land use and summer bird populations in northwestern galicia, spain

THE I B I S , JOURNAL OF THE BRITISH ORNITHOLOGISTS' UNION, VOL. 124, NO. I , 1982

LAND USE AND SUMMER BIRD POPULATIONS IN NORTHWESTERN GALICIA, SPAIN

SALVATORE F. BONGIORNO

Received 20 June 1980

The bird populations of Iberia have not been so well investigated as those of many other parts of western Europe. Studies of the interaction of land-use and avifauna in this region provide data which can be compared with those collected elsewhere in Europe and North America. Seven main land uses in northwestern Spain can be identified: heathlands, which occupy 27% of the area, farmland 26%, pine plantations 23%, eucalypt plantations 2%, deciduous woods 1-2y0, pasture 3% and roads, villages, urban areas, etc. 17%. This paper analyses the summer bird populations of the first five.

METHODS AND STUDY AREAS

The area in this study is located in northwestern Galicia within the province of La Corufia, near Cab0 Finisterre (42"52'N, 5"35'W) (Fig. 1). The distance separating the two farthest study sites is about 40 km.

The climate of littoral Galicia is temperate oceanic, with Mediterranean influence evidenced by mild, wet winters and cool, dry summers. The 30-year average January and July temperatures for the city of La Corufia are 9 Y C and 19"C, respectively. Annual precipitation varies from 800 mm at Cab0 Finisterre to 1100-1200 mm at Sierra de Outes and Cot0 de Muifio.

The bedrock underlying much of the province of La Coru5a is a mixture of schist and granitic gneiss which, together with the humid climate, produce more type soils low in Ca, Mg and K. This soil is characteristically granular with little aggregate structure, well drained with poor water retention capabilities in the inorganic horizons and with pH values between 4.5 and 5 or less (Muiioz Taboadela et al. I 966).

I selected ten sites for study: two heathlands; two pine plantations; four eucalypt stands of one, four, seven and 13 years of age; one climax deciduous wood; and one farmland site. Table 1 summarizes the characteristics of each site. The symbols used to identify the study sites in other tables and figures are: E l , E4, E7, El3 (one, four, seven, 13-year-old eucalypt stands); HS (heathland on Mt Seoane); HP (heathland on Peton Blanco); PS (pine stand on Mt Seoane); PP (pine stand on Peton Blanco); DO (deciduous wood near Sierra de Outes); FA (farmland).

VEGETATION S A M P L I N G

I recorded tree density, basal area and DBH (diameter breast high) of stems thicker than 2.5 cm using 10-m2 quadrats placed alternately right and left along transects, with a 10-m gap between successive quadrats. Table 2 summarizes this information.

Secondly, to render a more informative picture of the habitats, I recorded the per cent vegetation cover at 21 height intervals over 100 points spaced 4 m apart along several transects laid out at random throughout each site. The height (m) intervats selected were: ground, 0.25, 0.5,0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3, 3.5, 4, 4.5, 5, &8) 8-10, 1CL14, 1418, 18-22, 22-26. I used a calibrated pole for heights up to 8 m and

001 9- 10 19/82/01000l+ 2 1 $OZ.OO/O 0 1982 T h e British Ornithologists' Union

A VOL. 124

2 S. F. BONGIORNO IBIS 124

estimated by eye above that. I constructed vegetation profiles (Figs 2 and 3 modified after Karr 1971) for each site by determining the percentage of points of each plant species at each height interval.

Lastly, to describe the farmland study area, I mapped the entire site over five days using a compass and metre tape. Each plot, land, hedgerow, thicket, etc. was

-43-

S i e r r a

F I n i s t e r r e

t Fic;i'iw I . >Lip of Iberia showing the Galician country, the province of La Coruiia, the Cabo Finistcrrc

region and the location of the study areas Sardiiieiro, SIt Seoane, Peton Blanco, Cot0 de Muiiio and Sierra de Outes.

1982 BIRDS OF GALICIA 3

TABLE I

Characteristics of study sites in northwestern Galicia

Number Elevation Ha of Shape of

Habitat (4 censused censuses study site

Farmland Sardikiro

Heathland Mt Seoane

Heathland Pet& Blanco

1 -year-eucalypt Cot0 de Mu50

4-year-eucalypt Cot0 de Muifio

7-year-eucalypt Cot0 de Muiho

13-year-eucalypt Cot0 de Muiiio

Maritime pine Mt Seoane

Maritime Pine Pet6n Blanco

Deciduous woods Sierra de Outes

20-50

247

200

300

300

300

300

90

90

130-1 80

18.4

5.9

5.3

4.7

5

5

5

2.5

2

2.9

11'

14'

123

105

I 0 7

I 09

64

116

lo8

131°

irregular polygon

Irregular polygon

Irregular polygon

215m x 220m

1 k m x 5 0 m

1 km x 50m

1 km x 50m

irregular polygon

200m x 100m

Irregular polygon

Notes: June-July 1976. June-July 1973. 'July 1972, June-July 1973. ' June-July 1973.

