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1, Tomás SANTAMARÍA. 2, Francisco GUTIÉRREZ- ... 1European University Miguel de Cervantes, Calle Padre Julio Chevalier 2, E-47012 Valladolid, SPAIN, e-mail: ... predation rate (Tellería & Santos 1992, Mazgajski ... included in the Red List as a result of its popula- ..... From nest-building to fledging of young in Great.
ACTA ORNITHOLOGICA Vol. 46 (2011) No. 1

Breeding success of Southern Grey Shrikes Lanius meridionalis in agricultural areas: the influence of nest site characteristics Francisco CAMPOS1, Tomás SANTAMARÍA2, Francisco GUTIÉRREZ-CORCHERO3, M. Ángeles HERNÁNDEZ4 & Pedro MAS2 1European University Miguel de Cervantes, Calle Padre Julio Chevalier 2, E-47012 Valladolid, SPAIN, e-mail: [email protected] 2Catholic University of Ávila, Calle Canteros s/n, E-05005 Ávila, SPAIN 3Joan Maragall 19, E-17002 Girona, SPAIN 4Department of Zoology and Ecology, Faculty of Sciences, University of Navarra, E-31080 Pamplona, SPAIN

Campos F., Santamaría T., Gutiérrez-Corchero F., Hernández M. Á., Mas P. 2011. Breeding success of Southern Grey Shrikes Lanius meridionalis in agricultural areas: the influence of nest site characteristics. Acta Ornithol. 46: 29–36. DOI 10.3161/000164511X589884 Abstract. Three hundred and thirty nests of Southern Grey Shrike Lanius meridionalis were located during 1998–2010 in two highly fragmented Mediterranean agricultural areas of Spain (Olite in Northern and Toro in Western part of the country). Most nests (ca. 85%) were built in thorny shrubs (mainly Dog Rose Rosa sp. and Blackberry Rubus ulmifolius) and thornless shrubs (mainly Holm Oak Quercus rotundifolia and Kermes Oak Quercus coccifera). Based on a logistic regression, three factors had an influence on breeding success: study area, breeding phenology, and plant cover type. Location of the nest inside the shrub, as well as luminosity inside a shrub varied between studied shrub species, but did not significantly affect breeding success. The percentage of successful nests was significantly higher in Olite than in Toro (64.5% and 37.9% respectively). In Olite, but not in Toro, clutches laid early (first egg laid prior to 1st May) were more successful than clutches laid late. The highest breeding success was recorded in nests located in thorny shrubs and, especially, in those found in Blackberry bushes. Conservation of Blackberry shrubs appears to be an appropriate measure to increase breeding success of the Southern Grey Shrike. Key words: agricultural areas, breeding success, Lanius meridionalis, Southern Grey Shrike, predation Received — Sept. 2010, accepted — April 2011

INTRODUCTION Agricultural intensification has reduced the extent of natural vegetation, especially trees and shrubs, to the point of relegating it to small plots of land located within large areas dedicated to cultivation. This has taken place over the last decades in Central and Western Europe (Stoate et al. 2009), with special relevance in Mediterranean countries (Falcucci et al. 2007). Reduction in habitat heterogeneity is probably the cause of the loss of biodiversity in agricultural areas (Benton et al. 2003). It has been demonstrated that bird diversity is lower in cultivated areas than in areas where the vegetation has not been altered (Collard et al. 2009, Orłowski & Ławniczak 2009, etc.). Birds living in agricultural areas have to adapt to the changes in vegetation structure, searching for the most favourable areas in which to breed

successfully (Douglas et al. 2010) or in which to feed (Silva et al. 2007). Nest predation may induce changes in nest location (Söderström & Karlsson 2011) and it forces many species of passerines to lay replacement clutches, with the resulting energy expenditure that this represents. Mammals and corvids have been cited as usual passerine nest predators (Weidinger 2009), and their action may occasionally reduce the number of breeding pairs although this is under discussion for farmland passerines (White et al. 2008). Birds tend to have breeding strategies that minimize the action of predators as much as possible, especially if predation rate is high (Lima 2009). In the Iberian Peninsula, agricultural land covers large areas. The sclerophyllous forests of the central plains of Spain have been largely fragmented and the effects of this fragmentation have

