Succession of Breeding Bird Communities After the Abandonment of Agricultural Fields in South-East Portugal

Ardeola 47(2), 2000, 171-181

SUCCESSION OF BREEDING BIRD COMMUNITIES AFTER THE ABANDONMENT OF AGRICULTURAL FIELDS IN SOUTH-EAST PORTUGAL

Carlos Pedro SANTOS*

SUMMARY.—Succession of breeding bird communities after the abandonment of agricultural fields in south-east Portugal. A study on the bird communities associated with a succession of vegetation in Baixo Alentejo, Portugal, was carried out. Five stages of vegetation were identified: fallow, fallow with shrubs, low shrubs, tall shrubs and mixed shrubs. The abandonment of the agricultural fields started this succession, which were then occupied by two species of the rock-rose family (Cistaceae), Cistus ladanifer and Cistus monspeliensis. The density of passerines increased with succession. In fallows, the most common species belonged to the family Alaudidae, while Sylviidae were the most frequent in the shrub stages. Generally, the numbers of passerine species and the total number of species increased with successional stage while the number of non- passerine species decreased. Migratory and granivorous/insectivorous passerines were the most abundant in sparse fallow but were replaced by resident and insectivorous species in later stages. The proportion of passerines feeding on the ground or in the air decreased while the proportion of passerines feeding on vegetation increased with successional stage. Passerines nesting on the ground decreased with succession but passerines nesting in shrubs increased. Protecting the fields with the highest densities of non-passerine birds and letting the fields with steep slopes undergo a long-term vegetation succession seem to be the best measures for the management of these areas. Key words: Agricultural abandonment, Alentejo, bird communities, Cistus ladanifer, Cistus monspeliensis, old fields, succession.

RESUMEN.—Sucesión de las comunidades de aves nidificantes tras el abandono de cultivos en el sureste de Portugal. Se ha llevado a cabo un estudio de las comunidades de aves asociadas con los diferentes estadí- os de la sucesión vegetal tras el abandono de cultivos en el Bajo Alentejo (Portugal). Se identificaron cinco es- tadíos de vegetación: barbecho, barbecho con arbustos, matorrales bajos, matorrales puros altos y matorrales mixtos. La sucesión comienza con el abandono de los campos cultivados, que son ocupados por dos especies de jaras (Cistaceae), Cistus ladanifer y Cistus monspeliensis. La densidad de paseriformes aumentó con la su- cesión. Las especies más comunes en los barbechos pertenecieron a la familia Alaudidae, mientras que la familia Sylviidae fué la más frecuente en las etapas de matorral. Por lo general, el número de especies de paseriformes y el número total de especies aumentaron con la etapa sucesional, mientras que el número de especies de aves no paseriformes disminuyó. Los paseriformes migradores y granívoros/insectívoros fueron más abundantes en barbechos ralos, siendo reemplazados por especies residentes e insectívoras en etapas sucesionales posteriores. La proporción de paseriformes que se alimentaron en el suelo o en el aire se redujo con la etapa sucesional, mientras que la proporción de paseriformes que se alimentan en la vegetación aumentó. Por otra parte, la proporción de paseriformes que anidan en el suelo disminuyó con la sucesión, mientras la de los que que anidan en los arbustos aumentó. La protección de los campos de cultivo que mantengan las mayores densidades de aves no paseriformes, junto con el abandono a largo plazo de los campos con pendientes acu- sadas, favoreciendo la sucesión de la vegetación, parecen ser las mejores acciones de manejo en estas áreas. Palabras clave: Abandono agrícola, Alentejo, barbechos, Cistus ladanifer, Cistus monspeliensis, comunidades de aves, sucesión.

