances were fairly skewed, and we log10-transformed these variables prior to all analyses. The data were analysed using the generalized linear model with the appropriate error distribution using R ® Development Core Team 2004). For a multivariate analysis we decided to use hierarchical partitioning (MacNally 1996, 2000) as implemented in the package hier.part (Walsch and MacNally 2003) within R ® Development Core Team 2004). This method does not aim to identify an appropriate regression equation, but identifies those variables that have high and independent correlations with the dependent variable and, therefore, likely to be most influential on the independent variable.
The number of recordings ranged from one individual to nearly 1000 individuals (Fig. 3). Across all species recorded in Central Europe, we found a correlation between the number of recordings in Central Europe and the size of the distributional range.
One of the factors responsible for the transport between the breeding range and Europe are bad weather conditions. In the aftermath of storms, North American species are often recorded on the other side of the Atlantic Ocean (e.g. Hering and Alfrey 2006). One might expect small birds to be more often affected by winds than large birds. However, we found no influence of body size on the probability that a species occurs as a vagrant in Central Europe. Firstly, we found no difference in that probability between thrushes and Phylloscopus, although thrushes are much larger than leaf warblers. Secondly, within the genus Phylloscopus, body size had no influence on this probability. However, the role of body size may not be so simple (see also McLaren et al. 2006). Small birds have a higher probability of death from starvation on the way from the breeding range to Central Europe. Calder (1974) reports a positive correlation between body size and survival time of fasting birds. According to the equations reported by Calder (1974), a bird of 10 g may survive 1 day without food, a bird of 50 g almost 3 days. Clearly, small species should have a lower probability of surviving a long flight without stops for feeding than large species. Interestingly, the percentage of passerines (small birds) versus non-passerines (larger birds) in the list of vagrants differs for species from East Asia and North America. In The Netherlands (van den Berg and Bosman 1999), eight of 40 North American vagrants were passerines (20%), whereas 32 of 46 East Asian species were passerines (70%; χ² = 21.2, P < 0.001). This difference in the composition of vagrants between North America and Asia indicates that the causes for the occurrence of North American and East Asian vagrants may be different.
We found that the probability that a species occurs as a vagrant in Central Europe increases with increases in the ratio of the distance between breeding and wintering range to the distance between breeding range and Central Europe. This relationship still holds when we corrected for abundance, suggesting that the migration programme and associated restlessness are important factors that determine whether a species may reach Europe. Errors in the migration programme (switch of the seasonal programme or reversed orientation in autumn) drive species on the great circle route from their breeding range to Central Europe (Alerstam and Petterson 1991). However, only those birds that are sufficiently restless arrive in Europe. Species with a short migration distance would stop far short of Central Europe.
Vögel aus Ostasien in Europa: Gründe für das Auftreten von Irrgästen
Alljährlich werden von Feldbeobachtern Irrgäste gemeldet. Wir untersuchten das Auftreten von ostasiatischen Arten der Gattung Phylloscopus, Turdus und Zoothera als Irrgäste in Mitteleuropa. Wir fanden, dass das Auftreten zunächst von der Häufigkeit der Arten abhängt, aber nicht von deren Körpergröße. Darüber hinaus fanden wir, dass das Auftreten einer Art als Irrgast mit zunehmendem Verhältnis zwischen der Zustrecke und der Distanz zwischen dem Brutareal und Europa wahrscheinlicher wurde. Das spricht dafür, dass letztlich die genetisch determinierte Zugunruhe dafür verantwortlich ist, ob eine Art Europa überhaupt erreicht. Das Auftreten von ostasiatischen Irrgästen beruht demnach eher auf Fehlern im Zugablauf, denn in schlechten Wetterbedingungen während des Zuges.
Two anonymous referees provided valuable comments on an earlier draft of this paper. Jonathan Guest polished our language.
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