The Census and Distribution of Wintering Woodpigeons Columba Palumbus in the Iberian Peninsula

Ornis Hungarica 12-13: 157-167. 2003

The census and distribution of wintering woodpigeons Columba palumbus in the Iberian peninsula

A. Bea, R. Beitia and J. M. Fernández

Bea, A., Beitia, R. and Fernández, J. M. 2003. The census and distribution of wintering woodpigeons Columba palumbus in the Iberian peninsula. – Ornis Hung. 12-13: 157-167.

Throughout the 1997-1998, 1998-1999 and 1999-2000 seasons, Woodpigeon population censuses were carried out in their traditional Iberian wintering area, which comprises the southwestern quadrant of the Peninsula. The method applied was the direct counting of flocks in communal roosts, of which 210 have been checked so far. Four counts were carried out per season in November, December, January and February. As an average, 95% of the roosts were counted simultaneously. Between 140 and 230 collaborators participated in each one of the censuses. These collaborators were mainly wardens from the Spanish autonomous regions and the Portuguese Government. Quantitative data suggests that the wintering population in the study area would total about 2.5-3 million birds, not including inter-yearly oscillations asso- ciated with reproductive success in the breeding areas. Their gregariousness remained relatively constant between November and January, 40-70% of the birds being counted in roosts of more than 400 000 individuals (1-2% of the number of roosts). Numbers were stable in this period, in spite of some inter-monthly differences attributed to difficulties in obtaining full cover. However, February offered a variable pattern. This phenomenon shows that dates of start of the spring migration must vary from year to year. The distribution of these contingents was not homogeneous within the study area, a massive presence occurring in Portugal and Extremadura-Toledo alternately. The evaluation of acorn production in Iberian dehesas and montados (pastureland) during 1999 provided interesting points for interpretation. For example, the low availability of food in cork tree woods during that winter explained the absence of birds in the Portuguese districts, and may have stimulated the start of migratory movements. The seemingly direct response shown by wintering Woodpigeons toward the nutritional capacity of the dehesa suggests that the analysis of this factor could allow predictions of the spatial and seasonal distribution of the population.

A. Bea and J. M. Fernández, International Wildlife Investigation Group, Plaza del Caddie, 1. 20160 Lasarte (Gipuzkoa), Spain. E-mail: [email protected] R. Beitia, International Wildlife Investigation Group, Maison de la Nature, 12 Boulevard Hauterive, 64000 Pau, France. E- mail: [email protected]

1. Introduction and Scandinavia to the Atlantic coasts and Mediterranean peninsulas (Saari 1997). Previously regarded as a woodland bird, The Woodpigeon Columba palumbus is a its recent adaptation to, and trophic depen- member of the family Columbidae, a dence upon cultivated fields has led to Palearctic, though mainly European, numerical prosperity in several European species. The subspecies C.p. palumbus countries, allowing phenologic, behav- breeds across the continent, from Russia ioural and demographic changes to be 158 ORNIS HUNGARICA 12-13: 1-2 (2003)

C AN TABRIA LA CO RUÑA ASTU RIAS BIZKAIA GIPU ZKO A LU G O

ALAVA PO N TEVED RA LEÓ N N AVARRA

G ERO N A O REN SE PA LEN C IABURGOSLA RIOJA H U ESC A LÉRIDA

VIAN A DO C ASTELO ZAMORA VILA REAL BARCELONA ZARAG O ZA VA LLA D O LID SO RIA BRAG A BRAG AN Ç A PO RTO SEG O VIA TARRAG O N A

