Plasticity of habitat selection by red-backed shrikes (Lanius Collurio) breeding in different landscapes.
|Abstract:||Environmental parameters in different breeding habitats of Red-backed Shrikes (Lanius collurio) in central Italy were examined at altitudes ranging from 0 to 1,200 m. The most suitable habitats for breeding were: (1) cultivated areas with hedgerows, and (2) high altitude grasslands. Similar population densities were recorded in both habitats (0.27 pairs/10 ha in farmland vs. 0.30 pairs/10 ha in meadows) and as were the number of fledged young per breeding pair (3.38 in farmland vs. 3.75 in meadows). The structural characteristics of 'open space' and 'edge density' differed in the two breeding habitats. Use of species of trees and bushes for nesting depended on habitat type, but nests were in the more abundant thorny shrubs (blackthorn [Prunus spinosa] in farmland and juniper [Juniperus communis] in meadows). Red-backed Shrikes in farmland appear to prefer to nest in the most heterogeneous territories, those with the presence of uncultivated areas and shrub patches. Plasticity of habitat selection by the species was evident.|
|Subject:||Landscape architecture (Research)|
|Publication:||Name: The Wilson Journal of Ornithology Publisher: Wilson Ornithological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2012 Wilson Ornithological Society ISSN: 1559-4491|
|Issue:||Date: March, 2012 Source Volume: 124 Source Issue: 1|
|Topic:||Event Code: 310 Science & research|
|Geographic:||Geographic Scope: United Kingdom Geographic Code: 4EUUK United Kingdom|
Characteristics of breeding territories of Redbacked Shrikes
(Lanius collurio) have previously been described in a number of studies
(Cramp and Perrins 1993, Olsson 1995b, Lefranc and Worfolk 1997, Harris
and Franklin 2000, Guerriere and Castaldi 2006, Casale and Brambilla
2009). The greatest density of breeding pairs in Europe is in farmland
ecosystems (Cramp and Perrins 1993, Farkas et al. 1997, Golawski and
Golawska 2007) with these landscapes providing highly varied ecological
conditions for many bird species and their prey (Golawski 2006). The
Red-backed Shrike has been shown to prefer agricultural landscapes that
are 'not intensively farmed' and are characterized by reduced
functional heterogeneity. This is especially true when there are
hedgerows and shrub patches that function as both key habitat for many
plants and animals, and wildlife corridors that enable dispersal and
movement between habitats (Vermeulen 1994, Vermeulen and Opdam 1995).
Habitats used for breeding have an essential role in the life-cycle of a bird, and many factors can influence the selection of breeding territories; the two most important of which are minimization of predation (Cody 1985, Martin 1995, Roos 2002) and presence of adequate food (Martin 1987, Cramp and Perrins 1993, Golawski and Meissner 2008).
The Red-backed Shrike is considered a farmland breeding species in the Marche Region of central Italy (Pandolfi and Giacchini 1995, Forconi 2007, Morelli et al. 2007). Hedgerows are a familiar feature of most farmlands, and were traditionally planted as boundaries by both farmers and townspeople, albeit for different purposes. Currently, in areas which once had much greater coverage, they are now only present as residual boundary lines of trees and shrubs, and are slightly >20 m in length and <5 m in width. Recent censuses indicate that Red-backed Shrikes may, however, use other habitat types for breeding in the Marche Region of central Italy (Forconi 2007, Morelli et al. 2007).
Detailed information about habitats and ecological requirements of a species is needed if it is to be successfully conserved. This is especially true for the Red-backed Shrike which, except for a few areas of relative stability (PECBMS 2009), has experienced a marked population decline in western and northern Europe over the last three decades. The Italian population has been estimated at 50,000 to 120,000 reproductive pairs (Birdlife International 2004), but with a negative trend (Meschini and Frugis 1993, Pandolfi and Giacchini 1995, Dinetti 1997).
The causes of this decline are poorly understood, and it has been suggested that reduction in suitable habitats, habitat modifications, use of intensive agriculture systems, decline in food resources, and climatic changes are the main reasons (Tucker et al. 1994, Yosef 1994, Fuller et al. 1995). Rapid changes in agricultural ecosystems can result in the loss of a bird species in just a few years. Thus, it is important to examine the ability of a species to colonize and breed in areas other than agricultural farmland. This is because the capacity to breed or forage in a greater number of habitat types can improve the survival potential of birds which settle in a territory that includes an optimal proportion of habitats suitable for successful breeding and survival (Soderstrom 2001, Golawski and Meissner 2008).
