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Prey of Harris' hawks (Parabuteo unicinctus) during autumn and winter in a coastal area of central Chile.

Harris' hawk (Parabuteo unicinctus) is distributed from the southwestern United States to southern Patagonia (Brown and Amadon, 1968; Couve and Vidal, 2003), inhabiting mainly vegetated open lands such as semi-arid deserts, shrublands, savannahs, open woodlands, and prairies, as well as agricultural areas with trees (Bednarz, 1995; Ferguson-Lees and Christie, 2001). According to studies conducted in North and South America, this hawk preys on a wide spectrum of prey including lagomorphs, small mammals, birds, reptiles, and insects (Jaksic et al., 1980; Whaley, 1986; Bednarz and Ligon, 1988; Jimenez and Jaksic, 1993; Bednarz, 1995). However, information on prey of Harris' hawks in South America is scarce with reports only from Argentina (Aguilar and Kowalinski, 1996), Brazil (Silva e Silva and Olmos, 1997), and Chile (Jaksic et al., 1980; Jimenez and Jaksic, 1993; Figueroa and Gonzalez-Acuna, 2006). Although diet of Harris' hawks is relatively well known for areas adjacent to the Andes Mountains (pre-Andean areas) of central Chile (Jaksic, 1997), no information is available from other geographic areas with potentially different prey. Here, we describe prey of Harris' hawks in a coastal area of central Chile.

Our study was conducted in Reserva Oasis la Campana (2,500 ha), Valparaso region, central Chile. This reserve is adjacent to La Campana National Park (31[degrees]55'S, 71[degrees]08'W) in the coastal mountain range. The study site had uneven topography (elevation 300-1,800 m, slopes to 40[degrees]) and vegetation comprised of sclerophyllous forests and shrublands (Gajardo, 1994). Forests were composed mainly of Cryptocarya alba, Peumus boldus, Lithraea caustica, Schinus latifolius, and Quillaja saponaria that mostly covered ravines and lowest sites on south-facing slopes. Shrublands were composed mainly of Trevoa trinervis, Colliguaya odorifera, and Puya berteroniana, and extensively covered flatlands and hills. Climate is Mediterranean-arid (di Castri and Hajek, 1976) with mean annual rainfall and temperature of 109 mm and 20[degrees]C, respectively (Saiz et al., 1989; Pliscoff, 2002).

During autumn-winter 2006 (May) and autumn-winter 2007 (April-August) we collected pellets (n = 94) and remains of prey (n = 128) beneath trees and utility poles used as perches by [greater than or equal to] 2 groups of Harris' hawks. These hawks probably represented two family groups, each consisting of [greater than or equal to] 5 individuals and including inmature, subadult, and adult birds. While one group used the northwestern area of the reserve, the other used the southeastern portion. To ensure that pellets at collection sites were from Harris' hawks, we focally watched individual hawks on a regular basis. We recorded information about behavior of hawks during 23 days (sampling effort of 199 h). During this period, we only observed one attempt of cooperative hunting by one group of Harris' hawks, which exhibited behaviors similar to those described by Bednarz (1988). Mammalian prey were identified on the basis of pieces of skulls and dentaries following keys in Reise (1973) and Pearson (1995). Avian prey were identified based on color, size, and shape of feathers (Figueroa and Corales, 1999; Figueroa and Gonzalez-Acuna, 2006). With both types of prey, identification was corroborated by comparison with reference collections of mammals and birds maintained in the Instituto de Ecologia y Evolucion and Instituto de Zoologia at the Universidad Austral at Valdivia, respectively. Insects were identified on the basis of head capsules, elytras, or legs following keys in Pena (1986). Remains of prey were identified from feathers, skeletons, skulls, beaks, mandibles, feet, tails, and fur. We identified prey to the lowest possible taxonomic category. Because Harris' hawks plucked and quartered large prey at a number of butcher sites, remains that we found were not necessarily independent from prey represented in pellets. Thus, we estimated minimum number of individual prey from both pellets and remains by reconstruction based only on identifiable body parts (Bednarz, 1988). To assess biases in diet, results obtained separately from pellets and remains were compared, and then, data were pooled, as recommended by several authors (e.g., Simmons et al., 1991; Mersmann et al., 1992; Redpath et al., 2001).

