Biodiversity and Conservation 14: 81–95, 2005.
# Springer 2005
Using satellite imagery to assess breeding habitat availability of the endangered loggerhead shrike in Que´bec BENOIˆT JOBIN*, MARCELLE GRENIER and PIERRE LAPORTE Canadian Wildlife Service, Environment Canada, 1141 route de l’E´glise, P.O. Box 10100, Sainte-Foy, Que´bec, Canada G1V 4H5; *Author for correspondence (e-mail:
[email protected]) Received 1 April 2003; accepted in revised form 29 September 2003
Key words: Habitat use, Landscape, Lanius ludovicianus, Loggerhead shrike, Pastureland, Que´bec, Satellite imagery Abstract. The loggerhead shrike (Lanius ludovicianus) is a grassland bird species whose preferred nesting habitat in eastern Canada is pastureland. This species has been extirpated from much of its historical range in this region, and breeding habitat loss is suspected to be an important cause of this decline. We evaluated the availability of suitable breeding habitats in Que´bec using satellite imagery. Because this species no longer breeds in Que´bec, we established habitat selection criteria from known nesting sites in the adjacent province of Ontario, from analysis of a Landsat-TM satellite image, and applied these criteria to Landsat-TM images covering southern Que´bec. We developed regional landscape criteria in 100 km2 plots and patch indices criteria at the pasture level. Spatial analyses were conducted to characterize plots and pastures on the basis of pasture availability and spatial distribution. Pastures suitable for nesting loggerhead shrikes were those fulfilling patch criteria at the pasture level and located in plots fulfilling regional landscape criteria. Overall, 310 out of 1700 plots located in the historical breeding range of the loggerhead shrike in Que´bec fulfilled landscape criteria, supporting 3988 pastures that fulfilled patch criteria. More than 500 of these pastures were visited to validate their current status. The Outaouais region would be the most suitable region for nesting loggerhead shrikes in southern Que´bec, where suitable breeding habitat still remains because more than two-thirds of visited sites were still pastureland, hawthorns were well-distributed in the region, and pastureland fragmentation was lowest. We conclude that the availability of breeding habitat does not limit the establishment of a breeding population of loggerhead shrike in southern Que´bec, as we estimated that thousands of hectares of suitable habitat still remain in that province.
Introduction The loggerhead shrike (Lanius ludovicianus) is an open-landscape passerine whose former breeding range extended throughout the US, southern Canada and northern Mexico. In the early 20th century, the subspecies L. l. migrans was a common breeder in southern Que´bec and Ontario, having extended its breeding range in the northeast along with the expansion of agricultural lands. Populations started to decline in the middle of the century in Que´bec (Robert and Laporte 1991) and recent population estimates have shown continuous decreasing trends in Canada (Downes and Collins 1996) and rangewide (Pruitt 2000). The loggerhead shrike has now become one of the rarest breeding bird species in Que´bec (the last nest was
82 discovered in 1995), and only solitary individuals have been reported each spring in this province in recent years. In Ontario, where the loggerhead shrike was also common in the past, range contraction has also been extensive, and the current breeding population is estimated at 30–40 pairs, concentrated in the Napanee (northwest of Kingston) and Carden (north of Toronto) regions. The migrans subspecies is now designated as endangered in Canada (COSEWIC 2003). Destruction of breeding and wintering habitats, direct and indirect impacts of pesticide use, intra- and interspecific competition, pastureland fragmentation, and roadkills have often been suspected to be responsible for the loggerhead shrike population decline (Cadman 1990; Robert and Laporte 1995; Yosef 1996; Cade and Woods 1997; Pruitt 2000). However, Cade and Woods (1997) argued that habitat modification on both breeding and wintering areas is the main reason for this decline. The preferred breeding habitat of the loggerhead shrike in eastern Canada consists of open areas with scattered thorny shrubs or coniferous trees, hawthorn (Crataegus spp.) being the shrub species mostly used for nest support in eastern Canada and adjacent New York state (Novak 1995; Robert and Laporte 1995; Chabot et al. 2001b), and a low herbaceous layer for foraging. Pastureland is particularly suited to the loggerhead shrike because livestock grazing maintains the vegetation layer at low heights throughout the nesting season. However, as observed in other agricultural regions of North America, agricultural intensification has greatly modified the rural landscape of southern Que´ bec over the last 50 years with concurrent impacts on the breeding avifauna (Jobin et al. 1996). The mosaic pattern of small cultivated fields intermixed with pastureland has turned into vast and uniform row crop fields, mostly corn and soybeans, which now cover roughly 85% of the agriculture land, leaving only 