VISEU AVEIRO SALAMANCA G U AD ALAJARA GUARDA TERU EL

ÁVILA MADRID

CO IMBRA C ASTELLÓ N

LEIRIA CUENCA C ASTELO BRAN C O TO LED O

C ÁC ERES SAN TARÉN VALENCIA

PO RTALEG RE LISBO A CIUDAD REAL ALBACETE

ÉVO RA BAD AJO Z ALICANTE

SETU BAL

CÓ RDO BA JA É N MURCIA

BEJA HUELVA SEVILLA

FA RO G RAN AD A ALM ERIA

MÁLAGA CÁDIZ

Fig. 1. The study area in the Iberian Peninsula and the range of dehesas and montados. explained through shifts in agricultural species in many European regions as techniques (O’Connor & Shrubb 1986, shown both by the total hunting bag across Inglis et al. 1990). Europe, estimated at 9.5m birds yearly Recognised migration patterns include during the 1980s, (Purroy et al. 1984) and partial or basically migratory populations, by such as social and cultural events relat- such as Fennoscandian and eastern ed to the shooting season, for example European, as well as short-distance migra- Région Cynégétique du Sud-Ouest (1994). tory and resident populations, as in Accurate population size information Western Europe. This pattern has been is a basic requirement for establishing a described as a general alohiemism (Bernis reliable monitoring program. Tracking this 1966), because northern and eastern popu- variable (and others such as reproductive lations occupy winter quarters further success or survival rates) over periods of south and west than intermediate popula- time will give early indication of populations. In this sense, the important role of tion trends, whose appropriate interpreta- the Iberian Peninsula as a wintering tion may suggest managing or conserva- ground was first pointed out by Bernis tion measures. Thus, monitoring is a pro- (1967) and strengthened by Purroy (1988). cedure to measure the history of variables The Woodpigeon is a prominent game in a systematic way, but with explicit aims A. Bea, R. Beitia and J. M. Fernández 159

(Spellerberg 1991, Gilbert et al. 1998). these habitats in the Iberian Peninsula The present study is a contribution to the depends on the defining criteria, but c3.1m quantitative knowledge and distribution of hectares is a reasonable estimate (Díaz et Woodpigeon population wintering in the al. 1997). Iberian Peninsula, and it also explores Of course, other areas in Iberia hold some of the applicable environmental fac- wintering Woodpigeons, and these have tors. also been studied. However, winter densities there are much lower than in spring 2. Study Area and Methods and summer (Díaz et al. 1996), and it is possible that wintering birds belong to the more sedentary native Iberian populations. The study area approximates to the south- That some from these populations move western quadrant of the Iberian Peninsula towards the southwestern quadrant and partially includes the Spanish regions (Gallego 1985) would explain the density of Castilla y León, Castilla-La Mancha, reductions. Lack of ringing effort applied Madrid, Extremadura and Andalucía, and to resident populations obscures knowl- Portuguese Alentejo and Ribatejo (Fig. 1). edge about their migration patterns. The quadrant was defined after the analy- The method employed to undertake a sis of the geographic distribution of recov- census of wintering Woodpigeon popula- eries during the winter months of tions in the study area was adapted from December and January of birds ringed in that used in France for the same purpose many European countries, 88% of recov- (Région Cynégétique du Sud-Ouest, eries being entered in databases up to 1997 1994). The first phase, an inventory of (N=58) of Spanish and Portuguese ringing roosting sites, was carried out in 1997, and schemes (Dirección General de was achieved through personal inquiries to Conservación de la Naturaleza, Instituto administration officials, wardens, shoot- da Conservaçao da Natureza and Sociedad ing federations, birdwatchers, naturalists, de Ciencias Aranzadi) were from inside local people and others involved in the quadrant. Bernis (1967) had used the wildlife management. A database was same method to define the wintering quar- compiled of ‘sites’ mentioned by any ters of trans-Pyrenean migratory informant. A ‘site’ was defined as any Woodpigeons. location noted ever to have been used as The area of the quadrant corresponds roosts by Woodpigeons. The database was largely to those regions whose landscape constantly updated to delete ‘sites’ that mostly comprises dehesas and montados. had been felled or burnt down and to These habitat types are principally wood- include newly discovered roosts. At the ed pasturelands with holm oaks Quercus time of writing, the total set comprises 218 ilex and cork oaks Quercus suber in densi- roosts (including former ‘sites’), 118 of ties of 20-60 trees/ha, and have multifari- them being in Spain and 100 in Portugal. ous productive uses, cattle raising in holm It was not possible to use the data from oak woodland and cork exploitation in inquiries to make rough estimates of pop- cork oak woodland being relatively impor- ulation sizes and trends, because many tant (Gómez 1997). The total extent of observations were inaccurate and roosting 160 ORNIS HUNGARICA 12-13: 1-2 (2003) behaviour was not consistent at any partic- dent rough estimatesof numbers at dusk in ular site. case the weather prevented proper counts The population census method chosen the next morning. was to count directly the birds gathered in Observers received training sessions to known roosting sites, a technique applied explain study goals, the species’ natural generally to censuses of gregarious win- history, census procedure and the methods tering birds such as herons, gulls and star- to be employed to count large flocks lings, because it allows absolute numbers (divide them into lesser units; Bibby et al. to be established by taking into account 1993). Great emphasis was placed on the contiguous distribution of birds and these sessions in order to stimulate separating the counts into detectable responsible and effective participation and recognisable units (Tellería 1986). An to reduce bias due to unsound counts. important requirement is for counts to be Good observer training is one of the fac- simultaneous, so that possible biases, due tors that increases counting accuracy to bird mobility, dispersion or interchange (Erwin 1982, Cantos & Tellería 1985). between roosts, are avoided. Because of As well as this net of collaborators, we the high number of roosts to be visited, formed a mobile team of experienced 140 to 230 observers (mainly official and observers, its task being to detect and wardens) collaborated in each census peri- count bigger roosts whose size exceeded od, so that an average of 95% of counts normal counting skills. Alternative tech- were done simultaneously on the planned niques were used, such as counting differ- dates. ent flocks of birds by different groups of Censuses were performed in the win- observers, and taking advantage of bound- ters of 1997-1998, 1998-1999 and 1999- aries as marking points while flocks 2000. In each winter four census periods passed them. were established, in November, December, During the overall counting period, we January and February (except 1997-98, began in 1999 an evaluation of acorn pro- when there was no February count). Dawn duction in Iberian dehesas and montados. counts are preferable, because the move- We selected at random 25 routes on sec- ment to the feeding areas is direct and ondary and country roads within the study occurs in large flocks at sunrise and over a area (18 in Spain and 7 in Portugal). In short timescale. Dusk counts occur over a each, 25 sample points were selected, longer period, but because the birds often evaluating acorn production in a random- use pre-roosting sites and are unsettled, ly-selected tree to produce a semi-quanti- the risk of double-counting is high. In any tative index. The average acorn crop in case, the census methodology included each route or wood came from the expres- visiting the roosting sites the previous sion: evening, when the arrival headings to Pi=Σ(Np×Ip)/25 roost usually were but one or two, the next morning’s departure headings being the and exact opposite. Knowing these routines 2× Np=Ap/[(Dp/100) (π/4)] helped the choice of the most favourable observation points. Observers made pru- where Pi is the average crop in the A. Bea, R. Beitia and J. M. Fernández 161