The importance level of each prey item in the diet was evaluated by frequency and biomass (Marti, 1987; Marti et al., 2007). Frequency was calculated by dividing number of individuals in each identifiable category by total number of individuals of all prey. Biomass was estimated by multiplying number of prey taken by mean mass of each type of prey (Bednarz, 1988; Marti, 2007). Masses of prey were taken from published literature and unpublished data of the authors (Table 1). We assumed that masses of unidentified prey were similar to mean mass of the most closely related taxon that was identified. Geometric mean mass of vertebrate prey (Marti, 2007) was calculated as antilog ([summation][n.sub.i][log.sub.10] [w.sub.i]/[summation][n.sub.i]), where [n.sub.i] was number of individuals of the ith species and [w.sub.i] was mean mass (Figueroa and Gonzalez-Acuna, 2006; Marti et al. 2007; in other publications geometric mean mass of vertebrate prey often is given as geometric mean weight of vertebrate prey). Only prey identified to species were included to estimate geometric mean weight of vertebrate prey. Because of the limited sample, information from years was pooled, and biomass and geometric mean mass of vertebrate prey were estimated only from pooled results of pellets and remains of prey.

Whole pellets (mean [+ or -] SD) averaged 40.6 [+ or -] 11.5 mm in length, 22.3 [+ or -] 4.6 mm in width, and 3.6 [+ or -] 2.9 g in dry mass (n = 64). Dimensions were intermediate to those obtained by Jaksic et al. (1980) in pre-Andean areas of central Chile (44.4 [+ or -] 1.7 mm length by 20.5 [+ or -] 0.9 mm width, n = 34) and Figueroa and Gonzalez-Acuna (2006) in a suburban area in southern Chile (37.2 [+ or -] 2.0 mm length by 23.0 [+ or -] 1.2 mm width, n = 17). Pellets from Oasis La Campana were somewhat heavier than those collected by Jaksic et al. (1980; 3.0 [+ or -] 0.4 g, n = 34), but markedly heavier than those reported by Figueroa and Gonzalez-Acuna (2006; 2.32 [+ or -] 0.25 g, n = 17). Pellets we collected may have heavier because Harris' hawks in our study consumed more larger prey than was recorded in previous studies.

Compared with pooled results, mammalian prey was overrepresented by both pellets and remains of prey (Table 1). Avian prey was well represented in remains of prey, but greatly underrepresented in pellets with the opposite being true for reptiles and insects. Although frequencies of mammalian, reptilian, and insect prey were higher in pellets than in remains of prey, differences were not great (Table 1). These results are coincident with those reported by Jaksic et al. (1980) and Jimenez and Jaksic (1993) for a pre-Andean area of central Chile, where they determined that mammalian prey was equally represented in pellets and remains of prey, but that pellets under estimated avian prey. Although Figueroa and Gonzalez-Acuna (2006) also noted that pellets greatly under represented avian prey, they noted that remains greatly underrepresented mammalian prey; a result that simply could be an artifact due to the smaller sample. Several studies have demonstrated that biases are common when using different methods to assess diets of raptors (Marti et al., 2007). Thus, as recommended by various authors (e.g., Simmons et al., 1991; Mersmann et al., 1992; Oro and Tella, 1995), we pooled results of pellets and remains of prey to better evaluate diet of Harris' hawks.

In all, we identified 112 individual prey in the pooled samples of pellets and remains including 5 species of rodents, 1 species of lagomorph, 10 species of birds, 1 species of snake, 1 species of lizard, and 1 order of insect. By number and biomass, mammals were the most common prey (Table 1). Birds were the second most-frequently encountered prey, but comprised only a small fraction of total biomass (Table 1). Among mammals, rodents were an important part of the diet, with degus (Octodon) being the most frequent (Table 1). However, the European rabbit (Oryctolagus cuniculus) accounted for most of the biomass, contributing almost 60% to dietary mass (Table 1). A similar pattern in diet of Harris' hawks has been reported elsewhere; mammals, particularly lagomorphs, were important prey in number and biomass, and birds consistently were secondary prey (Bednarz, 1995).

