69

Irruption and wintering ecology of the Great Spotted major

KALERVO ERIKSSON

ERIKSSON, K. [Research Laboratories of the State Alcohol Monopoly (Alko), Box 10350, 00101 Helsinki 10, Finland] 1971. - Irruption and wintering ecology of the Great Spotted Woodpecker Dendrocopos major. Ornis Fenn. 48 :69-76. The study is based on winter censuses carried out in Finland in the years 1956-68, cone crop figures of the and for the same period, and recoveries of Great Spotted ringed in Finland. It is statistically demonstrated that the density of wintering populations of Great Spotted Woodpeckers depends on the cone crop of the spruce (r = -I- 0.32 ; p < 0.05). The irruptive populations of the Great Spotted Woodpecker fluctuate synchronously in Finland and Norway (r = + 0.74 ; p < 0.01) . It is claimed that young birds tend to erupt annually. High population density and early territoriality of the young release a mass emigration, the extent of which is regulated by the cone crops.

Only a few authors have studied the on the age and sex ratios among the autumnal eruptions of the Great Spotted irrupting . The great irruption ir, Woodpecker, which are sometimes Fennoscandia in autumn 1968 was exceptionally large. PYNNÖNEN (1939) studied in detail by H ILDEN (1969), reports that Great Spotted Woodpeckers who claims on the basis of recoveries of moved along the Southern coast of birds ringed in Finland that - Finland in greater numbers than usual peckers coming from the east performed in August 1930 and 1935 . He connects a loop movement in Fennoscandia . these autumn emigrations with the However, it is possible to clarify the unusually poor seed crops of the spruce picture of the wintering and irruption (Picea abies) and the pine (Pinus sil- ecology of the Great Spotted Wood- vestris) in those years. SVÄRDSON pecker on the basis of winter bird (1957) in his extensive survey of bird censuses carried out in Finland, data invasions also deals with the Great about the seed crops of , and Spotted Woodpecker, and he states that Finnish ringing recoveries. the influx of this species in Central Europe in the autumn of 1929 led to breeding in the alpine region, where Material and methods spruce had a good seed crop. The fol- The material showing the fluctuation of lowing autumn there was a new invasion wintering populations of the Great Spotted which took the birds to Italy. He thus Woodpecker is based on winter bird censuses considers the invasions to be emigration organized by the Zoological Museum of the University of Helsinki. These have been leading to the occupation of new breed- carried out in various parts of the country ing areas. In the autumn of 1962 there since the winter of 1956/57. The present were very extensive irruption move- paper deals with the censuses for 13 successive ments in Fennoscandia and the British winters, the last one being 1968/69. The censuses were carried out between 25 Decem- Isles, which have been studied in detail ber and 6 January. They were always made only in Britain (WILLIAMSON 1963) along exactly the same routes and normally and on Helgoland (VAUK 1964) . also by the same persons each year (see also WILLIAMSON claims that the weather HILDEN & KOSKIMIES 1969) . In recent years the total number of routes has been about was a releasing factor for the flight to the 450. In the present study the routes were , while VAUK concentrated grouped according to the zoo-geographical 70 ORNIs FENNICA Vol. 48, 1971 regions of Finland (MERIKALLIO 1958), and Numbers of wintering population and three regions were chosen for consideration because their census scores were the most cone crops reliable. The seed crop figures examined were those from the corresponding years and The best-explored parts of Southern regions. Since the variances of the statistical Finland, i.e. South-western Finland, functions were roughly equal, the figures of all three regions were pooled in the regression Lake-Finland and Ostrobothnia, were analysis. The methods were otherwise the taken into consideration . Fig. 1 gives same as those described in detail earlier the mean numbers of Woodpeckers per (ERIKSSON 1970a) . participant and ten census routes for In order to eliminate the effect of weather on the results, the observers were instructed each region in each of the winters of to carry out their census in good weather, e.g. 1956-68 . The material indicates clear- avoiding strong winds or blizzards. It may be ly that the Great Spotted Woodpecker assumed therefore that the number of parti- was most abundant in Finland in the cipants per route and the length of the routes 1956/57, 1958/59 are the main causes of error in the census winters of and results. Using multiple regression analysis the 1967/68 . The greatest autumn erup- effect of the length of the route and the tions during this period occurred in number of participants on the number of 1962 and 1968 but in neither year was birds observed was studied. The analysis showed that these errors are very small and the number of wintering Woodpeckers of no importance . above average. In autumn 1967 there Details of the cone crop of the spruce and was also a clear, although weaker exodus pine were obtained from the Finnish Forest HILDEN 1968, 1969), which was Research Institute, where the cone crops are annually classified into six categories (0-5) followed by abundant wintering . on the basis of inquiries made each autumn in Fig. 1 . shows that on the average some 400 localities in Finland (see also a slightly smaller number of Wood- ERIKSSON 1970a) . peckers was recorded in South-western The data on migration is based on 35 recoveries of Great Spotted Woodpeckers Finland than in the other regions . This ringed in Finland before 1 February, 1970. is due to the fact that the parts of the The whole material has been checked and the routes traversing forest are smaller. Oth- study includes all but unimportant records, erwise the population fluctuations in dif- such as short-term local controls, etc. None of the recoveries excluded contradict the ferent winters seem to be similar in the results presented. different regions . An exception is the The fluctuations of the irruptive populations astonishing abundance of Great Spotted of the Great Spotted Woodpecker were exam- Woodpeckers in Ostrobothnia in the ined by studying the number of birds ringed in Finland and Norway in the years 1956-68. winter of 1958/59 . This winter was Since the majority of Great Spotted Wood- a good one for wintering Great Spotted peckers are ringed when full grown, (the pro- Woodpeckers everywhere in Finland portion is especially large in irruption years) but on the basis of other figures, the these figures should give a fairly good idea of the strength of the autumn movements. expected density figure for Ostrobothnia Since it is obvious that the number of Great should have been around 20, whereas Spotted Woodpeckers ringed is connected in fact it was 168 .8, the highest density with the intensity of ringing activity, the figure for this region for the whole figures were made comparable by relating them to the total number of birds ringed each study period. year and expressing them as the number of Fig. 1 . presents the cone crop figures Woodpeckers per 10 000 birds ringed. of the spruce and pine for the corre-

