Northern Bullfinch Pyrrhula P. Pyrrhula Irruptive Behaviour Linked to Rowanberry Sorbus Aucuparia Abundance
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Ornis Fennica 86:51–60. 2009 Northern Bullfinch Pyrrhula p. pyrrhula irruptive behaviour linked to rowanberry Sorbus aucuparia abundance Anthony D. Fox, Sverre Kobro, Aleksi Lehikoinen, Peter Lyngs & Risto A. Väisänen A. D. Fox, Department of Wildlife Ecology and Biodiversity, National Environmental Re- search Institute, University of Aarhus, Kalø, Grenåvej 14, DK-8410 Rønde, Denmark. E-mail: [email protected]. (correspondence author) S. Kobro, Bioforsk, Plant Health and Plant Protection Division, Høgskoleveien 7, N-1432 Ås, Norway A. Lehikoinen, Bird Ecology Unit, Department of Biological and Environmental Sci- ences, P.O. Box 65 (Viikinkaari 1), FI-00014 University of Helsinki, Finland P. Lyngs, Christiansøs Naturvidenskabelige Feltstation. Christiansø 97, DK-3760 pr Gudhjem, Denmark R. A. Väisänen, Zoological Museum, Finnish Museum of Natural History, P.O. Box 17 (P. Rautatiekatu 13), FI-00014 University of Helsinki, Finland Received 23 December 2008, revised 15 April 2009, accepted 28 April 2009 Autumn Northern Bullfinch abundance at bird observatories around the Baltic Sea con- firmed periodic irruptive behaviour outside of its normal wintering range, but age-ratio data suggested dispersal was not linked to reproductive success. Rowan berries are im- portant food of Fennoscandian Northern Bullfinches and show synchronised masting in annual fruit production over large spatial scales. Norwegian and Finnish annual berry abundance indices from 1972–2004 were used to test the hypothesis that poor rowanberry production in normal wintering areas was responsible for efflux of birds to other areas. Annual Finnish wintering bird surveys and catches at local bird observatories correlated with rowanberry abundance indices, supporting the prediction that highest Bullfinch abundance would occur in normal wintering areas in years with heavy rowanberry crops. Northern Bullfinch autumn abundance at Danish and Swedish bird observatories (outside the normal winter range) showed inverse correlations with Norwegian rowanberry crops, supporting the prediction of highest irruptions in years of lowest rowanberry abundance from the normal wintering range. These data suggest that at large spatial scales, berry masting can have profound effects on the annual distribution and migratory behaviour of birds consuming the crop. 1. Introduction mon among species feeding on seeds or berries of trees and bushes which show greater inter-annual Arrivals of large numbers of wintering birds out- fluctuation in their reproductive investment in side their normal range (irruptions) are a feature of seeds and berries than, for example, herbaceous several northern hemisphere species (Newton plants (Newton 1972). Unlike regular “obligate” 2006a, 2008). This behaviour is especially com- migrants, moving predictably and seasonally be- 52 ORNIS FENNICA Vol. 86, 2009 tween regular breeding and non-breeding quarters high relative to the food supply, or simply a re- (Alerstam 1990), irruptive species are “faculta- sponse to a lack of winter food supply? Some pas- tive” migrants, retaining flexibility in their migra- serines exhibit density-dependent dispersal tory response in the face of fluctuating food sup- (Matthysen 2005, Nilsson et al. 2006), especially plies. Irruptive species show greater annual vari- among juveniles (e.g., Greenwood et al. 1979, ability in proportions of the population that mi- Nilsson 1989). Could this be the case among grate, and greater individual and year-to-year vari- Northern Bullfinches? If so, more birds would be ations in timing, directions and distances of move- expected to appear outside normal wintering areas ments. Irruptive behaviour is generally driven by following summers with high reproductive suc- lack of food on the normal wintering grounds, cess (measured by the proportions of young in the causing birds to be displaced elsewhere. However, caught sample). this is not always the case, because it may also re- Much of the Northern Bullfinch range (e.g., sult from nomadic or irruptive behaviour in rela- Fennoscandia) lies beyond the present range of tion to breeding conditions (Newton 2006a). For many plant species exploited for food at lower lati- example, nesting Siskin Carduelis spinus densities tudes – such as the “keys” or seeds of ash Fraxinus correlated with local spruce-cone abundance excelsior and the berries of blackberries Rubus (Haapanen 1966) and Common Redpoll C. flam- spp. – and winter snow cover precludes feeding on mea flammea breeding densities with birch Betula herbaceous weed seeds, the staple diet of smaller spp. (Taylor 1954, Enemar 1969) or spruce Picea most southerly distributed races (Newton 1967). spp. seed stocks (Peiponen 1957, 1962). Many As a result, rowanberry Sorbus aucuparia seeds passerines respond to unpredictable local food (an important food item for Bullfinches through- abundance by nomadic shifts between wintering out their distribution) attain particular significance areas, which are not strictly eruptions from a tradi- in the northern part of the range, where they domi- tional winter range, for example, Bramblings nate the autumn diet of P. p. pyrrhula and are also Fringilla montifringilla aggregate in areas with taken in large amounts in winter (Erkamo 1948, densest beech Fagus sylvatica mast cropping Pulliainen 1978, Snow & Snow 1988, Raspé et al. (Lithner & Jönsson 2002) and Crossbills (Loxia 2000). spp.) may traverse continents in response to pre- Rowans rarely fruit heavily (Ahlberg 1927), ferred conifer seed abundance (Reinikainen 1937, and recent Norwegian studies have shown that the Newton 1972, 2006b, Benkman 1992, Adkisson rowan is a classic “masting” species, with large 1996). However, despite many local studies, large variations in berry production across years, highly scale studies of irruptive migrants remain scarce in synchronised at large spatial scales (Kobro et al. the literature, with the notable exception of Koenig 2003, Satake et al. 2004). It is tempting to specu- & Knops (2001) in North America. late, therefore, on whether the autumn irruptions The Bullfinch Pyrrhula pyrrhula seems an un- of Northern Bullfinches, witnessed outside of likely irruptive species; median documented re- Fennoscandia, coincide with years of poor rowan- covery distance of British-ringed birds is less than berry cropping there. 1 km (Newton 2002), and 67% of individuals Because of the level of cropping synchrony ringed in Denmark have been recovered within throughout the Nordic countries, we here use the less than 5 km (Bønløkke et al. 2006). Neverthe- time series of fruit setting in the rowan from Fin- less, the nominate race, the so-called Northern land and Norway to test the hypothesis that Bullfinch P. p. pyrrhula, is invasive (e.g. Newton Fennoscandian Northern Bullfinches are eruptive, 1972, 2006a), showing variable abundance out- departing their normal short migration wintering side of its normal winter range, ranking in the top areas in seasons when the rowan crop fails. ten migrants showing greatest annual fluctuations in catches at eastern European ringing stations We test three specific predictions: (Nowakowski & Vähätalo 2003). But is this irruptive behaviour the result of density-depend- 1. At bird observatories outside the normal range ent dispersal, with birds departing traditional win- of the Northern Bullfinch population, the spe- tering areas when reproductive success has been cies’ abundance should show wider annual Fox et al.: Northern Bullfinch irruptive behaviour 53 variations compared to more regular migrant an assessment of annual game statistics carried out species, consistent with an irruptive migratory by the Finnish Game and Fisheries Research Insti- pattern. tute amongst game wardens and hunters, generat- 2. Years with heavy rowanberry masting should ing some 500 responses annually (Wallenius coincide with high densities of Northern Bull- 1999). Observers assessed relative annual abun- finches remaining in Fennoscandia, as shown dance as average (score 2), or above (3) or below by comparing winter bird abundance in Fin- average (1), with zeros entered for areas where the land with indices of rowanberry abundance species did not occur and for crop failure; average there. scores were used to generate an abundance index 3. In years of poor rowanberry production, un- (AI) for the entire country in each year. Here, we usually large numbers of Northern Bullfinches use the data presented in Wallenius (1999) for the should be witnessed migrating outside of province of North Karelia, representative of Fin- Fennoscandia, consistent with an eruption, land as a whole. Such scores (in contrast to system- with few residual birds in remaining in core atic counting) likely overestimate abundance in winter areas because of lack of food there. inter-mast years, since flowers and berries in abun- dance are conspicuous, but sparse fruiting is more We use winter bird count data and ringing totals as easily overlooked. Although this introduces some measures of efflux of Northern Bullfinches from bias, the measures nevertheless provide a good Fennoscandia during their passage at four differ- measure of those years of greatest and poorest ent bird observatories around the Baltic to test pre- abundance. dictions about how the birds should distribute themselves in relation to rowanberry abundance in each year. 2.2. Measures of irruptive bird abundance Long-term changes of the abundance of different 2. Material and methods bird species have been monitored in Finland since the winter 1956/1957,