Study Area.--The study was conducted during 2009 in the Marche Region of central Italy, which is characterized by a temperate climate, high spring and summer temperatures, and a marked summer drought (Tomaselli et al. 1972). The study area contained two breeding zones of 5,573 ha total (farmland: 43[degrees] 47' 27.81" N, 12[degrees] 39' 39.91" E; 4,975 ha; high altitude meadows: 43 [degrees] 37' 51.84" N, 12[degrees] 45' 48.18" E; 598 ha). Monitoring Frequency.--The study area was monitored for breeding Red-backed Shrikes every 5 days from early May to mid-July 2009. Occupied territories were surveyed at least once every 3 days during the breeding period to find nest sites (Bibby et al. 1997). Red-backed Shrikes are particularly sensitive to disturbance during nest building, egg-laying, and early incubation (Olsson 1995a, Tryjanowski and Kuzniak 1999), and the number of visits to each nesting site was kept to a minimum. Location of each nest site was recorded using a Global Positioning System (GPS) and numbers of fledged young per pair near a nest were recorded. The density of Red-backed Shrike territories in the two habitat types was calculated as breeding pairs per 10 ha.
Environmental Parameters.--The typical size of a Red-backed Shrike territory varies from 1-2 to 5-6 ha (Cramp and Perrins 1993, Lefranc 1993, Olsson 1995b, Harris and Franklin 2000). A 3-ha territory size around the nest site was standardized to enable territory preferences to be studied. ArcGIS 9 software (ESRI 2009) was used to delineate a polygon with a 100-m radius around the nest site, and a land-use map (1:10,000) (AA.VV. 2001) was used to characterize the habitat in the polygons.
The environmental parameters studied for each nest site were: (1) nest shrub: the plant species where breeding pairs placed their nest and were active in defending the territory; (2) relative abundance of each plant species in the total study area as: very low (1 item), low (2 to 5 items), medium (6 to 20 items), high (21-50 items), and very high (>50 items); (3) the altitude of the nesting site (m above sea level); and (4) composition and percentage of land-use types around the nesting area (cultivated, uncultivated, vineyard, shrub, forest, reforestation, grassland, river, buildings, and roads)
The features and structural characteristics studied for each nest site were: (1) 'open space' around the nesting site: percentage of cover of roads, grasslands, uncultivated land, and shrubs; (2) 'edge density' as the sum of the perimeters of all polygons in the buffer zone per number of land-use types/100 (as a surrogate of the habitat fragmentation level in the buffer zone); (3) road type (paved, unpaved); (4) distance from the nearest road (m); and (5) distance to the nearest building (m).
Statistical Analysis.--Differences in the plant species used for nesting in the two habitat types were compared with Chi-square tests using standardized residual analyses to highlight the associations of plant species with habitat types. A Mann-Whitney U-test was also performed to examine differences in environmental parameters of breeding sites in farmland areas versus those in high altitude meadows. The structural characteristics (open space and edge density) between the two habitats were further compared with a Chisquare test.
I also performed a logistic regression analysis to identify which variables best explained the presence or absence of Red-backed Shrikes in the farmland study area. Presence and absence of active nests were used as response variables, while composition and percentage of land-use types around the nesting area, structural characteristics, and road and building distance were the explanatory variables. Internal validation was performed by calculating the non-parametric confidence interval of the AUC (Area Under the Curve) values. The AUC generally ranges from 0.5 for models with no discrimination ability to 1.0 for models with perfect discrimination. Data are presented as means [+ or -] SE and all tests were conducted with SPSS for Windows software, Version 17.0 ([c] SPSS Inc., Chicago, IL, USA. www.spss.com).
I monitored and characterized 150 Red-backed Shrike nest sites in two breeding zones: 132 in farmland (mean altitude = 347.2 + 138.8 m asl) and 18 in high altitude meadows (mean altitude = 837.6 [+ or -] 115.5 m asl) (Fig. 1). The density of breeding pairs per 10 ha was 0.27 for farmland and 0.30 for high altitude meadows, respectively. The number of fledged young per pair was similar in the two habitats: 3.38 [+ or -] 1.05 (n = 31) for farmland and 3.75 [+ or -] 0.86 (n = 12) for meadows. These differences were not significant (U = 148, P = 0.282).