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been analysed in passerines (Santos & Tellería 1998). Bird species of ecotone habitats and generalists can invade these fragments of woodland acting as competitors or predators (Janzen 1983, Yahner 1988). On the other hand, it is known that nest predation increases in fragmented landscapes, although patch size shows little relation to predation rate (Tellería & Santos 1992, Mazgajski & Rejt 2005). However, many of these studies have been based on artificial nests whose predation rates differ from those of real nests (Wilson et al. 1998, Haegen et al. 2002). In this scenario, the Southern Grey Shrike Lanius meridionalis is a shrub-nesting species that can travel through and/or feed in the cultivated land, which surrounds the patches of natural vegetation (Santos et al. 1998). Details on their nesting are relatively unknown as few data have been published (De la Cruz & De Lope 1985, Hernández 1993, 1994, Campos et al. 2006, 2007). In Spain, the Southern Grey Shrike has been included in the Red List as a result of its population decline recorded over the last decades (Hernández & Infante 2004). Nesting is a critical period for the species, and its conservation is largely dependent upon it. This is why it is important to know the causes which harm or benefit nesting, even more so in a habitat as extended as agricultural land. The objective of this study is to determine whether nest site characteristics have an influence on breeding success. Here we discuss data related to breeding success and problems faced by predators: 1) to trace/discover the nest and/or 2) to access the nest.

STUDY AREA AND METHODS The study was conducted in two areas almost 350 km apart: 1) in the surroundings of the village of Olite (hereinafter Olite, 42°29’N 01°39’W), Northern Spain, where an area of 52 km2 was sampled for five years (1998–2002). 2) In the surroundings of the town of Toro (hereinafter Toro, 41°31’N 05°24’W), Western Spain, where an area of 169 km2 was sampled for four years (2006–2008 and 2010). In both areas, the potential vegetation was the Querceto rotundifoliae S. series (RivasMartínez 1987) but very much altered by deforestation to cultivate the land. Islands of vegetation of various sizes (range < 1–10 ha) have remained among the cultivations, some with herbaceous plants only, others with bushes only (mainly

Kermes Oak Quercus coccifera in Olite and Holm Oak Quercus rotundifolia in Toro) and, finally, others with natural tree cover (mainly Holm Oak) or reforested with Stone Pine Pinus pinea. Agricultural land was dedicated to Vineyards Vitis vinifera, non-irrigated cereal crops (Wheat Triticum sp., Barley Hordeum sp.) and trees (Almond Amygdalus communis, Olive Olea europea). In these islands of vegetation, the most frequent thorny shrubs were Blackberry Rubus ulmifolius in Olite, and Dog Rose Rosa sp. in Toro. In both areas, predators such as mammals (mainly Weasel Mustela nivalis, and Small-spotted Genet Genetta genetta, Palomo et al. 2007) and corvids (Magpie Pica pica, and Carrion Crow Corvus corone) were present. Density of Southern Grey Shrike (as breeding pairs) is unknown in both areas. However, mean distance between nests was 599 m in Olite (Campos et al. 2006) and 748 m in Toro (Campos et al. 2010), suggesting the presence of a greater number of shrikes in the first area than in the second one. Field methods and data analysis Nests were found through observation of the adult shrikes’ behaviour and by searching the isolated shrubs found between cultivated areas or located in areas with natural vegetation. The nests from which at least one nestling fledged were considered as successful, and those from which no nestling fledged were considered as having failed. In addition to year, five other variables that can influence breeding success were analysed: 1) Breeding phenology: each nest was visited regularly once it was discovered, and therefore dates first egg laid and hatching were known. When this was not possible, it was assumed that one egg per day was laid, that incubation lasted for 16 days, and that the time that nestlings remained in the nest was 20 days (Yosef 1992). Nests were classified as having early clutches if the first egg was laid prior to 1st of May, and late clutches if eggs were laid at a later date. 2) Plant species used for nesting were pooled in two types: a) thorny shrubs, including Junipers Juniperus spp., Hawthorn Crataegus spp., Blackhorn Prunus spinosa, Blackberry, Dog Rose, Buckthorn Rhamnus lycioides, Boxthorn Lycium europaeum; b) thornless plants, either shrubs (Holm Oak, Kermes Oak, Evergreen Buckthorn Rhamnus alaternus), trees (Maritime Pine Pinus pinaster, Stone Pine, Almond, Olive), or piles of vine shoots (cut branches of vine).