July 1972, June-July 1973. 'July 1972, June 1973. June- July 1973. July 1972, June 1973. July 1973. l o June-July 1974.

recorded on a scale map and notes were kept on the physiognomy of wooded cover and type of cultivation.

Deciduous wood

Originally, oak Quercus robur covered most of Galicia as the climax vegetation extending from northern Iberia to Ireland and England and east to central Europe (Bellot Rodriguez 1966). Today, climax woodlands larger than I ha are rare in the province of La Coruiia because, as in Britain (Moore & Hooper 1975) and throughout much of western Europe, the deciduous forest gave way to agriculture and pasture, becoming reduced to tiny enclaves of vegetation isolated on steep slopes or generally inaccessible sites (Bellot Rodriguez 1966). The largest wood that I could locate (after considerable effort) is 6 km north of the Ria de Muros y Noya and about 22 km from the Atlantic coast in the Sierra de Outes (Fig. 1 ) on terrain that exceeded a slope of 20" in places. Oaks, some of centenary proportions, form the canopy between 9-20 m which also includes some chestnut Castanea sativa (Fig. 3c). Oak and chestnut, together with blackthorn Prunus spinosa and birch Betula vermcosa, make up the subcanopy between 5 and 9 m. Sweetbay Larus nobilis, holly Ilex aquifolium, hazel Corylus avellana, wild pear Pyrus communis, broom Sarothamnus scoparius, young oaks and chestnuts compose the 1-5 m shrub layer. Butcher's broom Huscus aculeatus 0.5-0.75 m is abundant and ivy Heaera helix covers more than 80% of the forest floor as well as a major part of oak trunks up to 2-3 m from the ground. Villagers own the wood and use it for occasional timber, to gather firewood and chestnuts.

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Heathlands Land exploitation in Galicia included burning to favour herbaceous cover for

pastoral purposes (Castroviejo 1973). As forest cover was destroyed, leaching of mineral nutrients in the humid climate resulted in impoverishment of the soil, producing the ‘disclimax’ (Odum 1971) of the Atlantic European heathlands which now cover vast areas (Heinrich 1973). One site is located on Mt Seoane (Fig. 1) about 1 km west of the village of Sardiiieiro (102 km on the main road from the city of La Coruiia to Finisterre) while the second site is on Pet& Blanco 1 km northeast of the village. The species composition of the vegetation for both heathlands is nearly the same with extensive patches of gorse Ulex europaeus interspersed with lesser amounts of heather Calluna vulgaris. Also present are the heaths Erica cinerea, E. ciliaris, E. umbelata and Dabeocia polifolia. Rockrose Helianthemum alyssoides grew sparsely on bare ground. Except for a few scattered maritime pines Pinus pinaster (Table 2), both heathlands present an aspect of open country (Fig. 3). Farmers periodically cut the heath vegetation every 4-8 years, allowing it to dry before collecting it as bedding for domestic animals. In areas not cleared, summer fires are common. Thus, the heathlands in the region are a patchwork of early stages of growth which rarely exceed ten years of age.

Pine and eucalypt stands Since the late 1930s extensive reforestation of the heathlands throughout much of

Galicia has occurred, especially in the province of La Coruiia where abundant rain and frost-free winters facilitate the cultivation of maritime pine. This species has become particularly widespread in the province (Bellot Rodriguez 1966) since the publication of Merino’s (1909) classical study of the flora of Galicia.

I selected two separate areas about 1.5 km apart, one on the east facing slope of Mt Seoane, the other on the west facing slope of Peton Blanco. Both sites are within a monoculture of maritime pine composed of scores of villagers’ private tracts that measure from 0.5 to about 3-5 ha usually with 1 m high stone walls separating them. The woods extend over various km2 as a single swath interrupted only by stone walls, footpaths and lanes. Both pine stands have an open aspect (Fig. 3a, b and Plate la) since farmers remove the dead lower branches for fuel, as well as assiduously clear away the bracken Pteris aquilina and gorse for use in stables and to prevent the build- up of flammable litter.