15% pastureland. In intensive farming regions, natural pastures have been converted to seeded pastures and hayfields to increase forage crop yields. Surface area covered by pastureland has thus been greatly reduced in Que´ bec during that period when it covered approximately 3000 km2 in 1990 as compared to nearly 20,000 km2 in 1941 (Robert and Laporte 1991). Concomitantly, many of the dairy farms located in less productive regions were abandoned and this has exacerbated breeding habitat loss. Because of its endangered status, a national recovery plan for the eastern Canadian population of the loggerhead shrike has been produced aiming at preserving and increasing the size of the breeding population (Johns et al. 1994). One objective of the plan is to evaluate breeding habitat availability in the species’ former breeding range, especially in southern Que´ bec where the spatial distribution of suitable habitat is largely unknown. We initiated this study to locate and quantify loggerhead shrike breeding habitats in southern Que´ bec. The objectives of our study were (1) to characterize, at both the landscape and the local scales, breeding habitats of the loggerhead shrike at known breeding sites in Ontario, (2) to identify regions of southern Que´ bec with suitable breeding habitats based on habitat selection criteria determined at Ontario nesting sites, and (3) to evaluate breeding habitat fragmentation in Ontario and Que´ bec.
83 Methods We used satellite imagery to locate regions with high pasture densities because the study area covers a large geographic region and this tool is very powerful to study habitat distribution over vast regions (Scott et al. 1993; Grenier et al. 1994; Dettmers and Bart 1999). Because breeding habitat selection acts at different spatial scales in birds (Cody 1985; Michaels and Cully 1998; Mazerolle and Villard 1999; Saab 1999), the approach we followed consisted of (1) determining loggerhead shrike breeding habitat selection criteria at both the pasture and landscape scales from known nesting sites in Ontario using satellite imagery, (2) applying these criteria to satellite images covering southern Que´ bec, (3) ground-truthing selected potential sites, and (4) mapping and identifying regions where suitable breeding habitats are located. Pastures suitable for breeding loggerhead shrikes in Que´ bec are those fulfilling patch criteria at the pasture level (local scale) and located in plots fulfilling regional landscape criteria.
Selection criteria at the local scale We first classified a Landsat-TM image covering the Kingston-Napanee region, Ontario, dating from June 1995, and using a supervised classification method (see Be´ langer et al. 2002). The classification was carried out with the six spectral bands and using a maximum likelihood algorithm. The area covered by the image was 7200 km2, extending from Brighton (448020 N; 788450 W) to Gananoque (448200 N; 768080 W) on the longitudinal axis and from Upper Rideau Lake (448400 N; 768200 W) to Picton (448010 N; 778080 W) on the latitudinal axis. Ground-truthing of sites classified as pastures in the Napanee region revealed no misclassification of this habitat class at nine visited control points. We overlaid on the image loggerhead shrike observations compiled in the region between 1991–1997 by the Ontario Ministry of Natural Resources and 14 pastures used as nesting sites during that period were located and characterized using Patch Analyst (Elkie et al. 1999) and FRAGSTATS softwares (McGarigal and Marks 1994). Spatial variables calculated for these 14 pastures were patch area (AREA), edge length (PERIM), perimeter– area ratio (PAR), shape index (SHAPE), fractal dimension (FRACT) and distance to nearest neighbor (NEAR, min ¼ 25 m). Because landscape indices are often inter-correlated (Riitters et al. 1995; Cain et al. 1997), we selected pasture criteria by analyzing a matrix of Pearson’s correlation coefficients calculated among these variables. Variables PERIM, SHAPE and FRACT were highly correlated with AREA (PERIM: r ¼ 0.97; SHAPE: r ¼ 0.85; FRACT: r ¼ 0.72) and were eliminated, whereas the variable NEAR had only minor influence on pasture selection and was also eliminated. We used minimum or maximum values of the two selected variables as selection criteria at the local scale to identify characteristics of all pastures that could potentially be used as nesting sites by loggerhead shrikes in the study region. These pastures had to be >5 ha in size (AREA) (min ¼ 4.6) with a low perimeter–area ratio
84 (PAR 250 m=ha; max ¼ 200). Setting the minimum pasture size at 5 ha is representative of the minimum pasture size used as nesting sites in Que´ bec (Robert and Laporte 1995) and elsewhere in North America (Yosef 1996). The other pasture selection criterion aimed at minimizing perimeter–area ratio, thus selecting sites with regular shape and rejecting elongated pastures. In their study of site selection by grassland birds in Nebraska, Helzer and Jelinski (1999) showed that this variable was a better predictor of grassland bird presence and diversity than pasture size, but the loggerhead shrike was not considered in their analysis.