INTRODUCTION many cases are the only species present. The causes for this abandonment were related to When the agricultural fields in Baixo Alen- soil exhaustion due to the wheat campaigns that tejo, Portugal, are abandoned, a succession of began in 1929 (Mascarenhas, 1981). The great vegetation takes place, dominated by two spe- resistance of the above mentioned plants to cies from the rock-rose family (Cistaceae), Cis- drought and the fact that they segregate alelo- tus ladanifer and Cistus monspeliensis. These pathic substances, which suppress the growth shrubs now occupy great tracts of land and in of other plants, allowed them to occupy and

* Av. Espanha n.o 63-3.o T. P - 5200 Mogadouro, Portugal. 172 SANTOS, C. P. dominate the fields (Dias et al., 1989). So, the plete one because no field with characteristics agricultural cycle of ploughed fields → of a climax stage large enough to be studied cereal → fallow → ploughed fields, typical of was found in the area. In each of these stages, this area, which allowed the restoration of soil density and species richness of nesting passeri- fertility, was interrupted (Mascarenhas, 1981). nes were assessed. The study focussed on pas- According to Nuñez et al. (1986), the areas of serines because they tend to have small territo- rock-rose will develop into mixed shrubland ries that are closely associated with the small with species like mastic tree Pistacia lentiscus, patches of shrubland. Density of non-passerines Phillyrea angustifolia, myrtle Myrtus commu- was not estimated because of the relatively lar- nis, holm oak Quercus ilex and wild olive tree ge dimensions of their territories. Non-passeri- Olea europaea. Rivas Goday (1964) argues nes actively using the vegetation patches of that succession in rock-rose areas tends towards each stage, however, were registered (Reynolds Oleo-Ceratonion or Quercion rotundifoliae ve- et al., 1980). getation. A point count method was used to estimate The reform of the Common Agricultural Po- the density of birds. Following this method, we licy (CAP), which may cause further abandon- recorded all the birds detected, by sight as by ment of agricultural fields or their conversion to sound, within a circle with a pre-determined ra- other uses (Baldock, 1991), and the fact that a dius (Bibby et al., 1992). Counts were made in great number of threatened species depend on the first 3 hours after sunrise on days without these types of fields (Santos, 1996), makes it rain or strong wind. The time period used was highly desirable to study the effects of field 10 minutes (Fuller & Langslow, 1984), with a abandonment on birds. The evaluation of the delay of 2 minutes after the arrival to the point, conservation importance of these fields and the to allow the birds to settle. Two series of counts consequences of their abandonment have been were made at each point. One between 21/4/94 published by Santos (1996). The present paper and 10/5/94, for residents and early migrants, gives quantitative data on the successional and another between 28/5/94 and 14/6/94 for changes in avian species composition, together later migrants. Twenty points were established with some ecological features of the species in each successional stage. In stage 1 (fallows), concerned. the points had a 100 m radius. Stages 2 and 3 (fallow with shrubs and low shrubs), had points with a 75 m radius. In taller shrubs and mixed STUDY AREA AND METHODS shrubs (stages 4 and 5) a radius of 50 m was used. Circles with different radii were used to The study site was located mainly in Mérto- allow for decreasing detectability of birds with la district (SE Portugal) and in some fallow vegetation growth (Reynolds et al., 1980). All fields in Castro Verde district, near the border points were marked with coloured ribbons, in of Mértola district (7°25´ - 7°56´ W, 37°26´ - the centre and in the limits in at least two direc- 37¡51« N). The climate is Mediterranean with tions. For the first three stages, the 50 m distan- continental features, with hot and dry summers ce from the centre was also marked. A range- and mild winters with little rain. Almost all finder was used to map all objects that could (99%) of the soils are poor, with no agricultural help to estimate the distance to the birds. Birds capacity, and most of them are exhausted and detected outside the circles but still in the same have problems of erosion. The majority of the patch of vegetation were also recorded. area is covered with Mediterranean steppe and The location of the circles was chosen by ÇsubxerophiticÈ shrublands dominated by rock- the following criteria: (1) Vegetation patches rose species. There are also areas of holm oak were characteristic of their successional stages: ÇdehesaÈ and pine Pinus sp. and eucalyptus fallow had only herbaceous species; fallow Eucalyptus sp. plantations (Pena et al., 1985; with shrubs had patches of herbaceous species Cancela, 1993). with 25% to 75% of the area covered by small Five stages were studied in this succession: patches of scattered rock-roses; low shrubs had (1) fallow, (2) fallow with scattered shrubs, (3) more than 90% of the area covered with rock- low shrubs, (4) tall shrubs and (5) mixed roses up to 1 m; tall shrubs had more than 90% shrubs. The succession studied is not a com- of the area covered with rock-roses taller than SUCCESSION OF BREEDING BIRD COMMUNITIES AFTER THE ABANDONMENT 173