More recently, vast monocultures of eucalypt or gum tree Eucalyptus globulus, indigenous to the Australian region, are being planted. North of the Rio Xallas about 25 km inland from the coast (Fig. 1) is the Cot0 de Muiiio, a large corporate owned eucalypt plantation which extends over 8 km2. Trees are planted 2 m apart. This species grows exceptionally rapidly in the marine climate. Seedlings reach 2 m height in one year and trees may attain 22-26 m in 13-1 5 years. Clear-cutting is followed by burning of slash and shrubs. Within a year of logging and burning, the cut stumps regenerate between 5-20, 1-2 m long sprouts (Plate lb) which are all pruned except one that then becomes the leader developing the next tree crop (Plate lc). I selected sites in one, four, and seven-year-old regenerated stands and in a 13-year-old stand that had not yet been harvested since planting (in the background of Plate 1 b). Tree heath E. arborea and broom mingle with gorse to form the bulk of the shrubs. Bracken, E. cinerea and D. polifolia form low ground cover with bramble Rubus spp forming dense clumps in clearings. I found that the shrub cover was very similar in height and density in the seven and 13-year-old stands despite the difference in stand ages (Figs 2e and 3d). A herd of 50-60 horses, reared principally to graze on the gorse, roams over most of the plantation. The gorse would otherwise grow into


impenetrable thickets, since local farmers make little use of the shrubs on the plantation.

Bellot Rodriguez (1966) pointed out that eucalypt plantations (and pine) are essentially heathlands with a canopy of trees overhead, but he found that the eucalypts reduced the number of species of ground cover from the 22 normally found in the region to six or seven species.

Table 2 illustrates the uniformity in DBH (and tree ages) among the eucalypt stands. By comparison, the pines show somewhat more variation in DBH than the eucalypts because small-scale, selective harvesting is practiced in the pine tracts according to an individual farmer's needs, while large-scale, clear-cutting is done in the industrial exploitation of eucalypt plantations. Although the average DBH of trees in the deciduous wood is substantially greater than in the eucalypts and

2

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Eucalypt

Gorse and Heather

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0 20 40 60 80 100

Vegetation cover ( O h )

F J ( ; c HI,; 2. Vegrtation profiles of (a) heathland, M t Seuanr-HS, (b) heathland, Peton Blanco-HP, (c) one->-ear-old regenerated, eucalypt stage-E I , (d) four-year-old regenerated eucalypt stand-E4, ( e ) seven-vear-old regenerated eucalypt stand -1<7.

PLATE 1. (a) Maritime Pine Pinus pitzaster plantation on west slope of Pet6n Blanco. (b) One-year-old regenerated eucalypt Eucalyptus globulus plantation, about 13-14 months after cutting and burning, Coto de Muifio. In the background is the 13-year-old stand. (c) Four-year-old regenerated eucalypt stand. A black and white section of the range rod equals 1 m.


somewhat greater than in the pine stands, the latter two evergreens have a total basal area (m2/ha) which may exceed that of the deciduous wood (Le., the 13-year-old eucalypt stand has a basal area of 33.5 m2/ha while the deciduous wood has 32.2 m2/ha). In effect, current silvicultural methods practiced on eucalypt and pine try to maximize net production in the shortest period of time, with eucalypts routinely

0 20 40 60 80 100

22

18

14

10

6 Gorse

E 13

. . Grass I,?:.. -

Bracken

I

0 20 40 60 80 100

Vegetation cover (O/J

FIGURE 3. Vegetation profiles of (a) pine plantation on east slope of Mt Seoane-PS, (b) pine plantation of west slope of Peton Blanco-PP, (c) deciduous woods, Sierra de Outes-DO, (d) 13-year-old eucalypt stand-E13. The category 'others' in the deciduous woods includes Prunus, Betula, Laurus and Ilex.


clear-cut for pulp wood after 13-16 years of growth and pines harvested for lumber after approximately 2 5 4 0 years.

Farmland Small-scale farming characterizes all of Galicia. Relatively high population

densities (135 persons/km2) occupy coastal regions, and Mensua (1966) considered Galicia to have an over-populated rural area, since 45", of the labour force engage in agriculture, silviculture or ranching. By comparison, 24'1, of the labour force in Spain as a whole and only 5 " , of the U.S. labour force engage in such activity. The large rural population results not only in extensive use of all arable lands, but also in an intensive subdivision such that the average sized farm plot in Galicia is only 0.28 ha, and the average total amount of land per farmer is 4.3 ha with less than 0.3:; of all farms exceeding 50 ha (Consejo Economico Sindical del Noroeste 1968). An average farmer owns about 16 plots, most of which may be scattered and bordered by a neighbour's plots (Paz Andrade 1959). This subdivision creates numerous peri- meters between plots that are often marked by hedgerows and vegetation covered stone walls, which produce 'edges' for birds (Chapman 1939, Moore, Hooper and navies 1967).

The farmland stud?; site is located within approximately 50 ha of terraced farms that occupy a gently sloping valley between Mt Seoane and Peton Blanco just north of the village of Sardiiieiro (Fig. 1). One barn was located within the perimeter of the study tract. The tract contains 138 farm plots whose average size is 0.13 ha exhibiting the typical regional pattern of numerous small land holdings. Several varieties of field crops, hay and meadows occupy about 86'" of the area (15.8 ha), while fallow fields occupy 6",, ( 1 . 1 ha) . Farmers ser\Tice their individual plots from a labyrinth of lanes designed to accommodate ox carts or small tractors. The lanes, whose average width is 2.2 m, have a combined length of 2.8 km (1.5 km of lane length/lO ha of farmland) and they occupy 4"" (0.7 ha) of the study site.