FIG. 1 . The numbers of wintering Great Spotted Woodpeckers per participant and ten census routes in Finland in three zoo-geographical regions in the winters 1956-68 (upper). The relative numbers of Great Spotted Woodpeckers ringed per 10 000 birds ringed in Norway and Finland (middle). The cone crop of the spruce and pine in respective periods and regions (lower). The regions studied are SW-Finland (X), Lake-Finland (0) and Ostrobothnia (A) .

K. Eriksson : Irruption and wintering ecology 71

72 ORNIS FENNICA VOI. 48, 1971

sponding regions during the period tion (r = + 0 .74; p < 0.01 ; df = 12) 1956-68. The best cone crop of the between the relative numbers of Wood- spruce seems to have been that of peckers ringed in the two countries. autumn 1967 . The seed crop records The fluctuations in the numbers of were also better than average in the Woodpeckers in Finland and Norway autumns of 1956, 1958 (especially in are thus synchronous. The mass irrup- Ostrobothnia), 1960, 1964 and 1965. tions of Great Spotted Woodpeckers Of these, the only years with abundant in 1962 and 1968 appear as clear peaks wintering Great Spotted Woodpeckers in the statistics of both countries . Large were 1956, 1958 (in Ostrobothnia ) irruptions seem to have occurred also and 1967 . The cone crop scores of the in the years 1956, 1957 and 1966. pine are on average a little higher than HILDEN (1968) considers the irrup- those of the spruce. Pine cones were tion in the year 1967 also to be more abundant in the autumns of 1959 and abundant than average. 1965, whereas in 1962, 1963, 1964 and It is appropriate to study the mass irrup- tions of 1962 and 1968 in closer detail . During 1968 the crops were rather poor. the summer of 1962 the mass movements of ERIKSSON (1970a) has earlier pre- Woodpeckers started as early as July . Great sented the numbers of spruce seeds shed Spotted Woodpeckers were found to be per square metre and demonstrated that exceptionally abundant in the Finnish forests, there are enormous differences especially in Lapland, around the middle of between July, though no abundance was yet apparent different years in this respect. The mean in Southern Finland. PULLIAINEN (1963) number of seed/m2 is much smaller in states that Great Spotted Woodpeckers began the pine than in the spruce and the to appear in great numbers in Lake-Finland variation between the years is also in August . The Woodpeckers seem to have come from the east along the coniferous zone, smaller. The differences between the arriving first in Northern Finland and then years are 10-30-fold in pine, whereas in Northern Norway, where many birds were for spruce they may be up to 300-fold. found around 20 July climbing the steep cliffs In the pooled material the numbers of Röst, as Toralf Mikalsen told me the following summer . It was observed that from of wintering Woodpeckers were found there the birds continued southwards, scat- to correlate significantly with the cone tering in different directions, and arrived in crop of the spruce (r = + 0 .32; greater numbers than normal in the British p < 0 .05; df = 37), but not Isles and Helgoland, where the first birds with the belonging to the irruptive northern race were cone crop of the pine (r = 0.04) . Their recorded on 15 July (WILLIAMSON 1963, dependence on the spruce crop can VAUK 1964). A very strong eastward move- perhaps be explained by a linear regres- ment was recorded at the Signildskär Bird sion model y = 4.7x + 0.88, since the Observatory in the Aland Sea, where some 130 000 Woodpeckers were recorded during coefficient (bx) is statistically nearly the period 25-27 July (WILLIAMSON 1963), significant (t=1 .8 ; 0 .05