1.5 m; mixed shrubs had more than 20% of the to separate the biotopes included in each stage area covered with species other than rock-roses aiming at a more accurate characterisation of taller than 1.5 m and had also a tree layer the bird communities. Eight biotopes were thus (> 3 m). (2) Elements not representative of each obtained: sparse fallows, dense fallows, fallows stage, e.g. trees, patches of shrubs or clearings, with C. monspeliensis shrubs, fallows with C. should not exceed 10% of the area. (3) The ve- ladanifer shrubs, low C. monspeliensis shru- getation patches in a nearby area, whether or bland, low C. ladanifer shrubland, tall C. lada- not of the same type, should allow the esta- nifer shrubland and mixed shrubland. The rela- blishment of at least 5 points, in order to opti- tionship of the eight biotopes to the five mise resources. (4) Points should be fairly ac- successional stages is shown in Table 1. The cessible. (5) Points should be positioned inside average slope increased with successional stage a patch of vegetation 25 m wider than the circle (Table 1) which is due to the fact that fields limits (50 m in fallows). located on steeper slopes were abandoned first. At each point the following measurements Many plain fields are still cultivated with whe- were made: slope, percentage of bare soil (fa- at today. llows), percentage cover of the different plant Substantial changes in bird species composi- species, percentage cover of trees, height of tion occurred with vegetation succession (Ta- herbs and shrubs, stonyness and presence of ble 2). Some species were only present in the stubble. Height of herbs in the fallows (stage 1) early stages, others in the late ones. Some spe- was measured each 10 m along a 50 m line cies appear in almost all stages and others only from the center of the point. In fallows with in some specific biotope. The Spectacled War- shrubs (stage 2), measures were taken each 5 m bler is one example of this last case, as this for herbs as well as for shrubs, along a 25 m species was only found in shrublands with C. line from the center of the point. The same was monspeliensis, being relatively abundant the- done to measure shrub height in the other sta- re. The presence of tits (Parus spp.) at some ges. Percentage of bare soil, vegetation cover points at low densities was due to the presence and stonyness were estimated visually. of trees nearby. To calculate the densities of the passerine For the most abundant species, significant species, the same procedure as Ferry & Fro- differences between biotopes and between sta- chot (1970) was used: 1 male singing, 1 pair, 1 ges were found, while for the less abundant nest or 1 family, means a pair; 1 bird calling or ones this was not the case (Table 3). Wood- seen means 0.5 pairs. Colonial passerines like lark and Stonechat were the exceptions because the Azure-Winged Magpie Cyanopica cyanus significant differences between biotopes were were not considered (Ehrlich et al., 1994), and found for the first species and among stages neither were aerial feeders like swallows or for the second one. Woodlarks have a higher swifts, because they do not have a close asso- abundance in biotope 2 C.m., but it becomes ciation with the vegetation below them when diluted when stages are compared. As for the they are feeding (Southwood et al., 1986). Be- Stonechat, the combining abundances of bioto- cause of the difficulties in identifying Crested pes 3 C.m. and 3 C.l. allow for the existence of Larks Galerida cristata and Thekla Larks Ga- significant differences among stages. lerida theklae, these species were combined as The total density of passerine birds increa- Crested Larks Galerida spp., although most of sed with successional stage from fallows to the birds seemed to be Thekla Larks. mixed shrubland. The total density in fallows was less than half of that found in any other stage whilst the density in mixed shrubland RESULTS AND DISCUSSION was twice as great as in any other stage. The total density in fallows was only 6% of the During the establishment of the points, dif- total density found in the mixed shrubland sta- ferences in the density or even species compo- ge. The pattern should be similar when based sition of vegetation within each stage were no- on densities for all nesting birds, including ticed (Table 1). Although the objectives of this non-passerines. study and the definitions of the stages were not This agrees with the results obtained by jeopardised by these differences, it was decided Johnston & Odum (1956), Haapanen (1965), 174 SANTOS, C. P. .] Cis- : holm Q.i. [piedras] 2% ; [herbáceas] . [Abundante] stones herbs Myrtus communis ; C.l.: jara pringosa Cistus ladanifer : L. M. c. Myrtus communis 0.3% 1,5% . Average values are presented C.l. Portugal. Sólo se dan valores me- ; ; M.c.: mirto : myrtle M.c. Cistus monspeliensis ; C. m. C. m. C. l. + m. [herbáceas] P. l. P. l. Pistacia lentiscus herbs Cistus monspeliensis : C.m. Pistacia lentiscus sp.; P.l.: lentisco 1 : mastic tree 1 [carlos] P.l. Lavandula ABLE bare ground sp.; T thistles 1 measured in shrub patches only). ; L.: lavanda 2 Lavandula [cardos] Ð Ð Ð Ð Ð Abundant 2% 1% 0.8% 0.2% 13% Q. i. Q. i. C. m. Q. i. O. e. Q. i. 3.5% 2% 2% herbs herbs herbs medido sólo en manchas de matorral). C.m.: jaguarzo 2 bare ground [suelo desnudo] [suelo desnudo] thistles : rosemary Olea europea L. ; 0 57% 43% ≈ ; O.e.: olivo measured in fallows only; 1 1 Olea europea medido sólo en barbechos, 1 [cardos] C. m. C. l. C. l. C. l. C. l. O. e. Ð+ Ð Ð 57% 69% 100% 3% 3% 92% 0.2% 0.5% 3% 16%15% 62% 29.5% 61% 32% 5% 3% 94% 5% 97% 2% 12% 66.5% herbs herbs Quercus ilex [herbáceas] [herbáceas] poco denso] denso] thistles : olive tree ausencia; Ð: O.e. ; 1 2 ; Q.i.: encina Stage Fallow Fallow with shrubs Low shrubland Tall shrubland Mixed shrubland [Etapa] [Barbecho] [Barbecho con arbustos] [Matorral bajo] alto] [Matorral [Matorral mixto] Biotope Sparse FallowDense Fallow FallowDense Biotope Sparse presencia; [Biotopo] [Barbecho [Barbecho C. m. C. l. C. m. C. l. C. l. [Rastrojo] +: Quercus ilex de estaciones point counts 8 12 11 9 14 6 20 20 o o Tree level[Estrato arbóreo] Stubble Ð Ð 1% Ð Ð 0,2% 0.25% 15% [Estrato herbáceo] de escucha] Stones and pebbles [Cobertura vegetal] [suelo desnudo] [herbáceas] [herbáceas] [herbáceas] Characterisation of the vegetation different successional stages identified in abandoned fields southeastern Portugal (+ means presence; Ð absence; Herbaceous level tus ladanifer [N. dios ( [Caracterización de la vegetación las diferentes etapas sucesionales identificadas en los campos abandonados del sureste [Piedras] Slope (grades)[Pendiente (grados)] Height herbs (cm)[Altura herbáceas] Height Shrubs (cm)[Altura arbustos] 15.8 ± 7.1 4 16.2 ± 7.2 12.1 ± 10.2 3 17.4 ± 15.9 6.7 79.6 ± 9.8 141.3 ± 39.2 8.5 88.8 ± 6.1 126.5 ± 17.7 8.5 168.4 ± 18.6 156.8 ± 8.5 21.1 16.6 21.5 Vegetation cover bare ground N. oak SUCCESSION OF BREEDING BIRD COMMUNITIES AFTER THE ABANDONMENT 175 , MS: MS 0.2 ± 0.6 0.1 ± 0.4 0.06 ± 0.3 0.06 ± 0.3 1.02 ± 1.06 Cistus ladanifer (1984, 1985). Diet, fe- , C.l.: et al. , MS: mixed shrubs) and the fenológica de las especies se (1994), Cramp (1988), ena et al. Cistus ladanifer .: Biotopes C.l Cistus monspeliensis [Biotopos] , C. m. C. l. C. m. C. l. C. l. 0.05 ± 0.2 Cistus monspeliensis .: 0.43 1.81 2.36 3.34 7.53 2 C.m 12345 ABLE T SF DF [Arboles] (1994), Cramp (1988), (1992) and & Perrins (1993). et al. Site Site Feeding Nesting [Lugar de [Lugar de [Vegetación] [Arbustos] alimentación] nidificación] . (1984, 1985). La dieta, lugar de alimentación y nidificación se tomaron Ehrlich Migrant Insectivorous Ground/air Ground MigrantMigrant InsectivorousMigrant Insectivorous Vegetation Insectivorous Ground/air Shrubs Ground/air Trees Ground 0.05 ± 0.2 0.03 ± 0.08 0.08 ± 0.4 0.06 ± 0.2 0.02 ± 0.08 0.2 ± 0.4 Migrant Granivorous. Ground/air Ground 0.6 ± 0.3 0.01 ± 0.05 0.1 ± 0.3 Migrant Insectivorous Vegetation Shrubs et al Resident Insectivorous Vegetation Trees 0.03 ± 0.09 ResidentResident InsectivorousResident Insectivorous Vegetation Insectivorous Ground/air Shrubs Vegetation Trees GroundResidentResident InsectivorousResident Insectivorous Ground/air Insectivorous Ground/airResident Shrubs Ground/air Insectivorous Shrubs Vegetation Trees 0.1 ± 0.2 Shrubs 0.4 ± 0.0 0.5 ± 0.3 1.7 ± 1.3 3.04 ± 1.6 0.09 ± 0.2 0.02 ± 0.08 0.1 ± 0.3 0.09 ± 0.2 0.3 ± 0.6 0.7 ± 0.9 0.05 ± 0.3 0.2 ± 0.4 0.2 ± 0.4 Resident Insectivorous Ground/airResidentResident Granivorous. Ground Insectivorous Ground/air Vegetation Ground Shrubs 0.04 ± 0.1 0.2 0.01 ± ± 0.3 0.04 0.2 ± 0.3 0.1 ± 0.4 0.4 ± 0.5 0.4 ± 0.4 0.4 ± 0.3 0.06 ± 0.2 0.8 ± 0.5 0.8 ± 1.1 0.9 ± 0.7 1.1 ± 0.9 0.99 ± 1.1 Resident Insectivorous Ground/air Trees sp. Resident Insectivorous Ground/air Ground 0.08 ± 0.2 0.8 ± 0.6 1.0 ± 0.3 0.5 ± 0.4 0.09 ± 0.2 0.03 ± 0.1 0.06 ± 0.3 Species Phenology Diet [Especies] [Fenología] [Dieta] dactylalandra [Migrador] [Granívoro] [Residente] [Suelo/aire] [Insectívoro] [Suelo] successional stages identified in abandoned fields southeastern Portugal. Phenology classification of species according to P eding site and nesting according to Ehrlich [Densidad de aves (parejas/ha) en los biotopos (SF: barbechos poco densos, DF: C.m.: (1992) y Cramp & Perrins (1993).] matorrales mixtos) y las etapas sucesionales identificadas en los campos abandonados del sureste de Portugal. La clasificación realizó según Pena Species density (pairs/ha) in the biotopes (SF: sparse fallows, DF: dense Parus caeruleus Sylvia conspicillata Lanius senator Oenanthe hispanica Lullula arborea Parus major Saxicola torquata Turdus merula Sylvia melanocephala Carduelis cannabina Luscinia megarhynchos Troglodytes troglodytes Miliaria calandra Sylvia undata Melanocorypha ca- Galerida Calandrella brachy- Carduelis chloris Hippolais polyglotta Total by biotopes[Total por biotopo] Total by stage [Total por etapa sucesional] 0.74 0.23 1.69 1.95 2.61 1.79 3.34 7.53 176 SANTOS, C. P.

