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 monspe- liensis. 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 pas- serines 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 monspe- liensis, 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 fa- milia Sylviidae fué la más frecuente en las etapas de matorral. Por lo general, el número de especies de pase- riformes 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 su- cesionales 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, comu- nidades 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 «subxerophitic» 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: «dehesa» 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.