Selection criteria at the landscape scale It is generally acknowledged that high pastureland coverage is an important factor in nest site selection by loggerhead shrike with nests located in pastures exceeding 5 ha and surrounded by a minimum of 50 ha of suitable habitats (Brooks and Temple 1990a; Gawlick and Bildstein 1990; Chabot et al. 2001b). To describe habitat use at the landscape scale, we divided the satellite image into 100 km2 plots (10 10 km) based on Universal Transverse Mercator (UTM) coordinates, and an arbitrary grid of 91 plots was developed starting at the bottom-left corner of the image. We calculated spatial features of pastures using FRAGSTATS software for the 12 plots where shrikes have been observed in pastures. These variables were total pasture area (CA), number of pastures (NP), pasture density (PD), total pasture core area (TCA; with edge effect estimated to be effective within 50 m), mean pasture area (MPS), overall pasture shape index (LSI), mean pasture shape index (MSI), mean nearest-neighbor distance (MNN; calculated within a 500 m radius based on the estimated 50 ha of suitable breeding habitats adopted in management plans for the loggerhead shrike in Ontario (Chabot et al. 2001b)); number of pastures for each of the following patch sizes: >5, >10, >25, >50, and >100 ha (referred to as nb_5 ha, nb_10 ha, nb_25 ha, nb_50 ha, nb_100 ha hereafter). As we did for pasture selection criteria, we discarded some landscape variables because they were highly inter-correlated. The variables NP, LSI, MSI, MNN, nb_5 ha, nb_10 ha, and nb_25 ha were eliminated because they were highly correlated with the variable CA (NP: r ¼ 0.79; LSI: r ¼ 0.83; MSI: r ¼ 0.87; MNN: r ¼ 0.85; nb_5 ha: r ¼ 0.92; nb_10 ha: r ¼ 0.96; nb_25 ha: r ¼ 0.94) whereas the variables nb_50 ha and nb_100 ha were too restrictive in plot selection and were eliminated. We thus selected four habitat selection criteria at the landscape scale using minimum or maximum values of these variables as the threshold to identify all 100 km2 plots that could potentially be used during the nesting season by loggerhead shrikes in the study region. Total (CA) and core area (TCA) cover of pastureland within these plots had to be 325 ha (min ¼ 345) and 65 ha (min ¼ 68), respectively, pasture density (PD) had to be 85=100 ha (max ¼ 81) with mean pasture size (MPS) 1.20 ha (min ¼ 1.20). These four variables thus aimed at maximizing pasture cover in 100 km2 plots while minimizing the number of selected pastures. We adopted a conservative approach in using minimum or maximum values as threshold to identify suitable plots and these values were somewhat
85 lowered (or increased) to take into account the precision of the satellite images and of the shrike observation coordinates.
Application of selection criteria to Que´ bec images We classified seven Landsat-TM images covering southern Que´ bec and dating from the summers of 1993 and 1994, using a supervised classification method (Be´ langer et al. 2002) with an average image classification accuracy of 86% based on calculated confusion matrices (Story and Congalton 1986). Pasture classification accuracy was also 86% with misclassified sites being largely confused with hayfields. The images covered the agriculture-dominated St. Lawrence Valley and the foothills of the Laurentians and the Appalachians mountains on the north and south shores of the St. Lawrence River, respectively. Intensive agriculture (corn, cereals, soybean, vegetables) prevails in the southwestern regions, whereas dairy farming dominates in eastern regions and in the Outaouais (Ottawa River Valley). We overlaid an arbitrary grid of 1700 100 km2 plots on the images and we calculated spatial indices describing pasture features in each plot and each pasture’s characteristics as on the Ontario image to identify suitable pastures located in suitable plots.