TABLE 3

Kruskal-Wallis tests for the differences in abundance of species by biotopes and by stages. (*: P < 0.05). [Pruebas de Kruskal-Wallis para las diferencias en la abundancia de especies por biotopos y etapas de la su- cesión (*: P < 0.05).]

By biotopes By stages Species [Por biotopos] [Por etapa] [Especies] H P H P

Short-toed Lark [Terrera Común] 79.07 0.000* 33.38 0.000* Calandra Lark [Calandria] 46.29 0.000* 25.25 0.000* Crested Larks [Cogujadas] 63.52 0.000* 55.86 0.000* Corn Bunting [Triguero] 17.99 0.012* 13.81 0.008* Dartford Warbler [Curruca Rabilarga] 36.36 0.000* 36.25 0.000* Sardinian Warbler [Curruca Cabecinegra] 79.76 0.000* 72.23 0.000* Woodlark [Totovía] 16.35 0.022* 8.08 0.089 Great Tit [Carbonero Común] 8.09 0.325 4.00 0.406 Blue Tit [Herrerillo Común] 8.09 0.325 4.00 0.406 Spectacled Warbler [Curruca Tomillera] 76.41 0.000* 48.97 0.000* Woodchat Shrike [Alcaudón Común] 9.91 0.194 8.92 0.063 Black-eared Wheatear [Collalba Rubia] 7.24 0.404 3.03 0.553 Linnet [Pardillo Común] 20.46 0.005* 20.43 0.000* Stonechat [Tarabilla Común] 12.16 0.095 11.51 0.021* Blackbird [Mirlo Común] 11.75 0.109 8.92 0.063 Nightingale [Ruiseñor Común] 48.79 0.000* 48.78 0.000* Wren [Chochín] 8.08 0.326 8.08 0.089 Greenfinch [Verderón Común] 4.00 0.780 4.00 0.406 Melodious Warbler [Zarcero Común] 8.08 0.326 8.08 0.089

Ferry & Frochot (1970), Jones (1972), Shugart in all biotopes, while other families were rela- & James (1973), Glowacinski (1975) and Lan- tively sporadic. yon (1981) for similar stages of succession to For the families represented by the most those examined here. These authors studied a abundant species significant differences were more complete succession, from the initial sta- found, while for the families with less abun- ges to the climax forest vegetation, which is dant species this was not the case (Table 4). not the case here. Three of the families were only represented by Larks (Alaudidae) were the dominant family one species (Emberizidae, Laniidae and Tro- in the early stages, being gradually replaced by glodytidae) so the test result is the same as for warblers (Sylviidae). Larks almost disappea- the corresponding species. red by stages 4 and 5, which is to be expected The proportion of individuals belonging to because these species are very dependent on migratory species showed no obvious trend with bare ground (Cramp, 1988). Martínez & Pu- successional development (Fig. 1). By far the rroy (1993), reviewing results from studies on highest percentage of migrant passerines was birds in Spanish steppes, also found a domi- registered in biotope 1SF. Apart from biotopes nance of larks. Warblers appeared in stage 2, 1SF and 3C.m., the resident birds constitute their dominance increasing until stage 4 and more than 80% of the total of birds present. then decreasing in stage 5. This family thus Considering successional stages only, fallows showed a great association with the shrub sta- have the highest proportion of migrants (36%), ges. Thrushes (Turdidae) only had some domi- followed by stage 3 (low shrubs) (20%) and nance in the last stage. Finches (Fringillidae) stage 5 (mixed shrubs) (17%). These values are only appeared from stage 3 on. Buntings (Em- due to the high densities of only three species: berizidae) showed almost the same dominance Short-toed Lark, Spectacled Warbler and Nigh- SUCCESSION OF BREEDING BIRD COMMUNITIES AFTER THE ABANDONMENT 177

TABLE 4

Kruskal-Wallis test for the differences in abundance of families by biotopes and by stages (* P < 0.05). [Pruebas de Kruskal-Wallis para las diferencias en la abundancia de familias por biotopos y etapas de la su- cesión (* P < 0.05).]

By biotopes By stages Families [Por biotopos] [Por etapa] [Familias] H P H P

Alaudidae 66.56 0.000* 56.03 0.000* Emberizidae 17.99 0.012* 13.81 0.008* Sylviidae 70.92 0.000* 70.60 0.000* Paridae 8.09 0.325 4.00 0.406 Laniidae 9.91 0.194 8.92 0.063 Turdidae 41.96 0.000* 40.90 0.000* Fringillidae 20.46 0.005* 20.44 0.000* Troglodytidae 8.08 0.326 8.08 0.089