100 3.000.000 80

2.000.000 60

roosts 40 of

1.000.000 % 20

0 0 November December January February XI XII I II Months

1997-1998 1998-1999 1999-2000 <1 000 1 000-5 000 5 000-10 000 Fig. 2. Results of Woodpigeon censuses in 10 000-50 000 50 000-400 000 >400 000 1997-1998, 1998-1999 and 1999-2000 winters. 100

80 route, Np the equivalent number of trees in 60 the 1ha plot throughout the sample point, birds

of 40 % Ip the semi-quantitative index of acorn 20

0 production in the particular tree, Dp its XI XII I II diameter (cm), and Ap the basal area in the Months 2 plot (m /ha). <1 000 1 000-5 000 5 000-10 000 This method, developed by Instituto de 10 000-50 000 50 000-400 000 >400 000 la Madera, el Corcho y el Carbón of the Fig. 3. Percentage of occupied roosts by roost- Junta de Extremadura, provides a simple size category (above) and percentage of birds counted by roost-size category (below). and rapid sampling technique to be used in field conditions, and gives accurate mea- problems of achieving observer cover or surements (C. Bernal pers comm). proper coordination in the first winter, Samples were taken in the first fortnight rather than to a lack of birds. of October, when fruit growth allows visi- The cover achieved of roosting sites bility from the ground, but the state of (sites planned to be visited/sites actually fruit maturity precludes it from being lost visited) averaged 88% in 1998-99, declin- through falling. ing to 65% in 1999-2000. This reduction in achieved cover could also explain the 3. Results decline in Woodpigeon numbers between November and December 1999, for it is difficult to attribute to factors other than Fig. 2 shows the total numbers of census performance. The November cover Woodpigeons counted in November, of 70%, declined to 63% in December. December, January and February for the Given the widespread gregarious distribu- winters of 1997-1998, 1998-1999 and tion of the birds, the level of cover acts an 1999-2000. The population size is indicator of counting effort, but not in a assessed at 2.5 to 3 million birds (disre- way proportional to the censused popula- garding inter-annual fluctuations) It has tion, because only a few unvisited sites been assumed that the number of birds holding significantly large roosts would using unknown or unvisited roosting sites, result in reductions to the overall totals or sleeping in dispersed flocks are not sig- counted. nificant, although these were not counted. Also relevant was the contrasting The lower figures for 1997-98 are proba- numerical stability during the winters of bly due to observer inexperience or to 1998-99 and 1999-2000 for the period of 162 ORNIS HUNGARICA 12-13: 1-2 (2003)

