Ibis (2006), 148, 782–788 Blackwell Publishing Ltd Movement patterns of Common Crossbills Loxia curvirostra in Europe IAN NEWTON* Centre for Ecology and Hydrology, Monks Wood Research Station, Abbots Ripton, Huntingdon, Cambs. PE28 2LS, UK Over much of Europe, Common Crossbills Loxia curvirostra depend primarily on the seeds of Norway Spruce Picea abies, and their breeding and movement patterns are governed largely by the cropping patterns of this tree species. Good cone crops occur only every few years in particular areas, but in different years in different areas. The main period of move- ment is in summer, when the previous year’s crop is coming to an end, and a new crop is forming in different areas. In years when the Norway Spruce crop is poor over a wide area, and when Common Crossbills are abundant, they leave the boreal forest in large numbers, and appear as irruptive migrants in southwest Europe, with at least 40 invasions reaching Britain in the 120-year period 1881–2000. On irruptions the main migration axis is from northeast to southwest or west. Recoveries of birds ringed mainly in Germany confirm that these birds do not return to the boreal forest in northern Russia in the same calendar year as their irruption, but do so only in a later calendar year, when a new Norway Spruce crop is becoming available. The findings agree with an earlier interpretation of Common Crossbill movements, but not with an alternative hypothesis that irrupting Common Crossbills return to their region of origin in the same calendar year as the outward movement. Recoveries also suggest that some individual Common Crossbills may have bred in widely separated localities in different years (records up to 3170 km apart), and sometimes in localities far removed from their natal sites (records up to 2950 km apart). The Common Crossbill Loxia curvirostra is well known in May, providing food for Crossbills from about as an irruptive migrant, every few years appearing in June into the winter, and particularly from January large numbers outside its regular range. This paper on, when the cones begin to open and shed seeds. By assesses evidence on the occurrence and timing of late May, most cones and seeds have fallen to the outward and return movements from recent European ground and are lost to Crossbills, which then switch ring recoveries, testing the views that (a) Crossbills in to alternative foods (Marquiss & Rae 1994, 2002), this region make only one major movement each and eventually to the new Norway Spruce crop. year, and that (b) most birds that leave the regular Most Common Crossbill breeding in Norway Spruce range on an irruptive migration do not return to that areas occurs in January–April when seeds are most range until one or more years later. readily available, but it may begin as early as Novem- ber in years of exceptionally good crops (Newton 1972). BACKGROUND In many regions, Norway Spruce grows alongside Over much of Europe, Common Crossbills feed other conifers, which also provide food for Common primarily on seeds of Norway Spruce Picea abies, Crossbills. Scots Pine Pinus sylvestris cones open later extracted from the cones, but not normally picked than those of Spruce, providing a food supply lasting from the ground (Newton 1972). The annual cycles into July, bridging the gap between successive spruce of these birds, including their movements, can best crops, and sometimes allowing Common Crossbills be understood in light of the fruiting patterns of to breed into May or June. Larch Larix decidua cones Norway Spruce. Each year, new cones begin to form form in May, open from late summer and in years of good crops allow Common Crossbills to breed in *Email: [email protected] September–October (Newton 1972). © 2006 The Author Journal compilation © 2006 British Ornithologists’ Union Movement patterns of Common Crossbills in Europe 783 Common Crossbills irrupted into Britain on at least Normal movements 40 occasions, at intervals of 1–9 years (updating Movement patterns of Common Crossbills in relation Newton 1972). Each time, the birds arrived during to Norway Spruce crops (Newton 1970, 1972) were late May to October, mainly in June–July. The fact that inferred from four types of observation: (1) in any trapped migrants differed slightly in bill dimensions one area, good Spruce crops are usually followed by from one irruption to another (Davis 1964, Herremans years with little or no seed production (Hagner 1965); 1988) suggested that not all invasions necessarily (2) in different regions, the year-to-year fluctuations originated from the same region (or that not all bill- in crops may be out of phase with one another, so size categories were stimulated to move in the same that poor crops in some regions may coincide with year). Most of these irruptions included small good crops in other regions (Formosov 1960, Hagner