Breeding success of Southern Grey Shrike

3) The position of the nest inside the plant cover: height above ground and depth inside the plant, i.e. the distance from the edge of the nest to the nearest point on the edge of the plant. 4) Nest visibility from the outside, measured indirectly by the amount of light penetrating in the shrub. Theoretically, the more the amount of leaf coverage, the lower will light intensity be inside and therefore, nest detectability inside will be lower. Light intensity (expressed in W-2) was measured on clear days in August using a radiometer Licor LI-188 B with a pyranometer sensor. Measurements were taken in specimens of Dog Rose, Blackberry and Kermes Oak chosen at random, on the outside of the shrubs and at the height of 25, 50, 75 and 100 cm above ground, always at a depth of 80 cm. This depth was chosen because it is the mean value at which the Southern Grey Shrike’s nests are found. The percentage of light intensity at each height was calculated in relation to the intensity outside the shrub. The final results are expressed as mean ± SD of these percentages. In the two studied areas, Dog Rose and Blackberry leaves are completely developed in the first days of May and remain that way during the entire summer (pers. obs.). Therefore, light measurements taken in August are representative of what happens in these shrubs in May and June. On the other hand, Kermes Oak and Holm Oak shrubs have a very similar branch and leaf distribution, and therefore light measurements in Kermes Oak can be extrapolated to Holm Oak. 5) Plant position, which was classified as a) isolated when the shrub was not in an area of natural vegetation and did not have any other shrub less than 50 m away; b) surrounded when they were in areas of natural vegetation and had other shrubs less than 10 m away. These data were recorded only in Toro. Not all data were collected for all nests and, therefore, the sample size varied. Statistical methods The influence of the variables mentioned above on breeding success was analyzed using a logistic regression model, with breeding failure as dependent variable, qualitative variables (area, breeding phenology and plant type) as factors, and the variables for year, nest depth and height inside the shrub as covariates. The significance of the coefficients obtained was assessed through their corresponding p-value. The odds ratio was used to assess probability of failure of a nest

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between categories of a particular variable. The Hosmer-Lemeshow test was used to study whether the observed and predicted values matched. The χ2 test was used for testing the frequencies of plant cover type used for nesting in each study area, and to assess the relationship between various pairs of variables. The analysis of luminosity in relation to height in the different types of shrubs was conducted using the Kruskal-Wallis test, applying the Z value for multiple comparisons (Dunn’s test). The reason for using nonparametric methods was that the data did not show homoscedasticity or normal distribution. The established significance level in the statistical analysis was p < 0.05, and all were carried out using MINITAB release 14.0 software.