Fig. 4. Distribution of wintering Woodpigeons across the study area during November, December, January and February of 1998-1999 (above) and 1999-2000 (below). Light grey squares stand for fewer than 50 000 birds counted in that particular square; dark grey squares stand for between 50 000 to 500 000 birds; black squares stand for more than 500 000 birds. A. Bea, R. Beitia and J. M. Fernández 163

January and February. In 1998-99, the ranges by Ceballos & Ruiz de la Torre numbers reduced by only 15.5%, but in (1971) and Montoya (1989). Nevertheless, 1999-2000, by 93.1%. This difference clear variable patterns between cork oak cannot be attributed to variations in count- and holm oak acorn production were ing cover, but reflected shifts in gregari- found. Average crops in cork-oak domi- ous behaviour that either stimulated a nated pasturelands 86.4kg/ha (σ=43.8, reduction in roost size, or initiated early N=5), against 438.2kg/ha (σ=233,4, return migration. In the first case, the birds N=20) in holm-oak dominated pasture- would not have been in the roosts to be lands, the difference being highly signifi- counted because of short-range dispersal cant (t=3.3, P<0.01). Furthermore, crop within the study area, and in the second, variability in cork oaks was low, with poor the birds would have left the study area production everywhere (apparently syn- entirely. chronized), but results from holm oaks To describe the gregarious behaviour pasturelands showed greater dispersion. of Woodpigeons, the roost occupation and Also of interest was that the acorn crop in population held in winter 1999-2000 were Spanish dehesas and Portuguese montados categorized by roost-size (Fig. 3). About (independent of tree species) differed sig- 80% of roosts each held fewer than 5000 nificantly (457.9 kg/ha against Woodpigeons, but less than 10% held 136.1 kg/ha; t=3.44, P<0.01). more than 50 000 birds each. Yet they held 80-90% of the censused population, 4. Discussion demonstrating the intense gregariousness of the species. Large aggregations of course are more detectable than small, but Through evaluation of the acorn crop in it is unlikely that significant numbers of Iberian dehesas and montados, the distrib- dispersed, smaller aggregations were ution of Woodpigeon population in the missed. This pattern clearly was modified study area can be explained satisfactorily. by February, as described above. Acorns from cork and holm oaks are a In Fig. 4, the distribution is shown of principle winter food for this population, Woodpigeon numbers within the study as are dicotyledonous leaves at the begin- area for 1998-1999. The most remarkable ning of the season (Purroy et al. 1984). feature during the four count periods was However, cork and holm oaks show a that the bulk of the population remained in complementary distribution in the Iberian Portuguese coastal districts. Fig. 4 also Peninsula as a consequence of different shows that the distribution during the ecological preferences. Cork oaks occupy 1999-2000 winter (except for February sectors possessing mild oceanic climate, 2000, as surmised above) was biased relatively high humidity and a low inci- towards the interior regions, mainly in dence of frost. Holm oaks are more resis- Spain tant to cold and temperature contrasts, so The average acorn crop estimated that their basic distribution lies across sec- throughout all the selected routes in 1999 tors of continental climate (Gómez 1997). was 367.5kg/ha, a value considered as In this way, the abundance of cork oak medium-low against accepted standard pasturelands in the landscape increases 164 ORNIS HUNGARICA 12-13: 1-2 (2003) from NE to SW, being dominant in shrub cover than holm oak pastureland, Portuguese districts, nearly exclusively so although the differences were statistically coastally. In the 1999-2000 winter, the significant only in the first and second study area lacked a cork oak crop, and so parameters (t test, P<0,01). These struc- the Woodpigeon distribution shifted to tural features could play a role at a regions where holm oak pasturelands were detailed scale in the way Woodpigeons dominant, mainly in Spain. Crop levels select feeding areas, but there are no real measured in 1999 in dehesas and monta- data on that topic. Basal area and tree den- dos were confirmed by other independent sity (closely and positively related; r=0.8, evaluation schemes (C. Bernal pers comm, t=6.2, P<0.001) influence production, and Vázquez et al. 2000). shrub cover would favour predation pres- This close fit between the spatial dis- sure on Woodpigeons and an abundance of tribution of Woodpigeons and their poten- seed-eating rodents. tial trophic resources (a provisional As a whole, even though at a detailed hypothesis that will require data series scale the above-mentioned factors are from several years) is observed at wider, responsible for great variability between landscape scale. Locally, the actual food plots, the close fit between the patterns of availability is influenced by some distribution of potential food resources extremely variable factors (Borchert et al. and of the birds is circumstantial evidence 1989). Woodpigeon food shortages may that such resources are limiting environ- arise from an abundance of seed