Bramble, oak, blackthorn, sweetbay, white willow Salix alba and crack willow S. fragilis are the predominant plants in hedgerows. The average hedgerow measures appro xi mat el!^ 10.7 m high and 1.2 m wide, while the combined length of all hedgerows in the farm plot is 3.4 km (i.e., 1.8 km of hedgerow per 10 ha offarmland); all the hedgerows in the study site occupy approximately 1 .9()., (0.4 ha) of the area. 'Together with the hedgerows, scattered wooded thickets and cover alongside field ditches and stream occupy a combined total of 4",, of the study tract.

H I K I ) CEXVSL-SES

I was unfortunately obliged to use study plots smaller than preferred (Robbins 1970) because I restricted myself to areas with as uniform a physiognomy as possible, and which had a minimum of 25 m of the same habitat around each site to act as a buffer strip in reducing the effect of ecotones. The number of hectares censused, the number of bird censuses, census dates and the shape of the study tracts are summarized in Table 1. I conducted all censuses between 0730-1 100 h on days when neither fog, rain, nor strong winds prevailed. I used the strip census method (Haapanen 1965) in the four, seven and 13-year-old eucalypt stands because the first area was long and narrow, while the latter two areas contained so much heavy gorse that I X V H S confined to patrolling established footpaths and fire breaks. I counted birds situated within 25 m of each side of the path I followed. All the other habitats werr censused by traversing the study areas and recording bird observations on prepared maps using the spot-map method of Williams (1936). All contacts with birds, Lvhether visual or auditory, were counted as a single bird except where a territorial male \vas involved, and then the contact counted as a pair. Since no


attempt was made to locate nests, I only happened to find one active nest during the study belonging to a pair of Mistle Thrushes (Turdus viscivorus) in the 13-year-old eucalypt site. I omitted birds of prey, swallows, swifts, ravens and crows from the censuses.

Bird species diversity was calculated using the Shannon-Wiener index ( H = - Z p i log, p i ) . An index of affinity between avian communities was calculated using 100 x 2C/(A+B) where A is the number of species in community (a), B is the number of species in community (b), and C is the number of species in common to both communities (Margalef 1974: 405; after Czechanovski).

RESULTS

Breeding bird densities, species composition and diversity indices for the ten study areas are given in Table 3. A total of 37 breeding bird species was recorded. One species not reported in any census, but common in the region, is the Magpie Pica pica.

NUMBER OF SPECIES

The farmland site had 24 species of birds, which is greater than the number found in any other habitat (Table 3, Fig. 4). The farmland site differed significantly in this respect from the one, four and 13-year-old eucalypt stands as well as the pine woods at Peton Blanco, all of which had 12 or less species ( P < 0.05 in x2 tests). It did not, however, differ significantly from the other study tracts, which supported 13 or more species. The one and four-year-old eucalypt stands had significantly fewer species than any of the other habitats (0.005 < P < 0.05 in x2 tests), although they did not differ from each other. The deciduous woods, the two heathlands, the two pine stands and the later aged eucalypt stands had between 12 and 17 species present, and in this respect were not significantly different.

BIRD SPECIES DIVERSITY AND PERCENTAGE VEGETATION COVER

Bird species diversity (BSD) is highest in the farmland site (2.62) and deciduous wood (2.52), and lowest in the one and four-year-old eucalypt stages (0.84 and 1.0 respectively, Table 3). Mean BSD (1.916) for both heathlands, both pine stands and the deciduous wood is significantly higher than the mean BSD (1.213) for the four eucalypt stands (0.02 < P < 0.05, in a two-tailed Independent t-test), even though percent vegetation cover (PCVC, data taken from Figs 2 and 3) is essentially similar for both groupings of habitats (Fig. 5).

25L 0 FA

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15 PS H Z * E7

pp' * El3 z

E 4 * - E l *

1 1 1 1 1 I I I I l 20 40 60 80 100 0

Individuals / 10 ha

FIGURE 4. Avian species-density relation of census plots in northwestern Galicia. See text for explanation of symbols.

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4

- -

12

0

8

08

12

-

-

-

-

__ -

__

-

28

14

44

8 -1

.521

2.

125

2,14

0

_.

42

07

2

4

20 2

6.9

vi

-

4.2

2.1

- 11

h)

92.7

P

2.

518

1982

2 2 -

18- D v) m

I 4 -

10-

BIRDS OF GALICIA - /

El?/’ t4

11

!.+ DO

I I I 5 10 35

0 6 L 2

PCVC

FIGURE 5. Bird species diversity (BSD) in northwestern Galicia against percent vegetation cover (PCVC) plotted on a log scale. The upper line is for the two heathlands, two pine stands and one deciduous woodland while the lower line is for the four eucalypt stands. The equation for the upper regression line is Y = 0.637+ 1.193Xwhile the lower regression line is Y = 0.611 +O,515X.

