1 1
POPULATION SURVEY OF THE COMMON BUZZARD
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BUTEO BUTEO ON MADEIRA ISLAND (PORTUGAL)
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ESTUDIO POBLACIONAL DEL BUSARDO RATONERO BUTEO
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BUTEO EN LA ISLA DE MADEIRA (PORTUGAL)
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Short title: SURVEY OF THE COMMON BUZZARD ON MADEIRA
9 10 Dorine Yvette Manon JANSEN 1, 2, *
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1
15
Chester Street, Manchester M1 5GD, U.K.
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2
17
Zoology, University of Cape Town, Rondebosch 7701, South Africa.
Manchester Metropolitan University, Division of Biology, John Dalton Building,
Present address: University of Cape Town, Avian Demography Unit, Department of
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* Corresponding author:
[email protected]
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SUMMARY. — A survey was conducted on Madeira Island (Portugal) from 9 May to
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2 June 2008 to determine population size and distribution of the common buzzard Buteo
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buteo. Distance sampling on foot was carried out along 87.6 km of transects. The
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species was present in small numbers in all areas sampled. Population size was
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estimated at 409 (95% CI 275–608) individuals at a density of 0.55 buzzards/km² (95%
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CI 0.37-0.83), based on 39 observations. The present survey sets a baseline for future
7
studies monitoring changes in the common buzzard population on Madeira.
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RESUMEN.—Se realizó un estudio en la isla de Madeira (Portugal), desde el 9 de
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mayo al 2 de junio de 2008, para determinar el tamaño poblacional y la distribución del
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busardo ratonero Buteo buteo. Se recorrieron a pie 87.6 km para efectuar un muestreo
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basado en distancias. Se registró un número bajo de individuos en todas las áreas
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muestreadas. En base a 39 observaciones, el tamaño poblacional se estimó en 409 (IC
14
95%, 275-608) individuos y la densidad en 0.55 busardos/km2 (IC 95%, 0.37-0.83).
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Este estudio sienta las bases para futuros estudios para el seguimiento de cambios
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poblacionales del busardo ratonero en Madeira.
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The common buzzard Buteo buteo L. is one of the most common and widespread
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raptors in Europe (Ferguson-Lees and Christie, 2001). As such it has been extensively
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studied and comprehensive data exist on its life history, ecology, biology and
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population status (see for example Bielański, 2006; Krüger, 2004; Reif et al., 2004;
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Palomino and Carrascal, 2007). Sedentary subpopulations of this species inhabit the
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small islands of the Macaronesian archipelagos (Garcia-del-Rey, 2011). These endemic,
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traditional but now disputed subspecies (Ferguson-Lees and Christie, 2001; Dickinson,
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2003) were found to be genetically indistinct from the mainland forms but they could be
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differentiated morphologically by shorter, narrower wings and stronger talons
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(Kruckenhauser et al., 2004). The buzzard breeding on Madeira (32°52’N, 17°09’W)
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was first mentioned by Harcourt in 1851 and described as a distinct insular subspecies,
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B. b. harterti, by Swann in 1919 (Bannerman and Bannerman, 1965). In contrast to the
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mainland common buzzard, this subpopulation has never been the subject of a detailed
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study.
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Potential threats to the biodiversity of Madeira are posed by one of the highest
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human population densities in Europe, an annual influx of a million tourists, agriculture,
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exotic species, forest degradation and erosion (Aguin-Pombo and Pinheiro de Cavalho,
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2009). In addition, the common buzzard is still considered an ill omen and persecuted
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(Oliveira and Menezes, 2004). The common buzzard population on Madeira is
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vulnerable to extinction due to its restricted range (Manne and Pimm, 2001). This
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survey was conducted to gather data on its population status by determining the
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population size and distribution.