numbers of other crossbill species (Parrot Crossbill 1965); (3) in any one area, densities of Common L. pytyopsittacus or Two-barred Crossbill L. leucoptera) Crossbills fluctuate from year to year, in parallel with (Newton 1972). the Spruce crop (Reinikainen 1937, Formosov 1960, Large numbers of migrating Common Crossbills Petty et al. 1995, Thies 1996, Summers 1999); and ringed in 1959 and 1963 at the Col de Bretolet in the (4) in each year, Common Crossbills are seen to Swiss Alps provided the first firm evidence that some abandon areas where seeds are becoming unavailable individuals eventually returned to their presumed through seed fall in April–May, and from then on to area of origin (Newton 1972). All but one of 17 recov- accumulate in other areas where a new Norway eries obtained within a year of ringing were in the Spruce crop is forming (Newton 1972). Common ‘invasion areas’ of southwest Europe, at latitudes to Crossbills might be scarce or absent from an area in the south of Bretolet, while four recoveries obtained one year, and present in hundreds per square kilo- in later years were far to the northeast in Russia (the metre in the next (e.g. Formosov 1960). The inference direction from which the birds had come). These from such observations is that Common Crossbills various birds seemed to have followed a direct NE– move around from year to year, each summer con- SW path, and the most distant recoveries (from Russia centrating in areas where Norway Spruce cones are and Iberia, respectively) were more than 5000 km plentiful (Newton 1972). The birds have one major apart. From these observations, I concluded that: (1) period of movement in the summer of each year, as in invasion years, the movements occurred mainly in they leave areas where the previous year’s Spruce summer, at about the same time of year as normal crop was good but coming to an end, and concen- (although beginning earlier and ending later), but trate in areas where the current year’s Spruce crop is were more directional and over longer distances, and good but forming. Between these times, some birds (2) a return movement occurred, but not until a later concentrate temporarily in Scots Pine areas. Lesser calendar year, when a new Norway Spruce crop was movements occur at other times of year, in relation forming (Newton 1972). Unlike most other migrant to local changes in food availability, such as seed birds, Common Crossbills in Europe thus had only depletion (caused by consumption or seed fall). This one major period of movement each year, southwest behaviour was inferred entirely from observations, in invasion years and northeast in later years. and there were no supporting or contradicting ring Of 1008 Common Crossbills caught at the Col de recoveries. To judge from Norway Spruce cropping Bretolet during the summer invasion of 1963, many patterns (e.g. Hagner 1965), in most years birds had high weights and high fat scores, indicating moving from areas of poor crops would have to migratory fat deposition, whereas ten birds caught in travel up to several hundred kilometres to find areas Switzerland at other times of year were relatively where new Norway Spruce crops were good. low in weight, with little or no obvious fat (Newton 1972). Of 176 birds caught in Scotland in more recent years, only 30 had high fat scores, all from Irruptive movements April–July. The remaining birds caught at other In some years, apparently when widespread high times of year had very low fat scores (Marquiss & populations coincide with poor Norway Spruce crops Rae 2002). The apparent restriction of fat birds to a over wide areas, Common Crossbills leave the boreal single period of year provides further evidence for a forest, and move mainly southwestward through single main period of movement each year, centred Europe, appearing in many places that lack suitable on summer. This is a different situation to that in conifer habitat. For example, between 1880 and 2000, most other migratory birds, which show two distinct © 2006 The Author Journal compilation © 2006 British Ornithologists’ Union 784 I. Newton periods of migratory fattening, in spring and autumn, In the rest of this paper, these alternative ideas associated with their two periods of (outward and on the timing of return movements after irruptions return) movements per year. are examined on the basis of ring recoveries, mainly However, Gatter (1993) produced an alternative from the Vogelwarte Radolfzell scheme in Germany hypothesis on Common Crossbill movements (with (Schloss 1984), but also from other sources (Table 3). further data and analysis in Gatter 2004). On the Recoveries are examined not in terms of calendar basis mainly of birds seen flying overhead at one years but in terms of ‘Norway Spruce-years’, taken here locality (the Randecker Maar at the northern edge of as 1 June−31 May.