DENSITY

On the basis of total density, the study areas separate into four groups (Fig. 4). (1) The deciduous woods with 93 individuals per 10 ha has a significantly higher avian density than the next community with 60 birds per 10 ha ( P < 0-005 in x 2 test). (2) The bird densities on the farmland, seven and 13-year-old eucalypt stands, the two pine stands, and the heathland on Pet6n Blanco are not statistically different from each other ( P > 0.05 in x2 tests). (3) The one and four-year-old eucalypt stands with seven and 13 individuals per 10 ha respectively do not differ between themselves, but each has significantly lower densities than any other habitat (0.005 < P < 0.05 in x 2 tests). (4) The heathland on Mt Seoane with 26 individuals per 10 ha stands alone in terms of avian density, supporting more birds than either the one or four-year-old eucalypt stands, but fewer birds than the next habitat, which is the pine stand on Mt Seoane ( P < 0.05 in x2 test).

The two heathlands differ by only one in total numbers of species (1 3 v. 14) and they share proportionately more species in common than do any other two study areas, giving them the highest coefficient of affinity (Table 4). Despite these similarities, the heathland on Pet6n Blanco has twice the density of birds as the heathland on Mt Seoane (53 v, 26 birds per 10 ha, respectively) ( P < 0.005 in x2 test). This increased density is attributable to a 14-fold increase of Linnets Acanthis cannabina, a five-fold increase of wrens Troglodytes troglodytes and a three-fold increase of Dartford Warblers Sylvia undata, despite a concurrent 14-fold decrease in Skylarks Alaudu arvensis (Table 3).

The Pet6n Blanco heathland has denser, higher shrubs and less open ground than the Mt Seoane heathland because the latter had been cut more recently (Fig. 2a,b). The severe reduction of skylarks on the former tract is not unexpected, given the species’ preference for open habitats and bare ground (Venables 1939, Lack & Lack 1951). Apparently, at the same time, the denser shrubs contributed to the notable increase in wrens and Dartford Warblers which are partial to heavy cover near the ground (Purroy 1975, 1977, Cody & Walter 1976). This matches my observations of male Dartford Warblers flitting about the highest and thickest patches of heather- gorse. Figure 6 shows that on either tract, male Dartford Warblers during territorial

12 IBIS 124 S. F. HONGIORNO

I I

Mt Seocne Pet& Bioncc

at 275 sampling points (see methods) on both heathland study sites (clear figures). Height of heather-gorse from which male Dartford \Varblers .7),/?,in mdnta nerc seen and heard in te r r i to rd display (solid figure). Horizontal line indicates means, vertical linrs are standard deviations on either side o f mean and box W',, confdcncc. int tvxls . Bird sightings are 1 1 and 34 f o r X l t Seoane and Pet6n Ulanco respvctively

1 ~ [ ( ; ~ ~ 1 ~ t . . 6 . ' rop of heather-gorse shrubs in north\vestern Galicia measures

display, select higher branches of heather-gorse than the mean height of the highest shrub tip (Mt Seoane: t = 7 - 5 , P < 0.001; Peton Blanco: t = 7.4, P < 0.001 in two- tailed independent t-tests). In summary, the difference between the two heathlands is one of density. As the vegetation matures, it apparently affords few opportunities to new species as pointed out by Bas Lopez et al. (1978), but does make available to those species present in the earlier stages more resources (the exception is the skylark) providing for an increase in their density (Cody & Walter 1976).

Figure 7 suggests that the density of hole-nesting species diminishes where DHH

,

/* i,*/* /.

0 2 2 5 1

+ ' 5 m

E 7 E l 3 PS PP DO Youngest Oldest

Study sites < *

I.'i(;\ RI-. 7. Summed denslty per 10 ha for seven hole-nesting species: Short-toed Tree Creeper ('erthio hrnc-hyductj,la, Great T i t Przrus nmjor, Coal ' rit P. nter, Blue Ti t P. cnrrulus, Crested 'l'it P. rrtctatus, Cheat Spotted LVoodpecker Drvdroropus major and Green Woodpecker Picus viridus on t i \-e study sites with trees. From basal area measurements (see Table 2) the relative age of cach site is: seven-year-old eucallpt (E7) < 13-year-old eucalypt (E13) < pine stand-Mt Seoane (PS) < pine stand-I'eton Rlanco (PI') < deciduous wood-Sierra de Outes (DO). Selection of hole-nesting species taken from Iack & \'enables (1939), Haapanen (IY65), Edington & Edington (1972), Purroy (1977).