predators, mental factors (Newton 1998). In Great whether invertebrates (beetles and cater- Britain and Sweden, the effect of food pillars, [Vázquez 1998]) or vertebrates availability on Woodpigeon densities in (cattle, and to a lesser extent, rodents), and the post-breeding period repeatedly has their level of intake, which in turn is relat- been shown (Murton et al. 1964, Nilsson ed to the existence of alternative resources 1984, Inglis et al. 1990). In the Iberian for the seed predators (Pulido 1999). Peninsula, crop variability in holm oak In man-made ecosystems such as dehe- pasturelands would allow the maintenance sas and montados, management can deter- of body fitness and weight in wintering mine the productivity of the trees, by birds (Purroy et al. 1984) in spite of the external means (farming and forestry prac- greater reproductive synchronism of cork tices) and by exploitation of internal oak pasturelands. Both types of habitat (genetic) variations (Koenig et al. 1991), could be described as having almost com- thus determining fruit availability. For plementary roles, which probably allowed instance, the degree of shrub cover in the Iberian Woodpigeon winter population dehesas was positively related to rodent to evolve as a dynamically stable compo- abundance (Díaz et al. 1993). Moreover, nent of the species’ migration strategy. shrub regeneration depends on grazing We think it would be unreliable for pressure and the economic orientation of population trends to be drawn from popu- each individual dehesa (San Miguel lation figures from the present direct cen- 1999). sus and from the previous estimates In our samples, cork oak pasturelands obtained since the early 1980s (5-6 million showed greater basal area, tree density and wintering birds [Purroy & Rodero 1986]), A. Bea, R. Beitia and J. M. Fernández 165 because the respective methodologies The reproductive strategies of cork and were substantially different. Since 1981, holm oaks shows differing temporal pat- direct, season-long counts of migrating terns, the former having a sustained crop flocks of Woodpigeons have been made at for a longer period through the winter the Iraty pass in the French Pyrenees months (Cańellas et al. 1991, Cańellas (Région Cynégétique du Sud-Ouest 1994). 1993). Consequently, we suggest that the Trends show this migratory population to holm oak pasturelands crop would be be declining slightly, for reasons that are more likely to be consumed by seed preda- unclear; there may be a real decrease, or tors, especially when production is medi- the migration route may have changed. um or low, as has been observed in Nevertheless, the range of the winter- Fagaceae trees in temperate latitudes ing population has probably reduced, (Crawley 1992, Siscart et al. 1999) and because the presence of birds in southern particularly in dehesas of holm oaks dehesas was not verified in the 1997-2000 (Pulido 1999) and cork oaks (Herrera period, despite good food availability. 1995, Vázquez et al. 1997). Reproductive Twenty years previously, the use of this strategies and plant recruitment in these area was evident, at least in some winter species is generally interpreted under the months (Purroy et al. 1984). Fidelity to “satiation of consumers hypothesis” winter ranges could play a role in the reg- (Janzen, 1971), which is verifiable in ular occupancy of Tajo-Sado basin, highly-productive fruit seasons that between the Toledo and Setúbal. exceed the consumption capacity of crop There is some evidences that the start predators. of the return (spring) migration may Finally, the described fit between change between years, for in February Woodpigeon distribution and the species’ 2000 there were several observations and potential food resources, even with pre- reports of flocks migrating through the dicted capacity at meso-scale or regional central Spanish mountains. It would scale (Newton, 1998), would relegate to appear that these movements flout the secondary roles other factors sometimes hypothesis of dispersion of flocking birds, claimed to explain so-called spatio-tempo- and as yet there is no explanation. ral “irregular” wintering. Such factors Complex mechanisms, genetically fixed include shooting or the degree of protec- and of an hormonal nature, act to stimulate tion afforded by roosting sites against migratory behaviour in birds as proximate meteorological conditions (Purroy et al. factors (Berthold 1993). We note that the 1984), although these could have a local modulating or synchronizing effects of influence over habitat selection in relation some environmental conditions have been to feeding or roosting areas. proved, especially for species that have a Acknowledgements. Observers, without whom northerly breeding range, such as for this study would never have been possible, Common Crane Grus grus (Alonso et al. were mainly officials and wardens from the 1990). regional Spanish administrations of Andalucía, We suggest that food availability in the Castilla y León, Castilla-La Mancha, Extremadura, Álava, Bizkaia and Gipuzkoa, Woodpigeons Iberian wintering range from Patrimonio Nacional, from the influences the start of return migration. Portuguese Government department Direcçao 166 ORNIS HUNGARICA 12-13: 1-2 (2003)