RESULTS Location of nests Three hundred and thirty nests were found (184 in Olite, 146 in Toro), the majority in shrubs (84.8% in Olite, 84.9% in Toro). In Olite, the shrubs most frequently used were Blackberry and Kermes Oak (16.3% and 46.4%, respectively) and in Toro, the majority were Dog Rose and Holm Oak (34.2% and 43.1%). The remaining nests were built in trees (Olite 10.3%, and 6.9% in Toro) and piles of vine shoots (4.9% and 8.2% in Olite and Toro, respectively). There were no statistically significant differences between both areas in the number of nests found in the thorny and thornless plants (χ21 = 2.914, p = 0.087). Also within each area, the percentage of nests in thorny and in thornless shrubs did not vary significantly between early and late clutches (Table 1). Breeding success Overall, the percentage of successful nests in trees was 55.2% (n = 29), in shrubs was 54.7% (n = 278) and in piles of vine shoots was 23.8% (n = 21). The logistic regression analysis for the areas of Olite and Toro pooled together showed that, for equal years, depths, and heights, late clutches in the study area and in thornless shrubs had greater probabilities of failure (thornless plants, z = 2.69, p = 0.007, odds ratio = 2.42, 95% CI: 1.27–4.61; Toro area, z = 2.51, p = 0.012, odds ratio = 10.05, 95% CI: 1.66–60.88; late clutches, z = 1.87, p = 0.062, odds ratio = 1.85, 95% CI: 0.97–3.54). The remaining variables did not affect breeding

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Table 1. Location of Southern Grey Shrike nests in two study areas in relation to breeding phenology and shrub type. Number of nests — in parentheses. Phenology

Shrub type

Early clutches

Area Olite 63.9 (53) 36.1 (30) 62.9 (22) 37.1 (13)

Toro 55.5 (56) 44.5 (45) 64.4 (29) 35.6 (16)

χ21 = 0.011, p = 0.915

χ21 = 0.699, p = 0.403

thrornless thorny thornless thorny

Late clutches

success (p > 0.05 for all of them). This is why an analysis was performed for each area separately. In Olite, the model showed that late clutches had a higher probability of failure (z = 2.64, p = 0.008, odds ratio = 4.5, 95% CI: 1.47–13.76). In effect, early clutches showed a significantly higher breeding success (73.5%, n = 83) than late clutches (29.4%, n = 34, χ21= 18.551, p = 0.000). By contrast, in Toro, the breeding success was similar for both types of clutches (38.6%, n = 101 and 36.4%, n = 44 in early and late clutches respectively, χ21= 0.066, p = 0.797), and this is probably why the model did not identify the breeding phenology variable as having an influence in this area. The model also showed that, in Toro, the nest located in thornless shrub had the highest probability of failure. In effect, nests located in thorny plants showed a higher success rate than those located in thornless plants, although this did not occur in the Olite area (Table 2). In the four species of shrubs used most commonly for nesting, the percentage of successful nests was 78.9% (n = 33) in Blackberry, 50.0% (n = 66) in Dog Rose, 59.0% (n = 83) in Kermes Oak and 38.7% (n = 75) in Holm Oak, with significant differences between these values (Kruskal-Wallis test, H3 = 16.27, p = 0.001 adjusted for ties). Of the 51 nests located in thorny shrubs in Olite, 30 (58.8%) were found in Blackberry and 16 (31.4%) in Dog Rose, while of the 53 nests located in Toro, 3 (5.7%) were in Blackberry and 50 (94.3%) in Dog Rose. The location of the nest inside the shrub varied according to the species both in depth and in height (Table 3). Both variables reached their high-

est values in Holm Oak and their lowest values in Blackberry bushes. Mean value of luminosity inside a shrub varied significantly between and within Dog Rose, Blackberry and Kermes Oak shrubs (Table 4), luminosity being higher in the Dog Rose at the same height. Furthermore, the position of 145 nests was recorded in Toro, of which 55 were isolated and 90 were surrounded by other shrubs. Breeding success (34.5% in isolated shrubs and 40.0% in surrounded ones) did not vary significantly between both positions (Kruskal-Wallis test H1 = 0.43, p = 0.513 adjusted for ties). DISCUSSION Variation of plant cover type In the studied areas, the Southern Grey Shrike nested in two types of plant cover, which comprised at least 13 species. The ability of nesting in a large variety of plant cover is important in order to breed in different habitats. However, in both areas, the majority of nests were found in four species of shrubs only: Kermes Oak, Holm Oak, Dog Rose and Blackberry. In Olite, 86.6% of thornless shrubs used by the Southern Grey Shrike were Kermes Oak, while in Toro they were Holm Oak, as Kermes Oak is absent because climatic conditions are not suitable for this species (Ruiz 2006). In Olite, Blackberry and Dog Rose are present, and both species were used for nesting (50.8% and 27.1%, respectively) while in Toro, Blackberry (but not Dog Rose) is very scarce as it is eliminat-