TABLE 4

Index of affinity between avian communities. For explanation see text

Studysites HS HP El E4 E7 El3 PS PP D O FA - -

HS -

HP 82 - El 47 44 - E4 53 40 20 - E7 46 44 12 53 - El3 32 46 13 44 72 - PS 52 50 1 1 60 74 74 - PP 32 46 0 33 56 61 62 - DO 33 45 0 35 60 70 65 76 ~

FA 43 37 21 33 54 50 63 39 49 ~

is smaller (and trees younger). This is most likely explained by the absence of holes which depend upon the presence of older trees (HildCn 1965, Edington & Edington 1972). It appears that woodland management of both eucalypt and pine removes trees before they have aged enough to favour hole-nesting birds.

RELATIVE DENSITY

Relative density of individual species varied markedly in the ten study sites. Figure 8 (after Emlen 1972) shows the dominant species in each study area. A species is considered dominant when its density in a habitat is 5% or greater of the total avian density (Palmgren 1930). These dominants divide into five groups.

Eurytopic The wren was the most ubiquitous species occurring in nine of ten study areas. It

was the most dominant in five of ten sites and a strong dominant in three others. No other species came close to occupying so many habitats and in such densities as the wren.

Farmland wooded cover and woodlands The Robin Erithacus rubecula and Chaffinch Fringilla coelebs were not only

dominant in the woods proper (deciduous woods or pine stands or older eucalypt stands), but also in the farmland wooded cover.

Woodlands only The Coal Tit Parus ater was a dominant in all woodland habitats (deciduous, pine,

13-year-old eucalypt). The Blackcap Sylvia atricapilla appears as a dominant in two wooded areas (deciduous and 13-year-old eucalypt), while both the Great Tit Parus major and Crested Tit P. cristatus are relegated to one woodland tract each as dominants. Woodland species associated exclusively with only one habitat include the Long-tailed Tit Aegithalos caudatus and Firecrest Regulus ignicapillus in deciduous wood, and Mistle Thrush Turdus viscivorus in the 13-year-old eucalypt stand. This last species’ dominance may be an exaggeration, since this was the only nest I found and I ‘expected’ the pair each time I censused the tract.

Open habitats As noted earlier, the skylark dominated the more open heathland on Mt Seoane as

well as the one-year-old eucalypt stand. Here also appear the Stonechat Saxicola torquata and Linnet. The former is also among the dominants of the open farmlands while the latter appears in the heathland on Peton Blanco. The Dartford Warbler’s

14 S. F. RONGIORNO IBIS 124

exclusive association with the heathlands suggests that this species is quite stenotypic. Bas Lopez et al. (1978) found that it represented 5 2 O / , of the community in a heathland in eastern Galicia and they as well as Purroy (1977) found it confined to matorral (shrublands).

Farmland Species dominant in the farmland site only include, in order of decreasing density:

blackbird Turdus merula, Cirl Bunting Emberiza cirultts, Tree Sparrow Passer montanus, Dunnock Prunelln modularis and Greenfinch Carduelis chloris. Several others, none of which are dominant, are also associated exclusively with the farmland site: Chiffchaff Phylloscopus collybita, Serin Serinus serinus, Goldfinch Carduelis carduelis, House Sparrow Passer domesticus and White Wagtail Motacilla alba (Table 3 ) . Whereas the first three of this group were found throughout the entire study tract, the last two were confined to the locale of the barn in the study area (81 ')A of sightings of House Sparrows and all sightings of White Wagtails were within a 100 m radius of the barn).

In summary, percentage representation of the dominant avian species in the study sites show (Fig. 8) the farmland with a 14O,, dominance by its most dominant member, followed by the deciduous wood with 220" dominance, then both pine tracts a t 24 and 34", , both heathlands at 44 and 47:/0, and finally all eucalypt stages, which cxhibit between 50--75',, dominance by one species. Figure 8 also shows that the most dominant avian species of each study site equals or exceeds the 2.536 level of population density (after Weins & Nussbaum 1975) in eight of ten study areas; the two exceptions are the deciduous wood and the farmland site. That this phenomenon occurs in the one-pear-old eucalypt stage is automatic, since there are only four species present.

DISCUSSION

How do the ai.ian populations of this study compare with others from Europe, North America or elsewhere? To answer this question I selected a sampling of avian populations from boreal, temperate and llediterranean-like habitats, some of which were man-influenced, and I assigned them on the basis o f vegetation height as avian

E l HS HP E 4 E 7 E l 3 P S P P DO FA 1'ii iii X . I'crccntagc rcprcscntatilm iif nll diJminant epecics in t m study sitcs in northbvestern Galicia

{af te r Ilmlen 1972; fig. 5 ) Thr broken 1iiirizi)ntal line indicates the 25",, level of dominance.