FIG. 1.—Proportion of residents and migrants in the different biotopes (1SF: sparse fallow; 1DF: dense fallow; 2C.m.: fallow with shrubs of Cistus monspeliensis; 2C.l.: fallow with shrubs of Cistus ladanifer; 3C.m.: low shrubland of Cistus monspeliensis; 3C.l.: low shrubland of Cistus ladanifer; 4C.l.: tall shrubland of Cistus ladanifer; 5MS: mixed shrubland) Open bars: migrants, closed bars: residents. [Proporción de especies residentes y migrantes en los diferentes biotopos (1SF: barbecho poco denso; 1DF: barbecho denso; 2C.m.: barbecho con arbustos de Cistus monspeliensis; 2C.l.: barbecho con arbustos de Cistus ladanifer; 3C.m.: matorral bajo de Cistus monspeliensis; 3C.l.: matorral bajo de Cistus ladanifer; 4C.l.: matorral alto de Cistus ladanifer; 5MS: matorral mixto). Barras vacías: migrantes, barras rellenas: residentes.] tingale, respectively. These results are not coin- tion between percentage of migrants and latitu- cident with the ones reported by Ferry & Fro- de. In the Ferry & Frochot (1970) study, the chot (1970), who detected an increase in the number of migrant species found is more than percentage of migrants for similar stages of suc- twice the number of migrant species found in cession. One explanation may be the latitude this study —only six species— and not the most difference between France and Portugal. Herre- abundant ones. This may explain the irregular ra (1978) found a significant, positive correla- pattern of migrants found in this succession. 178 SANTOS, C. P.

If we consider only long distance migrants succession, inside and outside the point counts, (birds wintering south of the Sahara desert) the in the two counts. The highest number of pas- results are clearly different. In the present study serine species is found in the most heterogene- only two species, the Woodchat Shrike and the ous biotopes, like fallow with shrubs and mixed Nightingale, fulfil the above definition. Thus, shrubs. This last biotope has the highest num- the only significant percentage of long distance ber of passerine species. The number of non- migrants is 16% in stage 5. Helle & Fuller passerine species decreases from the fallows (1988), collecting data from successions in Eu- to the shrubland areas. The total number of spe- rope that include the results of Ferry & Fro- cies shows no clear trend with successional sta- chot (1970), verified that the proportion of long ge and is similar to that for passerines. A nega- distance migrants is higher in stages with ve- tive significant correlation was found between getation 1-2 m tall and then decreases in the number of species of non-passerine birds and following stages. Based on data from the nort- the successional stage (r = Ð0.9; P = 0.037), hern hemisphere, Mönkkönen & Helle (1989) while no significant correlation was found for only found this pattern for successions in Eu- the passerine birds (r = 0.6; P = 0.285) or for rope, while in North America and Asia long all bird species (r = Ð0.103; P = 0.870). distance migrants increased or did not decrease These results are not totally coincident with (Western North America) along the succession. the ones obtained by Ferry & Frochot (1970), Data obtained in this study seem to agree with Jones (1972), Glowacinski (1975), Lanyon the data from these authors, considering that (1981), Southwood et al. (1986) and Valvo & the present succession is not a complete one. Massa (1990). Their results showed an increase No obvious pattern was evident in the relati- of the total number of species with successional ve proportions of insectivorous and granivorous development. Blondel (1981) found a positive passerines as related to succession (Figure 2- relationship between the number of species and A). Granivorous birds dominated biotope 1 SF, the structural complexity of the vegetation. The while insectivorous birds made up more than reason for this difference in the results is pro- 80% of the birds present in the other biotopes. bably due to the fact that this succession co- Figure 2-B suggests a decrease in the proportion rresponds only to two or three stages of the of birds that feed on the ground and/or in the succession studied by the above authors. Pro- air, which is 100% in the fallows, and an incre- bably, the differences found here would be ase in the proportion of birds feeding on the ve- smoothed if we could have included the next getation. This is to be expected because the stages of the succession. Besides, if we consi- availability of bare ground decreases with suc- der stages 2 and 5 as edges, we may understand cessional stages. Nevertheless, in biotope 5 MS the higher number of species found there. Ed- there is a slight increase of birds feeding on the ges attract species from the two biotopes ne- ground, probably due to the height of the trees arby which add to the species from the edge and death of shrubs which allows them access itself. Martin (1960) also found the highest va- to the ground. Ferry & Frochot (1970) reported lues in forest edges. similar results. Figure 2-C suggests a similar Southwood et a.l (1986) also found that lar- pattern. There is a decrease in the proportion ger species, usually non-passerine birds, were of birds nesting on the ground, and an increase mainly associated with the open stages. Two of birds that nest on the vegetation. In biotope 5 hypotheses may explain this situation. Verner MS there is a slight increase of birds nesting & Wilson (1966) argue that, in early stages, all on the ground which may be explained by the primary productivity is concentrated in a na- reasons referred to above. In biotopes 2 C.m. rrow band just above the ground, while in the and 3 C.l. the presence of tree-nesting birds is woodland areas it is spread throughout a larger related to the presence of trees in the vicinity of volume of vegetation. Density of food is thus point counts. Ferry & Frochot (1970) and Glo- higher in the open areas and these species have wacinski (1975) reported similar results with a higher feeding efficiency. Lawton (1978, in respect to nest site selection in the same stages. Southwood et al., 1986) and Southwood et al. Figure 3 shows the number of passerine spe- (1978, in Southwood et al., 1986) suggest that cies, non-passerine species and the total num- only small animals can explore the great va- ber of species, in the different stages of the riety of niches in woodland areas. SUCCESSION OF BREEDING BIRD COMMUNITIES AFTER THE ABANDONMENT 179