[FIGURE 1 OMITTED]
Plant Species Used as Nest Sites.--All nest sites were in shrubs. Plant species used differed in the two breeding habitats ([chi square] = 71.3, P < 0.05) with blackthorn (Prunus spinosa) being most (48.5%) used in farmland followed by dog rose (Rosa canina; 25.8%), elm-leaf blackberry ( Rubus ulmifolius; 12.1%), and common hawthorn (Crataegus monogyna; 8.3%) (Table 1). Plant species used most frequently in high altitude meadows were juniper (Juniperus communis; 50%), blackthorn (22.2%), elm-leaf blackberry (11.1%), European beech (Fagus sylvatica; 11.1%), and dog rose (5.6%) (Table 1).
Land-use Cover in Farmland and High Altitude Meadow Breeding Sites.--The percent cover of the types of land-use around nest sites differed between the two breeding habitats studied. Up to six habitat types (mean = 3.7) were recorded for farmland with cultivated land, shrubs, and grassland dominating (Table 1). Five habitat types (mean = 2.9) were recorded in meadows, where grassland and forest prevailed (Table 1).
Characteristics and Structural Differences Between Breeding Habitats.--Open space and edge density values differed between the two areas used for breeding ([chi square] = 47.3, P < 0.05). The percentage of open space in high altitude meadows (67.0 [chi square] 28.2%) was higher than in farmland (31.3 [chi square] 26.5%). In particular, the main types of land-use cover functioning as open space in farmland were roads, and uncultivated land, while in meadows the main type was grassland. The edge density value in farmland (93.4 [+ or -] 45.8) was higher than in meadows (50.9 [+ or -] 54.3).
There were significant structural differences between the two habitats in terms of distance of nests from the nearest road and building (the shortest distance was in farmland) (Table 2). The model produced using stepwise logistic regression analysis was based on 35 nest sites selected at random from the farmland breeding habitat and 35 other sites where the species had not been present (real absence) within the same studied farmland. Only five variables had a substantial effect on probability of the occurrence of the species in farmland (Table 3). These were: edge density, open space, uncultivated land, and shrubs, forests, and grassland. The first three had a positive effect on the probability of the occurrence of Redbacked Shrikes, whereas the other two had a marginally negative effect.
Red-backed Shrikes seem to be much more common in low-altitude farmland than in mountain environments in the Marche Region of central Italy (Forconi 2007, Morelli and Pandolfi 2009, Morelli 2011a), as is also the case in other parts of the country. However, the results of my study indicate that Red-backed Shrikes can use very different environments and select, if available, high meadow areas for nesting.
The two populations studied were in very different altitude zones with farmland birds breeding at a mean elevation of 350 m, while the meadow population was at an elevation >800 m asl. However, population densities of shrikes were similar in both areas, as were numbers of fledged young per pair.
These results highlight the ecological plasticity of Red-backed Shrikes in selecting breeding habitats, as this species can use different types of landscape in central Italy for breeding. This suggests these two habitat types meet the ecological requirements of Red-backed Shrikes.
Farmlands where Red-backed Shrikes were detected were characterized by higher landscape heterogeneity (edge-density values, roads and buildings in the vicinity), reducing risks from predators or providing natural hunting perches (Lefranc 1993, Yosef and Grubb 1994, Lefranc and Worfolk 1997, Bechet et al. 1998, Tryjanowski et al. 2000, Roos 2002, Roos and Part 2004). Variables that appear to have made a major contribution to the suitability of farmland as breeding habitat for Red-backed Shrikes were edge density, which indicates a certain functional heterogeneity (Benton et al. 2003), open spaces, uncultivated land, and shrubs for nesting. In contrast, the species does not seem to favor farmland with forest patches or grassland cover.
Meadows frequented by Red-backed Shrikes were less fragmented than the farmland. They also contained virtually no roads or building structures, but abundant shrubs were uniformly distributed across open grasslands. These high altitude meadows can provide another important resource to maximize foraging ecology of shrikes in the increased availability of 'open space', as open grasslands at high altitude are optimal for finding and capturing prey (Fernandez-Juricic et al. 2004, Golawski 2006, Morelli 2011b). Recent studies reveal that meadows, in comparison with other habitat types, support the highest number and biomass of invertebrate prey of Red-backed Shrikes (Golawski and Meissner 2008).