communities of shrublands ( < 9 m) or woodlands ( > 9 m). It is evident (Fig. 9) that the shrubland communities exhibit a per cent dominance by one species equal to or greater than the 25% level of dominance in 59% of the cases (17 out of a sample of 29 studies, not including this study). By contrast, the woodland communities exhibit a per cent dominance by one species equal to or greater than the 25% level in less than 20% of the studies (ten out of a sample of 52, not including this study). Both Figure 9 and the species diversity indices (Table 3) suggest the following about the shrubland and woodland avifauna of this study:

6)

(ii)

(iii)

A

Silvicultural practices with monocultures of eucalypt tend to be accompanied by excessive dominance by one avian species. Whether as shrublands or woodlands, eucalypt stands are populated by a very dominant species with lower than ‘normal’ levels of community diversity. Management of pine stands and heathlands has tended to produce more equitable avian communities than eucalypt monocultures, but these habitats nonetheless show rather high (i.e., above 25%) levels of dominance by one species. The avian community of the deciduous wood, although used on a continuing basis by man, lies well within the expected norms for this habitat, based upon the other studies of mostly natural habitats.

second and related approach is to contrast this study with Udvardy’s (1957) ._ - . . ,

comprehensive analysis. He compared avian communities from various habitats in North America, ‘where substantial man-made alterations did not influence the bird populations’. With the exception of the deciduous wood (I omitted the farmland site from this comparison), Table 5 suggests that the man-managed shrublands and woodlands of this study all tend to have not only fewer species present, but also lower population densities than the ‘natural’ habitats summarized by Udvardy (1957).

Silviculture may increase or decrease both species richness as well as avian density depending on practices (Kilgore 1971, Recher, Thomas & Milledge 1971). This is understandable, due to the influence of the structural quality of the habitat on avian communities (Shugart &James 1973, Anderson & Shugart 1974, and others).

Specifically in the case of the eucalypt plantation, trees crowd so closely that lateral

TABLE 5

Tabulation of breeding bird censuses from North America (after Udvardy 1957) and northwestern Galicia in brush and scrub, coniferous woods, and oak dominated woodlands

Habitats studied n

Brush and scrub

Coniferous woods

Oak dominated woods4

19 4

56 43

30 1

Total species n

11-31 6 1 4

18-28’ 12-14

17 11-36

Birds/ 10 ha

49-146 IJdvardv 1957 7-53

73-244’ 45-58 35-185

93

This st idy Udvardy 1957 This study Udvardv 1957 This study

Notes: These numbers were most common, but young and uniform stands reported 7-13 species. ’ Udvardy (1957) reported: ‘the densities are mainly low (i.e., under 73 aves/lO ha) but, especially in the Northeastern area censuses, moderate and high densities occur’ (i.e., up to 244 aves/ 10 ha).

Includes both pine as well as the seven and 13-year-old eucalypt stands. Figure 2 from Udvardy (1957) shows 30 censuses for oak-dominated forests, 34 in beech-maple,

52 in mixed deciduous, and 20 for mixed deciduous on flood-plains. I selected the 30 censuses in ‘oak-dominated forests’ since these seemed to correspond most closely to the situation in the oak wood of this study.

16

60-

40-

20

0-

S. F. BONGIORNO

Woodlands *El3

c

- - - ~ ^PS 0

r PP

'* - - - A +-DrcL - - - - 7

0 . 0' - 0 . . 0 -

' ~ ;Gcc-- .A.

0 c - ? - 0- : - * b O

0 0 - 0

I I I I I I I I I

IBIS 124

Shrublands

60

- -

BIRDS O F GALICIA 17 1982

branches scarcely develop; those that do are wispy and unsuitable for nesting birds. Crotches for anchoring nests are uncommon in the mast-like trees. Per cent of vegetation cover (PCVC) as a measure of habitat distinctiveness would not reveal these kinds of differences between the eucalypt and other wooded tracts in this study. Unanswered questions raised by this study are: is the close planting of eucalypts a major factor interfering with nesting, feeding patterns (Holmes, Bonney & Pacala 1979) or both? Do eucalypt plantations depress avifaunal densities and variety as much as they appear to do because this tree is exotic to Europe and thereby produces fruits left unexploited by native birds? Or do eucalypts, by virtue of their eucalyptol, alter food webs or food levels for the insectivorous birds by affecting litter invertebrates? What effects will continued clear-cutting every 15 years have on soil nutrient levels and the subsequent long-term production of avian food items? Since the present trend toward more monocultures of eucalypt will undoubtedly persist in northwestern Galicia, it is well to anticipate the possible consequences of this activity on the avifauna. Current ecological perspective questions the wisdom of the accelerating world trend toward monocultures (Odum 1971, Rappaport 1971, Margalef 1974), mainly because of the impoverishing effect of less diverse ecosystems on the biotic resources of the earth (Woodwell 1970). The condition of impoverished avian communities, which reflects the high level of exploitation and chronic youth of the Galician eucalypt experiment, may not be easily resolved, given the presumed economic benefit derived from the present system of silviculture.