Our results revealed that the geometric mean mass of vertebrate prey (267.8 g) at Oasis La Campana was somewhat higher than reported by Jaksic and Braker (1983; 202.4 [+ or -] 1.5 g) and Jimenez and Jaksic (1993; 257.1 [+ or -] 4.0 g) for pre-Andean areas of central Chile, and markedly higher than reported by Figueroa and Gonzalez-Acuna (2006; 85.4 [+ or -] 5.1 g) for southern Chile. In the study by Jaksic and Braker (1983), relative similarities of geometric mean mass of vertebrate preys clearly are due to a similar frequency of the same species of prey. Both in our coastal area and pre-Andean areas, Harris' hawks preyed mainly upon fence degu rats (Octodon degus; 184 g), Bennett's chinchilla rat (Abrocoma bennetti; 219 g) and European rabbits (1,300 g). The somewhat higher geometric mean mass of vertebrate prey at Oasis La Campana may be due to consumption of moon-toothed degus (Octodon lunatus; 180 g) and occasional taking of birds as large as great egrets (Ardea alba; 950 g). The difference between geometric mean mass of vertebrate prey from central and southern Chile were discussed previously by Figueroa and Gonzalez-Acuna (2006).

Predation on species inhabiting sparse shrublands, dense shrublands, and open forests suggests that Harris' hawks profitably used the mosaic of habitats present in Oasis La Campana. Predation on moon-toothed degus and Bennett's chinchilla rats also suggests that Harris' hawks mostly searched for prey in highly vegetated sites. Both species live in caves among rocks covered by shrubs and trees (Mann, 1978) and are important prey of the rufous-legged owl (Strix rufipes), which is a forest-specialist (Diaz, 1999; Alvarado et al., 2007). Possibly, Harris' hawks captured these rodents on borders of forests, while the hawks searched for prey in open areas. However, in our study area, the small remnants of forest had a relatively open midstory, which may have facilitated movements by hawks. Thus, it is possible that some rodents were captured by Harris' hawks in the interior of forests.

Because we did not evaluate abundance of prey in the field, we could not assess whether Harris' hawks preyed on animals in an opportunistic manner, or if they selected them. However, Jaksic (1997), in a study of Harris' hawks in central Chile, did not detect a correlation between prey taken and abundance if prey, although hawks did frequent habitat with high abundance of small mammals. Among potentially available species of prey in Oasis la Campana, Harris' hawks appeared to consume those that were large and abundant more frequently (e.g., degus, European rabbits). More studies are needed to determine which attributes of prey (e.g., habitat, size, behavior, abundance) influence the diet of Harris' hawks, and of abundance of prey is influenced by amount of activity by humans in their habitat.

We thank M. Moreno and F. Gomez who kindly permitted access to the Reserva Oasis la Campana, and F. Mondaca and R. Schlatter for facilitating access to reference collections of mammals and birds in the Instituto de Ecologia y Evolucion and Instituto de Zoologiaat the Universidad Austral de Chile, respectively. We also thank J. A. Munoz for help in the field and D. Haughney who helped with English. During field work, FJS was funded by Fondo de Investigacion de Memorias y Seminarios of the Universidad Metropolitana de Ciencias de la Educacion, Santiago, Chile. During preparation of this article, RAF was supported by a doctoral scholarship from the Comision Nacional de Ciencia y Tecnologia and la Escuela de Graduados, Facultad de Ciencias, Universidad Austral de Chile. Comments and suggestions by J. Bednarz improved this paper.


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Submitted 24 June 2009. Accepted 12 June 2011.

Associate Editor was Gary D. Schnell.


Laboratorio de Zoologia, Departamento de Biologia, Universidad Metropolitana de Ciencias de la Educacion, Santiago, Chile, y Laboratorio de Ecologia de Vida Silvestre, Facultad de Ciencias Forestales y Conservacion de la Naturaleza, Universidad de Chile, Santiago, Chile (FJS)

Division de Epidemiologia, Escuela de Salud Publica, Facultad de Medicina, Universidad de Chile, Santiago, Chile, y Facultad de Ciencias de la Salud, Universidad de Tarapacao, Arica, Chile (SAA)

Escuela de Medicina Veterinaria, Facultad de Ciencias Silvoagropecuarias, Universidad Mayor, Santiago, Chile (PAR)

Escuela de Postgrado, Facultad de Ciencias, Universidad Austral, Valdivia, Chile

* Correspondent:
TABLE 1--Prey of Harris' hawks (Parabuteo unicinctus) in Reserva
Oasis La Campana, Valparaiso region, central Chile, based on 94
pellets and 128 remains of prey.