'Open space' had different features in each type of breeding habitat (mainly roads in farmland and grassland in meadows). The Red-backed Shrike used different kinds of shrubs in which to nest and perch in the two habitats; generally, the most abundant, suitable species (blackthorn in farmland and juniper in meadows).
These characteristics, which highlight the Redbacked Shrike's relative ecological plasticity, could be important when defining 'suitable habitat' and 'habitat availability' for the species. These findings should be used to identify new potential breeding sites and expansion areas for Red-backed Shrikes in central Italy. Future studies focused on breeding success of shrikes in different habitat types may be necessary if we are to obtain information required to understand the biological requirements of this species. This information may have an important role in the success of bird conservation programs (Hoffmann and Greef 2003, Salem 2003, Morelli et al. 2007).
We thank Maurizio Saltarelli and Chiara Tagnani for help in the field, and Marco Girardello and Maria Balsamo for valuable suggestions on the text. A special note of thanks is owed to the anonymous reviewers for suggesting several improvements in the original manuscript.
Received 21 June 2011. Accepted 14 September 2011.
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FEDERICO MORELLI (1)
(1) DiSTeVA, University of Urbino, Scientific Campus, 61029 Urbino, Italy; e-mail: email@example.com
TABLE 1. Plant species used for nest sites by Red-backed Shrikes in farmlands and high altitude meadows in central Italy. Farmland Species Name n % Abundance Clematis vitalba Clematis 3 2.3 Low Cratagus monagyna Common hawthorn 11 8.3 Medium Fagus sylvatica Beech Juniperus communis Juniper Paliurus spina-christi Christ's thorn 1 0.8 Very low Prunus spinosa Blackthorn 64 48.5 Very high Robinia pseudoacacia Black locust 1 0.8 High Rosa canina Dog rose 34 25.8 High Rubus ulmifolius Elm-leaf hackberry 16 12.1 High Sambucus nigra Elderberry 2 1.5 Medium High altitude meadows Species n % Abundance Clematis vitalba Cratagus monagyna Fagus sylvatica 2 11.1 Low Juniperus communis 9 50.0 Very high Paliurus spina-christi Prunus spinosa 4 22.2 Medium Robinia pseudoacacia Rosa canina 1 5.6 Medium Rubus ulmifolius 2 11.1 Medium Sambucus nigra TABLE 2. Characteristics of environmental parameters of farmland and high altitude meadows used by Red-backed Shrikes for breeding in central Italy. Characteristics Farmland (n = 132) Meadows (n = 18) Building, % 3.4 [+ or -] 6.8 0.8 [+ or -] 2.6 Cultivated, % 57.4 [+ or -] 26.1 2.4 [+ or -] 3.9 Vineyards, % 0.9 [+ or -] 3.6 0.0 [+ or -] 0.0 Forest, % 6.7 [+ or -] 13.2 27.6 [+ or -] 28.4 Grasslands, % 14.2 [+ or -] 21.5 65.2 [+ or -] 27.9 Uncultivated 11.0 [+ or -] 19.9 0.6 [+ or -] 2.1 and shrubs, % Water, % 0.3 [+ or -] 1.4 2.2 [+ or -] 8.4 Roads, % 6.0 [+ or -] 3.2 1.2 [+ or -] 2.6 Landuse 3.7 [+ or -] 1.2 2.9 [+ or -] 1.1 categories, number Near road, m 30.6 [+ or -] 43.7 145.6 [+ or -] 123.6 Near building, m 186.4 [+ or -] 136.7 781.6 [+ or -] 475.2 Characteristics U P Building, % 862.0 0.028 Cultivated, % 64.0 0.000 * Vineyards, % 1,080.0 0.184 Forest, % 490.0 0.000 * Grasslands, % 189.0 0.000 * Uncultivated 818.5 0.016 and shrubs, % Water, % 1,052.5 0.070 Roads, % 362.0 0.000 * Landuse 692.5 0.003 * categories, number Near road, m 369.5 0.000 * Near building, m 355.5 0.000 * * Significant at P < 0.01. TABLE 3. Logistic regression of presence and absence of breeding Red-backed Shrikes in farmlands (only significantly associated variables are shown). Variable B SE P Edge density 0.05 0.012 <0.001 Open space 45.12 17.67 0.011 Uncultivated and shrub 1.13 0.525 <0.001 Forest -1.12 0.436 0.012 Grassland -21.42 9.21 0.05
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