Table 2. Number (and percentage) of successful and failed Southern Grey Shrike nests in thorny and thornless plants in two study areas. Area

Thorny shrubs Successful Failed

Thornless shrubs Successful Failed

Olite Toro

43 (72.9%) 30 (50.0%)

75 (60.5%) 25 (29.4%)

16 (27.1%) 30 (50.0%)

49 (39.5%) 60 (70.6%)

χ21 = 2.169, p = 0.140 χ21 = 5.488, p = 0.019

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Table 3. Comparison of depth and height above the ground (mean ± SD) of Southern Grey Shrike nests build in thorny and thornless shrubs, with results of Kruskal-Wallis test (adjusted for ties). N — sample size. Shrubs Thorny Thornless

Nest Dog Rose (N = 45) Blackberry (N = 23) Kermes Oak (N = 61) Holm Oak (N = 64)

depth (cm)

height (m)

0.93 ± 0.27 0.60 ± 0.16 0.68 ± 0.25 1.08 ± 0.45 H3 = 64.58, p = 0.000

1.13 ± 0.31 1.00 ± 0.34 1.10 ± 0.30 1.46 ± 0.57 H3 = 21.50, p = 0.000

ed by the farmers who consider it as a harmful plant (pers. obs.). The preference for nesting in shrubs has also been recorded in Loggerhead Shrike Lanius ludovicianus (Woods & Cade 1996), a species genetically close to Southern Grey Shrike (Klassert et al. 2008). By contrast, Great Grey Shrike Lanius excubitor frequently nests in trees, especially conifers (Schön 1994, Tryjanowski et al. 1999, Antczak et al. 2004), perhaps as a result of substantial differences in vegetation cover in central Europe and Mediterranean habitats. However, shrikes may also prefer dense vegetation to increase the likelihood of extra-pair copulations (Tryjanowski et al. 2007).

use more thorny shrubs as the breeding season progresses as, by then, these shrubs have well developed leaves and detectability is lower. Breeding success varied between areas. In the Olite area, early clutches had a higher breeding success, especially when the nests were placed in Blackberry bushes. The shrikes did not use more thorny shrubs in late clutches than in early clutches, and their high failure rate may have been due to a higher activity of predators later in the breeding season (Wiklund 1984). This suggests that, in this area, the strategy followed by the shrikes was to use shrubs that effectively defend the nest, whether by hiding it from the outside or by making the access difficult for predators. Blackberry bushes provide both defences. In the Toro area, breeding success was low in early and late clutches. Shrikes mostly used thornless shrubs in spite of their poor result and, as the breeding season progressed, they did not use more thorny shrubs either. The reason may be that in this area, Blackberry is very scarce, and therefore shrikes are forced to use Dog Rose or Holm Oak to nest. Dog Rose has a smaller leaf coverage than Blackberry, and therefore nests are probably detected more easily by predators. Holm Oak shows a high leaf coverage, but the absence of thorns makes it more easily accessible for predators. In an attempt to reduce this high predation rate, the shrikes seem to choose to place their nests higher and deeper into both species of shrubs.