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The island of Madeira lies 635 km west of Morocco and 794 km south-west of
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Portugal (Aguin-Pombo and Pinheiro de Cavalho, 2009). It has an area of 736.4 km² (58
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x 23 km) and a high mean elevation: 90% of the island rises above 500m. A central
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mountain range, reaching 1,861m, divides the island into a warmer, drier southern half
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and a cooler and more humid northern half. Temperatures range from 5 to 24°C
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(Sziemer, 2000). Seven habitat types were distinguished in this study: coastal
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vegetation, agriculture (small-scale apart from large-scale banana plantations in the
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south), exotic forest, indigenous laurel forest, upland pasture, heath forest and the
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mountains above the tree line (Oliveira and Menezes, 2004).
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Distance sampling was conducted using variable width line transects surveyed on
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foot (after Bibby et al. 1998). Between 9 May and 2 June 2008 I surveyed 14 transects
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with a total length of 87.6 km (including repeats) (Fig. 1, Appendix 1). These were sited
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in coastal vegetation (6.9 km), agriculture in the north (4.5 km), agriculture in the south
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(13.83 km), exotic forest (31.48 km), laurel forest (18.84 km), heath forest (7.2 km) and
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the mountains above the tree line (4.85 km). The potential causes of bias in the
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estimation of population size in this study were identified as the inequality and
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inadequate lengths (< 10 km) of transects in some habitat types (Jones, 1998); their non-
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random siting (Fig. 1) and the curving of transects due to the rugged terrain (Buckland
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et al., 2001; Hiby and Krishna, 2001).
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Habitat type, time, elevation and visibility were recorded every 300m (transect
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segment) or when habitat type changed (Appendix 1). The following variables were
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recorded for each sighting: observer location and elevation, time, contact type (aural,
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visual), sighting angle and distance to the buzzard location (an approximate ground
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location was determined for flying birds), number of birds and habitat type. Survey
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efforts were concentrated around noon since buzzards are most active in the air, and
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thus most visible, when thermals and updrafts facilitate soaring (Tubbs 1974; Bautista et
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al. 2004). Distances were measured with a Bushnell laser rangefinder Yardage Pro 500
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(accuracy ± 1 m, up to 1000 m) and locations and elevations with a Garmin eTrex
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Venture GPS.
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Population density (buzzards/km²) was calculated using the DISTANCE 5.0
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Release 2 program (Thomas et al., 2010). Detections were entered as individuals.
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Histograms of the distance data at intervals of different sizes were examined to
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determine the number and size of intervals that would fit the data to the shape criterion
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of a broad shoulder, indicating no heaping on the line and certain detection near the line,
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and tailing off (detection gradually diminishes with greater distance) (Buckland et al.,
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2001), and to detect outliers. AIC (Akaike’s Information Criterion) was used for model
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selection. The smallest χ2-value and the p value closest to 1 of the goodness-of-fit test
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indicated the best fitting intervals and grouping.
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The influence of visibility on buzzard detectability was assessed with a χ2-test of the
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frequency of sightings per visibility category versus the expected frequency based on
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random distribution of observations. Minitab® Statistical Software Release 14 (Minitab
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Inc., Pennsylvania, U.S.A.) was used for statistical analysis and R 2.13.0 (R
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Development Core Team 2011) for graphs.
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A total of 39 sightings were made during the survey period (Fig. 1). One
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observation was not visual but aural. Buzzards were sighted from 07:45 until 17:09
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hours. Twenty-four buzzards were recorded above exotic forest; four above laurel
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forest; three above agriculture in the south and above coastal vegetation; two above/in
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heath forest and above agriculture in the north and one in the mountains above the tree
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line (Table 1). Visibility was rated from ‘completely obscured by vegetation’
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(minimum) to ‘not obscured’ (maximum) (Table 1). It was lowest in laurel forest
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(mode=1, range=1–5) and exotic forest (mode=3, range=1–5) (Table 1). In agriculture
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in the south (mode=5, range=2–5), agriculture in the north (mode=5, range=3–5) and
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heath forest (mode=5, range=3-5) visibility was barely impeded by vegetation (Table 1).