Discussion controls the extent of the exodus (KLUIJVER 1951, ULFSTRAND 1962) . A comparison of the figures of winter It is mainly young Tits which take part bird censuses and the seed crops of in these eruptions . A similar eruption conifers shows that the density of a mechanism has been found to exist in wintering population of Great Spotted the Sitta europaea, where a Woodpeckers is to some extent depen- growth of population leads to the dent on the cone crop of the spruce. emigration of young birds, which causes The results do not show any dependence an abrupt drop in population density of the wintering population level on the (BERNDT & DANCKER 1960, ERIKSSON cone crop of the pine. Nevertheless, the 1970b) . So it seems reasonable to cone crop of the pine may be a more or assume that regular eruption move- less important factor in the food ecology ments are part of the normal behaviour of Great Spotted Woodpeckers . It is of young Great Spoted Woodpeckers too. generally considered that poor cone crops The denser the population, the greater of the spruce and pine release an eruption is the proportion participating in the of Great Spotted Woodpeckers (PYNNÖ- emigrations, the extent and the duration NEN 1939, VOOUS 1947, PULLIAINEN of which seem to be regulated by the 1963, and HAAPANEN 1966) . Pynn6- availability of food. nen mentions that the eruptions in 1935 This assumption agrees with the were caused by unusually poor cone finding of PULLIAINEN (1963) that crops. Unfortunately, more recent esti- young Woodpeckers demonstrate clear, mates of fluctuations in the breeding territorial aggressive behaviour and try population are not available. According to occupy their territories as early as to the results, the eruptions seem to August. He also states that the terri- start in late July (PYNNÖNEN 1939, tories are larger in areas where food is WILLIAMSON 1963, VAUK 1964, HIL- scarce. They also seem to be larger in DEN 1968 and 1969), coinciding with pine forest than in spruce forest, which the time when the young birds begin to is due to the differences in the amounts move about . According to JOHANSEN of seed of these trees. Further, the (1955), young Great Spotted Wood- predominance of young birds in the age peckers head for western Siberia in late ratio (VAUK 1964, HI LDEN 1968) June or early July . PYNNÖNEN (1943 ) probably reflects high population den- has noted that, at that time of the year, sity during eruption. PYNNÖNEN'S the Woodpeckers still feed mainly on (1939) study shows that the breeding various insects and plants . Not until the population was rather large before the end of September do the seeds of coni- eruption of 1935 but dropped abruptly fers begin to form the main part of after the emigration. This is typical of their diet. Pynnönen also observed that species such as Tits and , the Woodpeckers seem to be very ver- whose eruptions are caused by high satile in their choise of food at the end population density. SVÄRDSON (1957) of July, when the culmination of erup- has already suggested that the emigra- tion usually occurs. This makes it hard tions of the Great Spotted Woodpecker to believe that the primary releasing lead to the occupation of new breeding factor of the emigrations is lack of food. areas. It has been demonstrated with a HAAPANEN (1966) states that the group that is ecologically closely related, nesting populations of this species fluc- namely the Tits, that the releasing tuated relatively little during his study factor of eruption is the density of period 1960-1964 in Southern Fin- population and that food supply only land. This may be due to the fact that