In situ validation of pasture status Because the inherent resolution of satellite images (30 m pixel) prevented us from detecting fine-scale habitat features important to nesting shrikes such as isolated shrubs or low grassy vegetation height, we conducted ground-truthing of potential nesting sites identified on the Que´ bec images in the summer of 2000. We first divided the study area into three dominant agricultural regions based on farming intensity and soil capability classification for agriculture (Environment Canada 1972) and following current agricultural landscape definitions for southern Que´ bec (Jobin et al. 2003). The St. Lawrence Valley was divided in two regions, dominated by cash crops and dairy farming, respectively, whereas the third region is the dairy farming-dominated region of the Outaouais located north of the Ottawa River (Figure 1). We randomly selected plots within each region, and we visited every pasture that fulfilled pasture selection criteria within these plots. We characterized each pasture visited either as a natural or seeded (tame) pasture, cash crop, hayfield, oldfield or other. Natural pasture refers to grazed sites containing isolated shrubs and rocky outcrops unsuitable for ploughing, whereas seeded pastures are generally part of crop rotations and occur on good quality soil that can be ploughed and receive subsurface drainage. Hawthorn presence was noted when observed. We used contingency table analyses to compare the relative importance of natural pastures, seeded pastures and other habitats among regions.
86
Figure 1. Plots (100 km2) fulfilling landscape selection criteria in southern Que´ bec (n ¼ 359) and in the Napanee region in Ontario (n ¼ 19). Density of pastures fulfilling pasture selection criteria is also illustrated, as are the three agricultural sectors of southern Que´ bec. The shaded region is the cash crop dominated agricultural region in the St. Lawrence Plain.
Estimation of total pasture cover and fragmentation Results from in situ validation of pasture status in the selected plots were extrapolated to all plots in southern Que´ bec and allowed us to estimate the total area of pastures and the total area of natural pastures in each plot. We developed two indices for each visited plot, the first to calculate the relative stability of agriculture and the second to evaluate the abundance of natural pasture. We defined the stability of agriculture as the ratio of the area covered with pasture as estimated visually during the summer 2000 visit relative to the pasture area as calculated from the satellite image (stability ¼ total pasture area in 2000=CA). Plots where crop rotation is not common practice should present a high stability index. The index developed to evaluate the importance of natural pasture was calculated as the area covered by natural pastures divided by the total pasture cover (natural þ seeded) in each plot as determined during the summer 2000 visit. We compared the mean value of these indices among regions with the non-parametric Kruskal–Wallis test (variables were not normally distributed and variances were not homogeneous due to low sample sizes).
87 Having calculated the mean value of both indices for each of the three agricultural regions and knowing the total area of pasture in each of the selected 100 km2 plots as calculated from the satellite images, we estimated the cover of ^ for each plot as: natural pastures (PN PN) ^ ¼ CA mean stabilityregion mean natural pasture importanceregion PN and mean values were compared among regions with the Kruskal–Wallis test. Finally, we assessed pasture fragmentation in each 100 km2 plot following Vogelmann’s (1995) index expressed as ln (SCA=STE); with CA ¼ total pasture area and TE ¼ total length of pasture perimeter (edge); a high value indicating a low level of pasture fragmentation. We anticipated that the satellite images used in this study would prove useful in evaluating pastureland fragmentation because the information gathered from the same images was very efficient at evaluating forest fragmentation in southern Que´ bec (Be´ langer and Grenier 2002). This fragmentation index was calculated for all plots located in southern Que´ bec and on the Ontario image and we used the Student t-test to compare the fragmentation index between selected and non-selected plots. Results Application of selection criteria to Que´ bec images Among the 1700 100 km2 plots covering southern Que´ bec, spatial features of pastures were calculated for each of the 823 plots holding at least one pasture. We applied selection criteria to these plots, and a total of 359 plots fulfilled these criteria, mostly distributed on the south shore of the St. Lawrence River and in certain areas of the Lower Laurentians north of Montre´ al and in the Outaouais (Figure 1). Only 5559 (2.4%) of the 230,209 pastures observed on the Que´ bec images fulfilled pasture selection criteria, of which 4460 (80%) were located in the 359 plots fulfilling landscape selection criteria. One-third of pastures >5 ha and 77% of pastures >25 ha were selected and most (>80%) were located in these plots. Regions with high numbers of selected plots were also those holding plots with high densities of selected pastures.