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The field of view was not obscured by vegetation in coastal vegetation and above the
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tree line (Table 1). Although 72% of the sightings (28 buzzards) were made when the
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field of view was ¼ obscured or not at all obscured, visibility did not significantly
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influence buzzard detectability (χ24 = 8.98, P = 0.062, N=39).
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TABLE 1
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Habitat details and number of buzzards sighted per habitat type of transects surveyed on
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Madeira (9 May to 2 June 2008). Visibility: 1 = completely obscured by vegetation;
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2 = ¾ obscured; 3 = ½ obscured; 4 = ¼ obscured; 5 = not obscured.
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[Detalles del hábitat y número de busardos ratoneros avistados por tipo de hábitat en
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los transectos muestreados en Madeira (9 de mayo a 2 de junio de 2008), Visibilidad: 1
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= completamente obstruida por vegetación; 2 = 3/4 de obstrucción; 3= 1/2 de
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obstrucción; 4= 1/4 de obstrucción; 5 = no obstruida.]
8 Habitat Laurel forest Heath forest Exotic forest Agriculture in the north Agriculture in the south Coastal vegetation Above the tree line 9 10
Distance sampled (km) 18.84 7.20 31.48 4.50 13.83 6.90 4.86
Sightings 4 2 24 2 3 3 1
Visibility range mode 1-5 1 3-5 5 1-5 3 3-5 5 2-5 5 5-5 5 5-5 5
The histogram (Fig. 2) showed no spiking at zero distance and obvious outliers at
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500 m (four observations, 10.3% of the data). The uniform key function model proved
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to be the most parsimonious model (lowest AIC) with χ2 = 1.83, N = 39, %CV = 25.0
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and P = 0.40 (Table 2). Without the four outliers (truncation at 360 m) in three equal
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intervals of 120 m the uniform distribution fitted the data better (χ2 = 0.06, N = 35, P =
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0.97) and the precision improved from 25% to 18.5% (Table 2). Detection probability
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on the line g(0) was 1.03 indicating certain detection of buzzards at zero distance
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(Buckland et al., 2001). The common buzzard on Madeira occurred at a density of 0.55
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individuals/km² with a variance of 18.5%. Projection of this density estimate across the
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entire area of the island gives a population of 409 individuals (95% CI 275-608) on
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Madeira.
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TABLE 2
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Summary of density estimates (individuals/km²) of the common buzzard on Madeira
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after different analyses based on surveys conducted between 9 May and 2 June 2008.
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CI: 95% confidence interval around the estimates, %CV: coefficient of variation of the
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estimate, χ2: chi-squared value of goodness-of-fit test: P the probability of a greater chi-
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squared value.
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[Resumen de las estimas de densidad (individuos/km2) de busardo ratonero en Madeira
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según diferentes análisis basados en muestreos realizados entre el 9 de mayo y el 2 de
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junio de 2008.]
9 10 Sample size
Buzzards/km² (95% CI)
%CV
χ2
P
Uniform (each observation a single buzzard)
39
0.65 (0.39-1.07)
25.0
1.83
0.40
Uniform (without outliers, truncated at 360 m) Uniform (without suspected duplicate observations) Uniform (as clusters pair/trio as one observation)
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0.55 (0.37-0.83)
18.5
0.06
0.97
32
0.51 (0.35-0.74)
17.4
0.25
0.88
30
0.55 (0.36-0.85)
20.4
0.60
0.74
Analysis (key function, data use)
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After elimination of three suspected duplicate observations (individuals seen twice
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on the same transect) the uniform distribution showed less variance (17.4%), but with χ2
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= 0.25 and P = 0.88 the fit was not as good (Table 2). In six instances buzzards were not
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observed singly, but in pairs and once in a trio (Fig. 1). Non-independence of
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observations could lead to underestimation of the variance (Buckland et al., 2001).