                                                        Frequency (%)
Prey item                                    (g) (a)   Pellets  Remains

Mammals                                                89.0      79.6
  Fence degu rat (Octodon degus)               184.0    6.1      13.2
  Moon-toothed degu (Octodon lunatus)          180.0    1.0      11.0
  Unidentified degus (Octodon) (b)             182.0   32.3      15.0
  Bennett's chinchilla rat (Abrocoma
    bennetti)                                  219.0    5.1      12.5
  Unidentified rat (Rattus)                    200.0    2.0       0.7
  Darwin's leaf-eared mouse (Phyllotis
    darwini)                                    66.0    0.0       0.7
  Unidentified rodent (b)                      169.8   25.3       5.2
  European rabbit (Oryctolagus cuniculus)    1,300.0   17.2      21.3
Birds                                                   5.0      19.0
  California quail (Callipepla californica)    200.0    1.0       4.4
  Great egret (Ardea alba)                     950.0    0.0       0.7
  Chimango caracara (Milvago chimango)         100.0    0.0       0.7
  Southern lapwing (Vanellus chilensis)        270.0    0.0       0.7
  Chilean pigeon (Patagioenas araucana)        300.0    0.0       0.7
  Fire-eyed diucon (Xolmis pyrope)              38.3    0.0       1.5
  Austral thrush (Turdus falcklandii)           94.3    2.0       2.2
  Austral blackbird (Curaeus curaeus)           90.0    0.0       0.7
  Long-tailed meadowlark (Sturnella loyca)     112.6    0.0       1.5
  Common diuca-finch (Diuca diuca)              31.0    0.0       1.5
  Unidentified passerines (b)                   73.2    2.0       4.4
Reptiles                                                2.0       5.9
  Lizards (Liolaemus)                            8.0    3.0       0.0
  Long-tailed snake (Philodryas chamisonis)    150.0    1.0       0.7
Insects                                                 2.0       0.7
  Scarabeidae                                    1.0    0.0       0.7
  Acanthinodera cummingi                         1.0    2.0       0.0
Total prey (n)                                         99       136

                                                    Pooled data

Prey item                                    Frequency (%)  Biomass (%)

Mammals                                       69.4          88.6
  Fence degu rat (Octodon degus)              11.6           6.5
  Moon-toothed degu (Octodon lunatus)          9.8           5.4
  Unidentified degus (Octodon) (b)             9.8           5.5
  Bennett's chinchilla rat (Abrocoma
    bennetti)                                 10.6           7.3
  Unidentified rat (Rattus)                    2.7           1.6
  Darwin's leaf-eared mouse (Phyllotis
    darwini)                                   0.9           0.2
  Unidentified rodent (b)                      9.8           5.1
  European rabbit (Oryctolagus cuniculus)     14.2          57.0
Birds                                         23.4          10.5
  California quail (Callipepla californica)    4.5           2.7
  Great egret (Ardea alba)                     0.9           2.6
  Chimango caracara (Milvago chimango)         0.9           0.3
  Southern lapwing (Vanellus chilensis)        0.9           0.7
  Chilean pigeon (Patagioenas araucana)        0.9           0.8
  Fire-eyed diucon (Xolmis pyrope)             1.8           0.2
  Austral thrush (Turdus falcklandii)          3.6           1.0
  Austral blackbird (Curaeus curaeus)          0.9           0.2
  Long-tailed meadowlark (Sturnella loyca)     1.8           0.6
  Common diuca-finch (Diuca diuca)             1.8           0.2
  Unidentified passerines (b)                  5.4           1.2
Reptiles                                       7.2           1.4
  Lizards (Liolaemus)                          2.7           0.1
  Long-tailed snake (Philodryas chamisonis)    1.8           0.8
Insects                                        2.7           0.2
  Scarabeidae                                  0.9           0.1
  Acanthinodera cummingi                       1.8           0.1
Total prey (n)                               112

(a) Masses from Jimeinez and Jaksic (1993) and Diiaz (1999) for
mammals, and Jimenez and Jaksic (1993), Egli (1996), and
Figueroa and Corales (1999) for birds, except for Ardea alba.
Masses of A. alba and remaining taxa are based on data gathered
by authors. Total biomass used to calculate percentage of biomass
was 36,553.24 g.

(b) Mean mass of most closely related and identified taxon.
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Title Annotation:Notes
Author:Santander, Francisco J.; Alvarado, Sergio A.; Ramirez, Piero A.; Figueroa, Ricardo A.
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:3CHIL
Date:Sep 1, 2011
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