In situ validation of pasture status From the 359 plots fulfilling landscape selection criteria, we eliminated 49 plots located outside the loggerhead shrike’s historical breeding range in southern Que´ bec (Cadman 1990; Robert and Laporte 1995), leaving 310 plots which could be selected for ground-truthing and which held a total of 3988 potentially suitable pastures. We randomly selected 40 plots, and 524 pastures were visited. More than two-thirds of sites in the two dairy farming-dominated regions were pastures (natural or seeded), whereas more than half of the visited sites in the cash crop
88
Figure 2. Status of the 524 visited sites in the three agricultural regions of southern Que´ bec in the summer of 2000.
region were not (Figure 2; w2 ¼ 26.98; P < 0.0001). Most sites identified as pastures on satellite images and which had a different status in the summer of 2000 were covered with hayfields in the two dairy farming regions, whereas cropfields were generally present on such sites in the cash crop region. Nine sites could not be visited because of road inaccessibility or permission was not granted by landowners. Natural pastures were more prevalent (w2 ¼ 41.08; P < 0.0001) in the Outaouais region (34% of all sites) than in the two agricultural regions of the St. Lawrence Valley, where only 22% and 8% of sites were natural pastures in the dairy farming and cash crop regions, respectively (Figure 2). We observed hawthorns in 37% (23=62) of sites in the Outaouais as compared to 19% (16=86) and 10% (19=182) of sites in the cash crop and dairy farming regions of the St. Lawrence Plain, respectively (w2 ¼ 22.77; P < 0.0001). Hawthorn presence was similar between the latter two regions (w2 ¼ 3.43; P ¼ 0.0641). The stability of agriculture index had similar mean values in the two dairy farming-dominated regions of the Outaouais and the St. Lawrence Plain (0.60, SE ¼ 0.02; and 0.62, SE ¼ 0.05; respectively) which were higher (H ¼ 5.58; P ¼ 0.06) than the mean value of the plots located in the cash crop region (0.43, SE ¼ 0.06). The mean value of the index developed to evaluate the importance of natural pasture index was 0.61 (SE ¼ 0.10) for the Outaouais region whereas it was only 0.34 (SE ¼ 0.05) and 0.12 (SE ¼ 0.05) for the dairy farming and cash crop dominated regions of the St. Lawrence Plain, respectively (H ¼ 13.11; P ¼ 0.001). These two indices allowed us to estimate the total area of natural pastures in each of the 310 selected 100 km2 plots. Estimates of natural pasture cover ranged from 0 to 481 ha per plot, the Outaouais region holding plots with high estimated cover values as opposed to plots in the cash crop region where natural pasture cover estimates are very low (Figure 3). As such, mean estimated cover of natural pasture per plot
89
Figure 3. Estimated cover (ha) of natural pastures in the 310 selected 100 km2 plots located in the loggerhead shrike historical breeding range in southern Que´ bec. The shaded region is the cash crop dominated agricultural region in the St. Lawrence Plain.
Table 1. Estimated cover (mean and standard error) of natural pastures in the 310 selected 100 km2 plots in the three main agricultural regions of southern Que´ bec. Region
Estimated cover of natural pasture (ha) n
Mean
SE
Total cover
Outaouais–dairy St. Lawrence–dairy St. Lawrence–cash crop
44 174 92
152.7 42.9 14.0
15.8 2.1 1.5
6719 7472 1292
Total
310
49.9
3.6
15483
was 10 times higher in the Outaouais than in the cash crop region of the St. Lawrence Plain (H ¼ 172.83; P < 0.001) (Table 1). Pasture fragmentation was high in the Appalachian regions along the Canada– USA border, where agricultural activity is fairly marginal and where few plots were selected (Figure 4). In addition, pastureland in plots where intensive agricultural activity dominated such as in the Richelieu drainage basin was also highly fragmented. On the other hand, pasture fragmentation was lowest in the Outaouais and in the lowlands of the Laurentians and the Appalachians. Plots fulfilling landscape selection criteria in southern Que´ bec in the shrike’s former breeding range (n ¼ 310) showed lower pasture fragmentation (3.31, SE ¼ 0.01) than non-selected
90
Figure 4. Pastureland fragmentation in the 100 km2 plots in southern Que´ bec and in the Napanee region in Ontario as calculated from Landsat-TM satellite images, 1993–1995 (a high value is associated with low pastureland fragmentation; see text for details).
plots (3.09, SE ¼ 0.01, n ¼ 312) (t ¼ 17.46; P < 0.0001), and this difference was also observed in each of the three agricultural sectors. Pasture fragmentation was also lowest in plots covering the Napanee region, where one of the core breeding populations of the loggerhead shrike still remains in Ontario, and the mean calculated index (3.39, SE ¼ 0.02, n ¼ 19) was significantly different than in plots not fulfilling landscape selection criteria (2.94, SE ¼ 0.03, n ¼ 69) (t ¼ 12.61; P < 0.0001).