K. Eriksson : Irruption and wintering ecology 75

adult birds mainly tend to stay in their strated also with Tits and Nuthatches territories. On the basis of ringing (KLUIJVER 1951, SVÄRDSON 1957, recoveries and the study of VAUK ULFSTRAND 1962 and ERIKSSON (1964), it seems that adult males are 1970b) . the most territorial ones, while young A detailed analysis of ringing re- birds show a strong urge to erupt. The coveries shows that, contrary to the assumption that population density, opinion expressed by H ILDEN (1969), is the primary cause of eruption explains even birds which have nested in Fenno- the apparently contradictory findings scandia may move eastwards . On the that in the autums of 1956 and other hand, adult males that have win- 1967 with good cone crops there tered in Fennoscandia may move east- were clear eruptions from Fennoscandia, wards in the following spring, appar- whereas in the very poor cone crop ently returning to their earlier nesting autumns of 1963 and 1966 there were places, which indicates a tendency to only weak eruptions . The lack of seeds true migration. This may also explain may cause some eruptive movements in the regular though small spring move- September-October when the birds ment of Great Spotted Woodpeckers settle down in their final winter terri- noted at bird observatories (HILDEN tory since, as noted by PULLIAINEN 1968 ; 1969) . (1963 ), the settling down is regulated by the cone crop. Such representatives of the floating population appear at the Acknowledgements observatories fairly often in September I wish to express my particular gratitude to -October. Mr. Pertti Saurola M.Sc., who made his obser- vation records available to me. This study was Owing to the poor cone crops in the partly supported by a grant from the Emil summers of 1962 and 1968, the irrup- Aaltonen Foundation. tive birds did not remain in Scandina- via, but performed a loop movement and returned to the east (HILDEN S e 1 o s t u s : Käpytikkojen vaellus- ja 1969) . The irruptions are always first talvehtimisekologiasta . observed in the north (HILDEN 1968, 1969) . This may be caused by the fact Käpytikan talvehtivien populaatioiden vuotui- sista ja alueellisista runsaudenvaihteluista esi- that the Great Spotted Woodpeckers tetyt luvut perustuvat vuosina 1956-69 suo- arrive from the east along the coniferous ritettuihin talvilintulaskentoihin . Mukaan on zone and are thus oriented north-west. otettu parhaiten tutkitut alueet : Lounais-Suo- It has been found that all mi, Järvi-Suomi ja Pohjanmaa . Vastaavalta the irruptions ajalta ja samoilta alueilta on (ERIKSSON esitetty myös of the Siberian Nuthatch Metsäntutkimuslaitoksen kokoamat tiedot kuu- 1970b) have a similar orientation . After sen ja männyn käpysadoista. Syksyisin vaelta- this the irruption moves southwards neiden käpytikkojen runsautta on tutkittu ver- dispersing in various directions, e.g, the tailemalla Norjassa ja Suomessa merkittyjen käpytikkojen suhteellisia lukumääriä. Edellä British Isles, Denmark and across the esitetyt tiedot on koottu kuvaan 1 . Aland Sea and Southern Finland to the Käpytikkojen talvinen kanta on ollut eri- east. A very large proportion of the tyisen runsas talvina 1956/57, 1958/59 ja birds remain somewhere along this route 1967/68 . Joitakin runsaita vaellussyksyjä on seurannut niukka talvehtiminen . Talvehtivan as indicated by ringing recoveries. The kannan runsaus näyttää korreloivan positiivi- settling in new areas is in accordance sesti kuusen (r = + 0.32; p < 0.05), mutta with the emigratory nature of the irrup- ei männyn (r = 0.04) käpysatoon . Näin siitä tion (SVäRDSON 1957), which means huolimatta, että mänty on lajin käyttämä ra- that vintokasvi . Männyn ja kuusen siemensatojen young birds try to nest in the absoluuttisissa määrissä on kuitenkin suuri wintering area. This has been demon- ero siten, että kuusen siemenmäärä on huo- 76 ORNIS FENNICA VOI. 48, 1971