FIG. 2.—Proportion of passerines with A) different diets (open bars: granivorous, closed bars: insectivorous); B) different feeding sites (open bars: ground/air, closed bars: vegetation); C) with different nesting sites (open bars: ground; striped bars: shrubs; closed bars: trees) according to biotopes. See table 2 for the classifications of bird species. [Proporción de aves paseriformes con A) diferentes dietas (barras vacías: granívoros, barras rellenas: in- sectívoros); B) diferentes lugares de alimentación (barras vacías: tierra/aire; barras rellenas: vegetación); C) diferentes lugares de nidificación (barras vacías: tierra; barras rayadas: arbustos; barras rellenas: ár- boles), según los biotopos. Véase la tabla 2 para la clasificación de las especies de aves.] 180 SANTOS, C. P.

FIG. 3.—Number of species of passerines, non-passerines and total species found in the successional stages of the abandoned fields of southeastern Portugal (open bars: passerines; closed bars: non-passerines; striped bars: all birds). [Número de especies de paseriformes, de no paseriformes y especies totales encontrados en las diversas etapas sucesionales de los campos abandonados del sureste de Portugal (barras vacías: paseriformes; barras rellenas: no paseriformes; barras rayadas: todas las especies).]

FINAL REMARKS aspects: a) a protection of the fields that support the highest densities of non-passerine birds, es- The wheat campaigns that started in 1929 in pecially the most threatened ones (Santos, Alentejo resulted in the cultivation of large are- 1996), which are usually lowland fields; and as of land, mostly with no agricultural capacity. b) to leave the fields with steep slopes to un- This fact led to an increase in soil erosion and to dergo long-term vegetation succession. Of the exhaustion of soil fertility, making it almost course, this management should be done consi- sterile. The progressive abandonment by far- dering the management of other conservation mers and the invasion of rock-roses started a interests, related to other species of fauna and process of succession that will eventually trans- flora, as well as the local population interests. form the rock-rose shrubwoods into a Medite- rranean forest. No one knows how long it will ACKNOWLEDGEMENTS.— I would like to thank take, but many decades will certainly pass be- Prof. Jorge Palmeirim for all the support that he gave fore we can see changes in some of these areas. to this work. To the Associação de Defesa do Patri- mónio de Mértola and Campo Arqueológico de Mér- The destruction of these shrublands should not tola for the facilities. To Dr. Susana Marques and occur, because they are the only protection of Dr. Robert Fuller and Dr. Mario Díaz for their most these soils, especially those on steep slopes. valuable comments and corrections of this study. Birds are one of the vertebrate groups most affected by these changes of vegetation. This paper shows that a succession of bird commu- BIBLIOGRAPHY nities follows the vegetation succession. An in- BALDOCK, D. 1991. Implications of EC farming and crease of passerine density and richness was countryside policies for conservation of lowland found as the succession developed. In contrast, dry grasslands. In, P. D. Goriup, L. A. Batten & J. the richness and abundance of non-passerine A. Norton (Eds.): The conservation of lowland birds decreased. This suggests that future ma- dry grassland birds in Europe, pp 111-117. ICBP. nagement of this area should be based on two Cambridge. SUCCESSION OF BREEDING BIRD COMMUNITIES AFTER THE ABANDONMENT 181

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