Variation of breeding success Breeding success in shrikes has been shown to be related to the type of shrubs used to nest. Thus, in Red-backed Shrike Lanius collurio, thorny shrubs enhanced breeding success in some areas but not in others (Farkas et al. 1997, Tryjanowski et al. 2000). In Loggerhead Shrike, breeding success was independent from the type of shrub used to nest (Woods & Cade 1996) while in Southern Grey Shrike from Israel the lack of foliage in plant cover negatively affects breeding success (Keynan & Yosef 2010). In our case, within each of the two areas the highest success was recorded in nests placed in thorny shrubs, probably because these shrubs’ thorns are an effective defence against predators. Shrikes should therefore be expected to

Table 4. Mean percentage (± SE) of light intensity at different heights above the ground in three species of shrubs. N — sample size, statistics — Kruskal-Wallis test (adjusted for ties). Shrubs Dog rose (N = 27) Blackberry (N = 27) Kermes Oak (N = 31)

Height (cm) 25

50

75

100

5.96 ± 0.80 1.79 ± 0.28 1.92 ± 0.07 H2 = 34.27, p = 0.000

10.18 ± 1.59 3.22 ± 0.32 2.78 ± 0.11 H2 = 44.21, p = 0.000

16.33 ± 2.33 5.75 ± 0.54 3.68 ± 0.15 H2 = 49.40, p = 0.000

21.60 ± 2.98 8.57 ± 0.58 4.56 ± 0.14 H2 = 57.63, p = 0.000

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It has been put forward that, in areas of fragmented vegetation, predation rate varies according to whether the nests are located on the edge of a forest patch or within it (Stephens et al. 2003). Our results show that in the Toro area breeding success of isolated nests and nests located within patches of natural vegetation did not differ. At other sites, linear habitats often force shrikes to nest in areas where their nests can be easily detected by predators (Major et al. 1999). In Toro, however, linear habitats are rare and were not used by the shrikes but, on many occasions, the lack of patches of natural vegetation forced them to use an isolated shrub or a pile of vine shoots. The low breeding success in vine shoots (29.4%) may have been due to 1) the short distance from the ground (mean ± SD: 0.72 ± 0.24 m, n = 17), which possibly made the nests more accessible to predators, 2) man’s action who burns these heaps every year, destroying also the nests. An even lower breeding success than that of Southern Grey Shrike in Toro (present study) was recorded for the Loggerhead Shrike in agricultural areas of the United States (25.6%, Walk et al. 2006, vs 78%–89% in non agricultural zones, Chabot et al. 2001). This confirms that natural habitat degradation favours higher levels of predation (Hoover et al. 1995). In order to minimize this impact, the shrikes try to nest in shrubs with difficult access for predators, choosing for example Blackberry bushes when they are present. Nests in Blackberry obtained the highest success even though they were placed at lower height and more accessible than in other species. Foliage was greater in Blackberry than in Dog Rose, as shown indirectly by the luminosity recorded inside both species. It may be that this is the reason why nests in Blackberry do not need to be as deep inside the bush as in Dog Rose in order not to be detected from the outside. Blackberry therefore appears to be an important species, which enables shrikes to achieve a high breeding success. Loss of habitat quantity and quality influences a species natality and mortality (Hanski 2005) and thus, population dynamics. Low breeding success may be the cause of the decline of Southern Grey Shrike in agricultural zones, although other causes (i.e. post-fledgling survival) may also reduce the size of the breeding population of shrikes (Tryjanowski et al. 2006). Therefore, in order to favour conservation of this species in the area studied, an effective and easy measure would be to preserve Blackberry bushes.

ACKNOWLEDGEMENTS Obra Social de Caja Espańa and Fundación Universitaria de Navarra subsidized the study. Amadeo Urdiain helped with the collection of light measurements in the shrubs. Miguel Miranda and Elsa Santos helped in the fieldwork. An anonymous reviewer made valuables comments and improved the submitted manuscript.