Geral das Florestas and from shooting federa- Díaz, M., Asensio, B. & J. L. Tellería, 1996. Iberian tions in southwestern France. The regions of Birds. I. Non Passeriformes. – J. M. Reyero, Aquitaine, Midi-Pyrénées and País Vasco also Madrid. (In Spanish). have to be thanked. Funds were provided by Díaz, M., Campos, P. & F. Pulido. 1997. The Spanish dehesas: a diversity in land-use and wildlife. pp. Fondo de Cooperación Euskadi-Aquitania, 178-209. In: Pain, D. J. & M. W. Pienkowski. Union des Federations Departementales des (Eds). Farming and Birds in Europe. – Academic Chasseurs and Federación Espańola de Caza. Press, London. Erwin, M. 1982. Observer variability in estimating numbers: an experiment. – Journal of Field References Ornithology 53: 159-167. Gallego, J. 1985. Note on migratory behaviour of native Iberian Woodpigeons (Columba palum- Alonso, J. C., Alonso, J. A., Cantos, F. J. & L. M. bus). – Ardeola 32: 379-383. (In Spanish with Bautista 1990. Spring Crane Grus grus migration English abstract). through Gallocanta, Spain. I. Daily variations in Gilbert, G., Gibbons, D. W. & J. Evans. 1998. Bird migration volume. – Ardea 78: 365-378. Monitoring Methods. – Royal Society for the Bernis, F. 1966. Bird Migration. Theoretical and Protection of Birds, Sandy. Practical Manual. Sociedad Espańola de Gómez, F. 1997. Iberian Woodlands. A Ornitología, Madrid. (In Spanish). Geobothanical Interpretation. – Planeta, Bernis, F. 1967. Iberian Migratory Birds. Volume 5. Barcelona. (In Spanish). Sociedad Espańola de Ornitología. Madrid. (In Herrera, J. 1995. Acorn predation and seedling pro- Spanish). duction in a low-density population of cork oak Berthold, P. 1993. Bird Migration. A General Survey. (Quercus suber L.). – Forest Ecology and – Oxford University Press, Oxford. Management 76: 197-201. Bibby, C., Burgess, N. D. & D. A. Hill. 1993. Bird Inglis, I. R., Isaacson, A. J., Thearle, R. J. & N. J. Census Techniques. – Academic Press, London. Westwood. 1990. The effects of changing agri- Borchert, M. I., Davis, F. W., Michaelsen, J. & L. D. cultural practice upon Woodpigeons Columba Oyler. 1989. Interactions of factors affecting palumbus numbers. – Ibis 132: 262-272. seedling recruitment of blue oak (Quercus dou- Janzen, D. H. 1971. Seed predation by animals. – glasii) in California. – Ecology 70: 389-404. Annual Review of Ecology and Systematics 2: Cantos, F. J. & J. L. Tellería. 1985. Errors in count- 465-492. ing flocks of birds in flight. – Ardeola 32: 392- Koenig, W. D., Carmen, W. J., Stanback, M. T. & R. 400. (In Spanish with English abstract). L. Mumme. 1991. Determinants of acorn produc- Cańellas, I. 1993. Acorn production in cork oak- tivity among five species of oaks in Central lands. pp. 223-226. In: Simposio Mediterráneo Coastal California. pp. 136-142. In: Symposium sobre Regeneración del Monte Alcornocal (In on Oak Woodlands and Hardwood Rangeland Spanish). Management. USDA Forest Service. Davis. Cańellas, I., San Miguel, A. & V. Del Río. 1991. Montoya, J. M. 1989. Holm Oaks and Holm Evaluation of production in a “dehesa” in Oaklands. – Mundi-Prensa, Madrid. (In Extremadura: grass, acorn and timber. pp. 234- Spanish). 240. In: XXXI Reunión Científica de la Sociedad Murton, R. K., Westwood, N. J. & A. J. Isaacson. Espańola de Estudio de los Pastos. – Gobierno de 1964. A preliminary investigation of the factors la Región de Murcia, Murcia. (In Spanish). regulating population size in the Woodpigeon. – Crawley, M. J. 1992. Seed predators and plant popu- Ibis 106: 482-507. lation dynamics. pp. 157-191. In: Fenner, M. Nilsson, S. G. 1984. The relation between the beech (Ed.). Seeds: the Ecology of Regeneration in mast crop and the wintering of Brambling, Plant Communities. – CAB International, Fringilla montifringilla, and Woodpigeon, Wallingford. Columba palumbus, in South Sweden. – Vär Ceballos, L. & J. Ruiz de la Torre. 1971. Trees and Fägelvärld 43: 135-136. (In Swedish with Shrubs of Peninsular Spain. – Escuela Técnica English abstract). Superior de Ingenieros de Montes, Madrid. (In O’Connor, R. J. & M. Shrubb. 1986. Farming and Spanish). Birds. – Cambridge University Press, Díaz, M., González, E., Muńoz, R. & M. A. Naveso. Cambridge. 1993. Effects of food abundance and habitat Pulido, F. J. 1999. Herbivorism and Regeneration of structure on seed-eating rodents in Spain winter- Holm Oak (Quercus ilex L.) in Woods and ing in man-made habitats. – Z. Säugetierkunde “Dehesas”. – PhD thesis. Universidad de 58: 302-311. Extremadura, Cáceres. (In Spanish). A. Bea, R. Beitia and J. M. Fernández 167