Comparison of suitable sites between Ontario and Que´ bec We compared landscape patterns and the spatial distribution of pasture in the selected 100 km2 plots between each of the three agricultural regions of southern Que´ bec and those located in the Napanee region in Ontario (Jobin et al. 2001). Mean values of the selection criteria determined at the landscape and the local scales, as well as pasture fragmentation and the abundance of large pastures (>50; >100 ha), did not differ (Student t-test; P > 0.05) between Ontario plots and those of the Outaouais region. Mean values of selection criteria at the landscape scale also did not differ between Ontario plots and those of the two agricultural regions of the St. Lawrence Plain, but pastures were more fragmented (P < 0.001) in the latter two regions than in Ontario plots. Area of selected pastures was lower (P < 0.0001)
91 in the cash crop and dairy farming regions of the St. Lawrence Plain and perimeter– area ratio of pastures was higher in these regions (cash crop region: P ¼ 0.09; dairy farming region: P < 0.0001) than values of selected pastures in Ontario.
Discussion Although there is no indication that loggerhead shrikes still breed in Que´ bec, we estimated that thousands of hectares of suitable habitat still remain in southern Que´ bec, and we thus concur with others (Cade and Woods 1997; Pruitt 2000) that the availability of breeding habitat does not limit the establishment of breeding populations of loggerhead shrikes. We also believe that remaining pastures are suitable for nesting shrikes because the reproductive success of the latest individuals recorded nesting in southern Que´ bec and eastern Ontario was similar to that observed elsewhere (Pruitt 2000; Chabot et al. 2001a). It has been suggested that loss of non-breeding habitat and low overwintering survival of both adult and juvenile shrikes would likely explain the observed population decline of the migrating populations (Brooks and Temple 1990b; Temple 1995; Cade and Woods 1997), but these hypotheses need further investigation. Threshold values set for landscape and patch selection criteria and fragmentation indices suggest that suitable pastures cover large areas in regions where pasture fragmentation is low, and that pasture shape and fragmentation may be important in nest site selection by the loggerhead shrike. As such, pasture fragmentation was less important in selected 100 km2 plots than that observed in non-selected plots, both in Que´ bec and Ontario. Several studies have shown that habitat selection and use by grassland bird species could be altered by the fragmentation of their preferred nesting habitat (Herkert 1994; Vickery et al. 1994; Freemark et al. 1995; Winter and Faaborg 1999; Johnson and Igl 2001). Habitat fragmentation has not been assessed for the loggerhead shrike in these studies, but its effect has been suggested as a possible factor responsible for the widespread population decline of this species (Johns et al. 1994; Novak 1995; Robert and Laporte 1995; Cade and Woods 1997; Pruitt 2000). Compared with other regions of southern Que´ bec, natural pastures with hawthorns and seeded pastures were more numerous in the Outaouais region located near Ottawa, especially in the western Pontiac region. Pastureland fragmentation was also low in that region, and cash crop fields were not prevalent in the agricultural landscape. The Pontiac region is the most important beef cattle production region in Que´ bec, where farming practices are similar to those observed in Renfrew County on the Ontario side of the Ottawa River where livestock raising also prevails (Statistics Canada 1997). Interestingly, sporadic loggerhead shrike nesting activity was observed over the past 10 years in sites located in this county (A. Chabot, personal communication), which is situated less than 20 km south of the Pontiac. Because suitable habitat still remains in the Pontiac region, it is likely that loggerhead shrikes may still be present there. However, none have been observed during recent bird surveys conducted in natural pastures in that region in June 2001 and 2002 (B. Jobin, unpublished data).