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STRESZCZENIE [Wpływ charakterystyki miejsca gniazdowego na sukces lęgowy dzierzby śródziemnomorskiej w krajobrazie rolniczym] Drapieżnictwo jest jednym z ważniejszych czynników kształtujących wybór miejsc na gniazdo u ptaków. Umieszczenie gniazda w miejscach trudno dostępnych dla potencjalnych drapieżników lub takich, w których trudno je odnaleźć, zwiększa sukces lęgowy. Badania prowadzono na dwóch, silnie pofragmentowanych powierzchniach w krajobrazie rolniczym Hiszpanii (Olite na północy oraz Toro na zachodzie tego kraju). W latach 1998–2010 łącznie znaleziono 330 gniazd. Za lęgi zakończone sukcesem uznano te, w których przynajmniej jedno pisklę opuściło gniazdo. Analizując czynniki wpływające na sukces lęgowy wzięto pod uwagę: fenologię lęgu (z podziałem na lęgi wczesne i późne), gatunek krzewu/drzewa, na którym umieszczone było gniazdo, lokalizację gniazda i jego ukrycie oraz rok i powierzchnię badań. Krzewy i drzewa, na których stwierdzano gniazda podzielono na dwie

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kategorie — cierniste, tj. takie, które mają ciernie lub kolce, utrudniające drapieżnikom dostanie się do gniazda, oraz bez kolców. Do pierwszej grupy zaliczono gniazda zbudowane m. in. w jałowcach, dzikich różach, jeżynach i głogach, do drugiej — gniazda na dębach (ostrolistnym i skalnym), szakłakach czy drzewach oliwnych. Do drugiej grupy zaliczono także gniazda zbudowane w stosach ściętych gałęzi winorośli. Opis lokalizacji gniazda obejmował jego wysokość nad ziemią, oraz głębokość jego posadowienia w roślinie — odległość między gniazdem a skrajem rośliny. Ukrycie gniazda mierzono w sposób pośredni. Zakładając, że większa ilość liści powoduje, że mniej światła dociera do wnętrza rośliny, a przez to możliwość dostrzeżenia gniazda jest mniejsza, mierzono intensywność światła docierającego do wnętrza rośliny (przy pomocy radiometru). Pomiary takie przeprowadzono tylko dla trzech najczęściej wybiera-nych na gniazda krzewów: dębu skalnego, jeżyny i dzikiej róży, a intensywność dochodzącego światła mierzono na wysokości 25, 50, 75 i 100 cm nad powierzchnią ziemi, i głębokości 80 cm w głębi krzewu (średnia głębokość zlokalizowania badanych gniazd dzierzby śródziemnomorskiej). Uzyskane wyniki odnoszono do intensywności światła mierzonego na zewnątrz rośliny.

Większość gniazd została znaleziona w krzewach — dębach skalnych, jeżynie i dzikiej róży. Nie stwierdzono, aby krzewy cierniste były wybierane częściej, czy też wcześniej niż rośliny bez kolców (Tab. 1). Sukces lęgowy różnił się pomiędzy powierzchniami Toro i Olite (odpowiednio 37,9% i 64,5%), był wyższy dla gniazd usytuowanych w roślinach ciernistych (tab. 2), a także dla lęgów wczesnych (ale tylko na powierzchni Olite). Stwierdzono, że gatunek rośliny, w której umieszczone było gniazdo wpływał na prawdopodobieństwo wyprowadzenia piskląt. Wśród gniazd zbudowanych w krzewach najwyższy sukces lęgowy (78,9%) odnotowano dla gniazd zbudowanych w jeżynach, najniższy (38,7%) — w dębie skalnym. Najrzadziej sukcesem kończyły się lęgi w gniazdach zbudowanych w stosach ściętych gałęzi winorośli (23,8%). Usytuowanie gniazd (ich wysokość nad ziemią i głębokość posadowienia) w poszczególnych roślinach różniło się istotnie (Tab. 3), jednak czynnik ten nie wpływał istotnie na sukces lęgowy. Intensywność światła dochodzącego do wnętrza roślin różniła się w zależności od gatunku i była największa dla krzewów dzikiej róży (Tab. 4). Autorzy wskazują, że ochrona krzewów jeżyn jest najlepszym sposobem na zwiększanie sukcesu lęgowego dzierzby śródziemnomorskiej na badanych terenach rolniczych.