Purroy, F. J. 1988. On the wintering of Woodpigeon Siscart, D., Diego, V. & F. Lloret. 1999. Acorn ecol- Columba palumbus in the Iberian Peninsula. pp. ogy. In: Roda, F., Gracia, C., Retana, J. & J. 137-151. In: Tellería, J. L. (Ed.). Invernada de Bellot. (Eds). The Ecology of Mediterranean Aves en la Península Ibérica. – Sociedad Evergreen Oak Forests. – Springer-Verlag, Espańola de Ornitología, Madrid. (In Spanish). Heidelberg. Purroy, F. J. & M. Rodero. 1986. Wintering of Wood Spellerberg, I. F. 1991. Monitoring Ecological Pigeons (Columba palumbus) in the Iberian Change. – Cambridge University Press, Peninsula. – Supplemento alla Ricerche di Cambridge. Biología della Selvaggina 10: 275-283. Tellería, J. L. 1986. Vertebrate Census Manual. – Purroy, F. J., Rodero, M. & L. Tomiałojc. 1984. The Raíces, Madrid. (In Spanish). ecology of woodpigeons Columba palumbus Vázquez, F. M. 1998. Seeds of Genus Quercus (L.). wintering on the Iberian Peninsula. – Acta Biology, Ecology and Management. – Junta de Ornithologica. 20: 111-146. Extremadura, Mérida. (In Spanish). Région Cynégétique du Sud-Ouest. 1994. Vázquez, F. M., Suárez, M. & E. Torres. 1997. Woodpigeon Passion. Woodpigeon Management. Limitations to natural regenerations of cork oak- – Deucalion, Biarritz. (In French). lands at Dońana National Park (Huelva, Spain). Saari, L. 1997. Woodpigeon. pp. 384-385. In: pp. 547-552. In: II Congreso Forestal Espańol, Hagemeijer, E. J. M. & M. J. Blair. (Eds). The (In Spanish). EBCC Atlas of European Breeding Birds. Their Vázquez, F. M., Doncel, E., Martín, D. & S. Ramos. Distribution and Abundance. – T. & A.D. Poyser, 2000. Estimation of the acorn crop in holm oak- London. lands in the province of Badajoz during 1999. – San Miguel, A. 1999. Cattle and natural regeneration Sólo Cerdo Ibérico 3. (In Spanish). of Mediterranean Quercus. – Congreso sobre Forestación en las Dehesas, Mérida. (In Spanish). 168 ORNIS HUNGARICA 12-13: 1-2 (2003)