Prey sizes of male and female northern goshawks
1996; Southwestern Association of Naturalists; Volume: 41; Issue: 4 Linguagem: Inglês
ISSN
1943-6262
AutoresClint W. Boal, R. William Mannan,
Tópico(s)Avian ecology and behavior
ResumoA common explanation proffered for reversed sexual size dimorphism among raptors is that size dimorphism allows the partitioning of prey size between the sexes and reduces intersexual competition for food. During the breeding seasons of 1990, 1991, and 1992, we compared prey captured by male and female northern goshawks (Accipiter gentilis) at 16 nests in northern Arizona. On basis of 192 prey items captured by males and 46 captured by females, we found no difference between the sexes in mean mass of prey captured, or in the distribution of prey sizes. There was no difference between the sexes in capture rates of mammals or birds; mammals accounted for 85% and 79% of the prey captured by females and males, respectively. The sexes had high dietary overlap (92%, Pianka's index), but males used the available prey species less equitably than did females (male = 0.37, female = 0.51; min 0.0, max 1.0). Prey captured in different foraging zones did not differ between the sexes. Our findings add to a growing collection of data investigating prey partitioning as an explanation for reversed sexual size dimorphism. Sexual size dimorphism is common among birds, with males typically being larger. This trait is reversed among most members of the Falconiformes. In addition, reversed sexual size dimorphism has evolved independently in the owls (Strigiformes), the jaegers and skuas (Stercorariidae), and the frigate-birds (Fregatidae) (Amadon, 1959), which suggests some association with a predatory lifestyle. The adaptive significance of reversed sexual size dimorphism remains an enigma; numerous explanations have been proposed but none has gained universal acceptance. One of the more commonly proffered explanations is that it allows prey size partitioning between the sexes by expanding the range of potential prey sizes (Selander, 1966; Storer, 1966; Snyder and Wiley, 1976; Newton, 1979). However, studies addressing differential prey size selection as an explanation for reversed sexual size dimorphism have been equivocal (Storer, 1966; Opdam, 1975; Balgooyen, 1976; Snyder and Wiley, 1976; Newton, 1978; Collopy, 1984; Kennedy and Johnson, 1986). Among diurnal raptors, reversed sexual size dimorphism is manifested most strongly in the genus Accipiter We investigated intersexual prey partitioning by analyzing dietary data collected at northern goshawk (Accipiter gentilis) nests during 1990, 1991, and 1992 in northern Arizona. METHODS-Our study area was on the North Kaibab Ranger District, Kaibab National Forest, Coconino County, Arizona. The district is approximately 259,000 ha located on the Kaibab Plateau in northern Arizona. The Kaibab Plateau ranges from 923 to 2,830 m, with a topography typified by gentle slopes interspersed with gentle to steep drainages. A description of the study area is provided in Crocker-Bedford (1990). We monitored activities at seven northern goshawks nests in 1990, seven in 1991, and six in 1992. Assuming northern goshawks used the same territories, we did not study any nesting pair for more than one successful breeding season. We conducted 1,539 hours of direct observations of northern goshawk activities from cloth blinds located on the ground or in trees near the nests. During these observations we used spotting scopes to attempt to visually identify all prey deliveries to species. Observations were conducted from the late incubation or early nestling stages until 5 to 7 days after fledging. We used only prey identified to at least genus in our analyses and categorized each identified prey item into the size category system developed by Storer (1966). Mass of mammalian prey was estimated by calculating the mean mass of specimens in the University of Arizona mammal collection. Mass for avian prey was taken from Dunning (1984). We considered all mammals as adult due to difficulty in distinguishing ages. Estimates of avian mass for the classes of nestling-fledgling, subadult and unknown age were computed following Bielefeldt et al. (1992). We assumed prey to have been captured by the male whenever we observed him deliver prey to the nest, THE SOUTHWESTERN NATURALIST 41(4):355-358 DECEMBER 1996 This content downloaded from 157.55.39.45 on Thu, 01 Sep 2016 04:46:51 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist TABLE 1-Numbers of prey in different Classes and from different foraging zones captured by male and female northern goshawks, North Kaibab Ranger District, Arizona, 1990, 1991, and 1992. Prey class Foraging zone (%) Sex of goshawk Bird Mammal Ground/shrub Shrub/canopy General Male 40 152 120 (62.5) 25 (13.0) 47 (24.5) Female 7 39 30 (65.2) 4 (8.7) 12 (26.1) surrender prey to the female in the nest stand, or when the female brought prey to the nest following vocalizations characteristic of prey exchanges (Schnell, 1958). With the exception of cached prey retrievals, we assume that all other prey brought to the nest by the female had been captured by her. We investigated diet differences between the sexes by analyzing data from 16 nests where both sexes were recorded as having cap-
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