On the other hand, traditional, small-scale, family farming which fragments the countryside into numerous tiny parcels that are worked mainly by animal and human labour produces an environmental patchiness which, although it does not support as high a density of birds as the deciduous wood, serves to maintain a relatively rich avian community in terms of species. These results seem in general agreement with the finding that the ‘edge effect’ increases numbers of avian species (Johnston & Odum 1956, Hogstad 1968) but that farm hedgerow may be suboptimal in terms of reproductive success (Williamson 1969, Krebs 1971, Gates & Gysel 1978). There is little literature concerning the breeding birds on farmlands. An ecosystem of mixed farmland plus hedgerow (e.g., Chapman 1939, Snow 1965) this study: Table 6, would seem, on the basis of these few analyses, to confer greater possibilities for avian communities than monocultures in agriculture (Tomek 1973, Wiens 1973, Wiens & Dyer 1975) this study: Table 6 . Wiens & Dyer (1975) found, in comparing the effects of agriculture on avian communities in North America, that avian breeding densities and species diversity were lowest in the more monocultured crop types (alfalfa, corn, wheat) which tended to be planted in ‘vast unbroken fields’, than in those cultivations which roughly approximated the native prairies (fallow fields, hayfields, pastures).

In conclusion, it is tempting to let avian dominance serve as a possible biological index of habitat perturbation in the sense of Bakelaar & Odum (1978). For these authors, ‘stress’ or negative effects are evidenced in shifts from equitable representation by many species to strong dominance of one or a few species, from specialized species to the generalists (Woodwell 1970), and from richer species diversity to a reduced diversity (Odum 1969). In this study, land use in northwestern Galicia produces a spectrum of effects with respect to the avifauna. Although it is a system completely dominated by man, it is capable of producing both ‘bad’ and ‘good’ effects on the avifauna which Bakelaar & Odum (1978) described as the ‘value judgements that man places on the environment’.

I thank Dr Jesus Garcia Devesa for assistance with plant identification, Don Jose Garcia Fernandez for helping solve transportation problems, Dr Ronald Salafia for suggestions about statistical procedures, Dr

B

18 S. F BONGIORNO IBIS 124

vi P

1982 BIRDS O F GALICIA 19

Juan Lopez Bermudez for making available his extensive personal library on Galicia, Dr Jake Rice and Michelle Buonocore for their helpful, critical reviews of an earlier draft of this manuscript. I benefitted much from the suggestions and comments of Santiago Bas Ldpez, August0 de Castro Lorenzo, Josi. Guitian Rivera, and Jost Luis Sanchez Canals of the Grupo Ornitoldgico Galego at the Unversidad de Santiago, Santiago de Compostela. I am grateful to them also for permission to use their unpublished data. A grant from the Frank M. Chapman Memorial Fund of the American Museum of Natural History, N.Y. and the Fairfield University Research Council supported part of this research. Finally, I am indebted to the farmers of Galicia, whose experience with the land is a rich, continuous source of knowledge and to Consuelo who was first to show me her verdant ‘ t e r m nai’.

SUMMARY Summer bird populations were studied in ten sites : two heathlands, two maritime pine Pinus pinaster

plantations, one, four, seven and 13-year-old eucalypt Eucalyptus globulus plantations, one climax oak Quercus robur woodland, and a farmland area in northwestern Galicia, Spain. The farmland and oak wood contained the greatest number of species (24 and 17 respectively) while the one and four-year-old eucalypt stands contained the fewest species (four and six respectively). Avian density was highest in the oak wood (93 birds/ 10 ha) and lowest in a heathland site, the four and one-year-old eucalypt stands (26,13, and seven bird/lO ha respectively). The wren was the most widespread and dominant species occurring in all study areas except the one-year-old eucalypt stand. Mean bird species diversity for the four eucalypt stands was significantly lower than mean bird species diversity for the other wooded habitats. The avian communities of the heathlands, maritime pine and eucalypt plantations, in contrast to the oak wood and farmland, show an excessively high dominance of one species. Using avian dominance as a biological index of habitat perturbation suggests that over half the landscape (4000 km2) in northwestern Galicia is suffering from chronic stress. The newest stress to the avifauna in a long history of land exploitation by man is the importation and cultivation on grand scale of eucalypts. Balancing this tendency, traditional, small-scale farming on the mountainous terrain leads to the development of many tiny parcels whose average size is 0.28 ha; this in turn creates much ‘edge’ in hedgerow which serves to maintain a relatively rich avian species diversity in farmland.

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orest biotopes. Ekologia Polska-Seria A 15: 64-695.

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of chaparral and pine forest bird communities on Santa Cruz Yt-a.t’i)\, K. I . 1974. r\n ecological anal-

Biology Department, Fairfield Unicersity, Fairfield, Connecticut, 06430, U.S.A.

land use and summer bird populations in northwestern galicia, spain

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