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However, when the data was entered as clusters (a pair or trio counting for one
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observation, N = 30), the shape of the data was worse and the fit was significantly
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worse with χ2 = 0.60 and P = 0.74 (Table 2).
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This study provided a rough population size estimate of 409 (95% CI 275-608) common buzzards on Madeira. Rodríguez et al. (2010) recorded 23.13 breeding
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pairs/100 km² of Buteo buteo insularum on the Teno massif on Tenerife. The only
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published estimate of the breeding segment of a buzzard population comes from
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England, where Wall et al. (2004) found that 25% of an adult spring population were
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breeding. Projecting this ratio to adult individuals/km² gives 1.85 adult individuals/km²
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on the Teno massif, which is much higher than the present survey's estimate of 0.55
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individuals/km² on Madeira. Rodríguez et al. (2010) noted that the 140 km², well-
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protected Teno massif offered preferred breeding sites, which could explain the much
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higher extrapolated density of this closely related subspecies, whereas the density on
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Madeira was calculated from observations in protected and non-protected areas that
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were treated as being representative of the entire island.
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The results of the present study could serve as a baseline for future surveys.
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Buckland et al. (2001) advise a minimum sample size of 60 to 80 observations for
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distance sampling. To obtain a sample size of 60 in a future repeat of this survey total
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transect length would need to be 134.8 km instead of the 87.6 km of the present survey
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(Buckland et al., 2001). Transects should then be more evenly distributed over the
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different habitat types and across the island. A larger sample size would improve
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assessment of the influence of visibility on buzzard detectability, since the apparent lack
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of any such influence could have been due to the small sample size of 39 observations,
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especially when taking into account that 28 buzzards were observed when visibility was
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largely or totally unhindered by vegetation. Furthermore, a larger sample size would
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enable habitat preferences of the common buzzard on the island to be determined, which
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is vital in ascertaining its conservation requirements.
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ACKNOWLEDGEMENTS. —I would like to thank P. Oliveira, director of the Parque
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Natural da Madeira, and rangers Nelson, Jao Paul, Claudio, Gil Pereira and Herculano
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for sharing their knowledge, their logistical assistance and hospitality. I am grateful to
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Dr M. Jones (MMU) for his advise, constructive comments and encouragement and to
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L. Kiff and T. Rosenberry for providing access to the Peregrine Fund Research Library.
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And thank you to P. Laiolo, T. Gottschalk and M. Vögeli for their constructive and
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thorough review of the manuscript.
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BIBLIOGRAPHY
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BAUTISTA, L. M., GARCÍA, J. T., CALMAESTRA, R. G., PALACÍN, C., MARTÍN, C. A.,
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traffic on the use of space by raptors. Conservation Biology, 18: 726-732.
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BIBBY, C., JONES, M. and MARSDEN, S. (Eds.) 1998. Expedition Field Techniques
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Bird Surveys. Royal Geographical Society. London. BIELAŃSKI, W. 2006. Nesting preferences of common buzzard Buteo buteo and
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Forest, Southern Poland). Biologia Bratislava, 61(5): 597-603.
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of Biological Populations. Oxford University Press. Oxford.
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DICKINSON, E. C. (Ed.) 2003. The Howard and Moore Complete Checklist of the Birds of the World. 3rd ed., Christopher Helm. London.
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Canary Islands, Cape Verde. Lynx Edicions. Bellaterra, Barcelona.
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HIBY, L. and KRISHNA, M. B. 2001. Line transect sampling from a curving path.
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Biometrics, 57: 727-731. JONES, M. 1998. Study design. In, C. BIBBY, M. JONES and S. MARSDEN (Eds.): Expedition Field Techniques, pp.15-34. Royal Geographical Society. London.
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KRUCKENHAUSER, L., HARING, E., PINSKER, W., RIESING, M. J., WINKLER, H., WINK,
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M. and GAMAUF, A. 2004. Genetic vs. morphological differentiation of Old
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World buzzards (genus Buteo, Accipitridae). Zoological Scripta, 33: 197-211.