noinakin siemenvuosina yleensä paljon suu- HILDEN, O. 1969. Activities of Finnish bird rempi kuin männyllä. stations in 1968. Ornis Fenn. 46 :179- Vaelluksen voimakkuuden vaihtelut Norjas- 187. sa ja Suomessa korreloivat positiivisesti HILDEN, O. & J. KOSKIMIES 1969. Effects of (r = + 0.74 ; p <0.01) . Erityisen runsaita the Severe Winter of 1965/66 upon massavaelluksia on ollut vuosina 1962 ja 1968, Winter Bird Fauna in Finland. Ornis mutta runsasta esiintymistä on ollut myös Fenn. 46 :22-31. 1956, 1957, 1966 ja 1967. Vaellusten runsaus- JOHANSEN, H. 1955. Die Vogelfauna Westsibi- huippu näyttää hyvin säännöllisesti sattuvan riens III Teil, Pici-Cuculi. J.f. Orn. 96: heinä-elokuun vaihteeseen. 382-410. Suomessa merkityistä käpytikoista tulleet KLUIJVER, H. N. 1951. The population eco- rengaslöydöt osoittavat, että Suomessa pesi- logy of the Great , Parus m.major L. neetkin vanhat linnut voivat vaeltaa kauas Ardea 39:1-135. itään ja vastaavasti Suomessa talvehtineet voivat MERIKALLIO, E. 1958. Finnish Birds. Their vasta keväällä muuttaa itään pesimäajaksi . distribution and numbers. Fauna Fenn. Vaellustikoista tehdyt iän ja sukupuolen mää- 5 :1-181. ritykset osoittavat, että vain noin 10 % yk- PULLIAINEN, E. 1963. Observations on the silöistä on vanhoja ja niistä enemmistönä ovat Autumnal Territorial Behaviour of Great vanhat naaraat. Tutkimuksessa tarkastellaan Spotted Woodpecker, Dendrocopos major vaellusten laukaisevia tekijöitä ja arvellaan (L) . Ornis Fenn. 40:132-139. nuorten lintujen loppukesäisen territoriaalisen PYNNÖNEN, A. 1939. Beitraege zur Kenntnis käyttäytymisen olevan laukaisevana tekijänä der Biologie finnischer Spechte. I. Ann. yhdessä populaatiotiheyden kanssa. Vaellukset Zool. Soc. "Vanamo" 7, 2:1-71 . olisivat tällöin emigraatioita, joiden ulottu- PYNNÖNEN, A. 1943. Beitraege zur Kenntnis vuutta siemensadot säätelevät . der Biologie finnischer Spechte. II . Die Nahrung. Ann. Zool. Soc. "Vanamo" 9, 4:1-60. SVÄRDSON, G. 1957. The "invasion" type of References . British Birds 50 :314-343 . ULFSTRAND, S. 1962. On the nonbreeding BERNDT, R. & P. DANCKER 1960. Der Kleiber, ecology and migratory movements of the Sitta europaea, als Invasionsvogel. Die (Parus major) and the blue tit Vogelwarte 20:193-198. (Parus caeruleus) in Southern Sweden. ERIKSSON, K. 1970a. The autumn migration Vår Fågelvärld, Suppl. 3 :1-145. and wintering ecology of the Siskin VAUK, G. 1964. Invasionen von Kreuzschnae- Carduelis spinus. Ornis Fenn. 47 :52-68. beln (Loxia) and Buntspechten (Dendro- ERIKSSON, K. 1970b. The invasion of Sitta copos major) . Die Vogelwelt 85:113- europaea asiatica Gould into Fennoscan- 120. dia in the winters of 1962/63 and WILLIAMSON, K. 1963. Aspects of Autumn 1963/64. Ann. Zool. Fenn. 7:121-140. Movements at the Bird Observatories HAAPANEN, A. 1966. Bird fauna of the Finnish 1962. Bird Migration 2 :224-251. forests in relation to forest succession II . Voous, K. H. 1947. On the History of the Ann. Zool. Fenn. 3 :176-200. Distribution of the Genus Dendrocopos. HILDEN, O. 1968. The bird stations of Fin- Limosa 20:1-142 . land and their activities in 1967. Ornis Fenn. 45 :58-65. Received 3 March, 1971 .

SLY:n kokoukset syyskaudella Suomen Lintutieteellisen Yhdistyksen syyskau- den 1971 kokoukset pidetään 23.9., 14.10., 17.11. ja 16.12. Helsingin Yliopiston Eläintie- teen laitoksessa, P. Rautatiek. 13, alkaen klo 19. - Sihteeri.