92 In the core breeding area of Napanee, Ontario, loggerhead shrike nests are generally found in eastern red cedars (Juniperus virginiana), whereas historic nesting records in other core areas of Ontario and Que´ bec reveal that nests were mainly built in hawthorns located in hedgerows (Chabot et al. 2001b). Specific pasture features such as nest tree species and hedgerow location are not visible on satellite images. Our study was not designed to identify potential site selection at such high resolution but aimed at locating suitable regions where landscape features would be similar to those of the current nesting regions in Ontario. Unlike the two farming regions of the St. Lawrence Valley, the spatial distribution of pastures in the Outaouais greatly resembles that observed in the Napanee region where the remaining shrike population in eastern Canada remains. We conclude that the Outaouais region is the most suitable region for breeding loggerhead shrikes in Que´ bec. Because our analysis was based on a few selected plots in the Outaouais region, a complete road survey coverage of the region was conducted in fall 2000 in order to map all agricultural habitats on 1:50,000 topographic maps to evaluate and analyze more thoroughly the spatial distribution of natural pastures in the region (Jobin 2003). This provided further support for the suitability of the Outaouais region for the loggerhead shrike, where more than 7500 ha of natural pastures were mapped; a figure similar to that estimated via the satellite image during this study (6700 ha). Because urban sprawl and corn production are rapidly expanding in the area, conservation and stewardship activities aimed at securing large and extensive natural pastures on private lands in the region should be given high priority. As such, landowners’ role in the conservation of endangered species should be recognized and encouraged (Morrison and Humphrey 2001).
Conclusions and conservation implications Because habitat selection in animals likely operates at different spatial scales, habitat management for selected species should consider habitat features per se in conjunction with the spatial distribution of the preferred habitat at the landscape scale (Mazerolle and Villard 1999). Using landscape indices calculated from satellite images was an effective tool to identify the most suitable regions of southern Que´ bec for breeding loggerhead shrikes. Aerial photographs, topographic maps, or finer-scale remotely-sensed images such as IKONOS images could also have been used to achieve similar purposes, but these media are more appropriate for habitat analysis at the regional or local scale (see Lauver et al. 2002) and are generally inadequate for habitat studies covering large areas, for which satellite images such as Landsat-TM images are best suited. Ground-truthing was necessary to validate sites identified as potentially suitable for breeding shrikes. Seven years had elapsed between the year the images were acquired and the ground-truthing campaign of 2000, thus providing ample time for habitat changes to occur due to crop rotation, a farming practice widely applied by cereal growers in the St. Lawrence Plain. On-site validation was also necessary
93 because habitats with similar physical features as pastures such as golf courses, city parks or recent forest clearcuts, may be identified as pastures on satellite images. Although some discrepancies existed between the status of several pastures between the information extracted from the satellite images and the actual status observed during the site visit, especially in cash crop regions, we showed that our approach of identifying loggerhead shrike habitat selection criteria at two spatial scales from known breeding sites, and subsequently applying them to regions where the species formerly bred, was appropriate to identify regions where suitable habitats were concentrated. This approach could easily be transposed to identify concentration of suitable loggerhead shrike breeding habitats in regions where the loggerhead shrike has been extirpated, such as in adjacent New England states (Novak 1995). The selection criteria used during our study could be applied to satellite images covering these regions or the criteria could be regionally adapted. After identifying regions with high potential for nesting shrikes, the distribution of suitable habitats then needs to be refined with the analysis of high resolution aerial photographs (Lauver et al. 2002) or on-site mapping (Jobin 2003). Evaluation of suitable habitat distribution of other species in any taxonomic group could also benefit from our approach, as long as adequate measures of habitat selection criteria can be determined and applied to remotely-sensed images covering large regions. It is especially appealing for studies of habitat use by threatened species because of the relatively few observation records that can possibly be compiled in a given area. Management and conservation activities as well as detailed habitat mapping could thus be oriented towards specific regions of interest. On-site validation of potential suitable sites is essential, especially if the remotelysensed images are outdated, if specific habitat features cannot be distinguished on the images, or if the habitat under study is embedded in highly dynamic landscapes such as agricultural landscapes.
Acknowledgements We thank Luc Be´ langer of the Canadian Wildlife Service for granting us access to Landsat-TM images. We also thank Don Cuddy, from the Ontario Ministry of Natural Resources, who gave us access to loggerhead shrike nesting records in Ontario. We are grateful to Luc Be´ langer, Amy Chabot, Michel Robert and three anonymous reviewers for providing comments on the manuscript.
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