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KRÜGER, O. 2004. The importance of competition, food, habitat, weather and phenotype
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for the reproduction of buzzard Buteo buteo. Bird Study, 51: 125-132. MANNE, L. L. and PIMM, S. L. 2001. Beyond eight forms of rarity: which species are threatened and which will be next? Animal Conservation, 4: 221-229. OLIVEIRA, P. and MENEZES, D. 2004. Birds of the Archipelago of Madeira. Madeira & Madeira. Funchal. PALOMINO, D. and CARRASCAL, L. M. 2007. Habitat associations of a raptor community
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in a mosaic landscape of Central Spain under urban development. Landscape and
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Urban Planning, 83: 268–274.
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REIF, V., JUNGELL, S., KORPIMÄKI, E., TORNBERG, R. and MYKRÄ, S. 2004. Numerical
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response of Common Buzzards and predation rate of main and alternative prey
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under fluctuating food conditions. Annales Zoologici Fennici, 41: 599-607.
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RODRÍGUEZ, B., SIVERIO, F., RODRÍGUEZ, A. , SIVERIO, M. , HERNÁNDEZ, J. J. and
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FIGUEROLA, J. 2010. Density, habitat selection and breeding biology of common
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buzzards Buteo buteo in an insular environment. Bird Study, 57: 75-83.
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SZIEMER, P. 2000. Madeira’s Natural History in a Nutshell. Fransisco Ribeira & Filhos, Lda. Funchal.
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THOMAS, L., BUCKLAND, S. T., REXSTAD, E. A., LAAKE, J. L., STRINDBERG, S., HEDLEY,
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S. L., BISHOP, J. R. B., MARQUES, T. A. and BURNHAM, K. P. 2010. Distance
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software: design and analysis of distance sampling surveys for estimating
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population size. Journal of Applied Ecology, 47: 5-14.
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TUBBS, C. R. 1974. The Buzzard. David & Charles Limited. Newton Abbot, UK.
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UNDERWOOD, J. and UNDERWOOD, P. 2008. Madeira - cars, tours and walks. Hunter
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Publishing Inc. Edison, USA. WALLS, S. S., KENWARD, R. E. and HOLLOWAY, G. J. 2004. How common is the common buzzard, Buteo buteo? Biologist, 51(1): 23-26.
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Received: 13 June 2011
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Accepted: 1 March 2012
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Editor: Paola Laiolo
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1
APPENDIX 1
2
Description of transects (numbering is shown in Fig. 1) with start and end coordinates, date surveyed, length, start and end time, and start and end
3
elevation. Rv = reverse, rp = repeat, data shown in brackets. Description names were taken from Reise Know-how (2007) and Underwood and
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Underwood (2008).
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[Descripción de los transectos (los números se muestran en la Fig. 1) con las coordenadas de los lugares de comienzo y finalización, fecha del muestreo,
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longitud, hora de inicio y finalización y elevación de los sitios de inicio y finalización. Rv = a la inversa, rp = repetición. Los nombres se tomaron de
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Reise Know-how (2007) y Underwood y Underwood (2007).]
8 9
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1 Transect number / description 1 2 3 4 5 6
7 8
9 10
11 12 13 14
2
Ponta de São Lourenço from Casa do Sardinha to ER109 road (rv) End tarmac road north of Caniçal post office to end dirt road Short-cut up from power station at Fajã da Nogueira to Levadinha João de Deus Dirt road from power station at Fajã da Nogueira to ER103 road Dirt road from continuation of ER218 road down to Lombo do Galego Levada do Pico Ruivo from the tunnels with railway tracks towards Fajã da Nogueira From Seixal south via short-cut through vineyards along ER221 road to its end Lombo do Pau Branco from Jardim da Serra via Fonteinhas to end dirt road above Cruz da Caldeira (rp) Levada do Norte from Ribeira da Quinta Grande to Boa Morte Levada dos Tornos from Jardim Botãnico cable car terminus to Hortensia Tea House (rv) Poiso to Pico do Arieiro (Miradour do Juncal) along ER202 road (rv) Levada do Furado and Levada da Portela (Ribeiro Frio to Portela) Levada dos Piornais from Quebradas, via Santa Quiteria, Levada do Curral Levada do Norte west from Boca da Encumeada, till second tunnel in Folhadal, Levada das Rabaças
Coordinates (longitude W, latitude N) Start End
Date (dd/mm)
Length (m)
Start
Time End
Elevation (m) Start End
3600 (3300) 1860
12h02 (07h35) 08h15
14h03 (11h07) 10h27
103
81
159
187
16 40 57.04
32 44 26.11
16 42 03.13
32 44 36.82
16 44 22.31
32 44 48.08
16 44 52.27
32 44 55.52
09/05 (10/05) 11/05
16 54 27.36
32 44 39.51
16 54 45.09
32 44 53.70
13/05
1780
07h38
09h45
686
968
16 54 20.46
32 44 36.56
16 52 50.36
32 45 14.41
14/05
4390
07h35
10h55
640
365
16 52 46.51
32 46 43.69
16 52 39.48
32 46 20.32
14/05
2400
11h50
14h13
699
460
16 55 15.69
32 45 19.06
16 55 14.97
32 44 36.34
15/05
3000
12h50
15h00
993
984
17 06 29.41
32 49 21.62
17 06 48.56
32 47 49.22
17/05
3600
08h01
10h25
150
477
16 59 54.25
32 41 55.78
17 00 11.35
32 40 11.77
21/05 (26/05)
5840 (5760)
09h05 (09h35)
12h24 (12h19)
926
674
17 00 21.03
32 40 01.78
17 02 20.55
32 40 45.11
22/05
8400
08h35
13h00
549
558
16 53 51.71
32 40 38.72
16 52 51.41
32 40 02.25
25/05 (31/05)
5700 (5700)
10h10 (10h05)
13h27 (12h53)
568
605
16 53 23.34
32 42 42.59
16 55 29.02
32 44 10.90
16 49 47.12
32 44 42.65
10h54 (15h19) 11h03
13h56 (17h19) 15h24
1783
32 44 05.83
6950 (7270) 8700
1400
16 53 10.31
28/05 (28/05) 30/05
871
699
16 57 30.95
32 39 07.20
16 57 22.95
32 40 36.02
01/06
6000
09h13
12h59
149
308
17 01 11.08
32 45 15.03
17 02 12.65
32 44 43.87
02/06
3300
10h12
12h36
1003
1026
14 1
FIGURE LEGENDS
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FIG. 1.— Madeira Island with transect locations (the numbers refer to the description in
4
Appendix 1) and distribution of buzzard sightings; open symbols are observations
5
outside the line transects.
6
[La isla de Madeira con la localización de los transectos (los números se corresponden
7
con los del Apéndice 1) y distribución de los avistamientos de busardos ratoneros. Los
8
símbolos vacíos son observaciones realizadas fuera de los transectos.]
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FIG. 2.— Detection distance (m) of buzzard sightings (N = 39).
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[Distancias de detección (m) de los avistamientos de busardos ratoneros (N = 39).]
12 13 14 15 16 17 18 19 20 21 22 23 24 25
15
1
FIGURE 1
2 3
Jansen, D. Y. M. Survey of the common buzzard Buteo buteo on Madeira
16 1
FIGURE 2
5 4 3 2 0
1
Number of buzzard sightings
6
7
2
0
100
200
300
400
Perpendicular distance of buzzard sightings from transects (m)
3 4 5 6 7 8 9 10
Jansen, D. Y. M.
11
Survey of the common buzzard Buteo buteo on Madeira
500