Parabuteo Unicinctus Research Articles

Pair and Group Reproductive Success, Polyandry, and Cooperative Breeding in Harris' Hawks

-The average size of Harris' Hawk (Parabuteo unicinctus) social units in New Mexico was 2.7 individuals, and pairs were most common (49%; n = 61). Groups of more than two hawks included both adult(74%) and immature-plumaged (26%) members (n = 76). Immatures rarely provided food to nestlings, but adult supernumeraries did. Electrophoretic analyses of two groups containing two adult-plumaged males did not suggest polyandry. At least in southeastern New Mexico, Harris' Hawk groups consist primarily of a monogamous pair with helpers. Pairs and groups showed no differences in clutch size, number of young produced per successful nest, or number of offspring fledged per year. Pair nests failed less often (16%) than group nests (46%) during the incubation period. Groups reared larger offspring and tended to initiate second nests more frequently than pairs. The overall lack of correlation between reproductive output and group size suggests that kin selection has not been a major influence in the evolution of the Harris' Hawk breeding system. Received 6 June 1986, accepted 22 December 1986. MADER (1975a, b) described a breeding population of Harris' Hawks (Parabuteo unicinctus) in Arizona that consisted both of pairs and of putatively polyandrous trios. Since publication of Mader's work, the Harris' Hawk often has been cited as one of only a few species known to exhibit polyandry or a cooperatively polyandrous breeding system (e.g. Woolfenden 1976; Brown 1978, 1983; Faaborg and Patterson 1981; Koenig and Pitelka 1981; Ligon 1983; Woolfenden and Fitzpatrick 1984; Oring 1986). In contrast, 18 of 19 (95%) Harris' Hawk nests in Texas were tended only by pairs, suggesting that polyandry and cooperative breeding are rare in at least one Texas population (Griffin 1976). Cooperative breeding should entail some fitness advantage over available alternative strategies. Benefits resulting from cooperation may be shared mutually by all group members or biased toward those individuals of the group that have greater behavioral leverage (Emlen and Vehrencamp 1983). These advantages may be expressed in terms of increased reproductive success, improved quality of young (e.g. production of healthier or heavier fledglings), decreased stress on breeders (Morton and Parry 1974, Ricklefs 1980, Emlen 1984), or increased probability of survival (Woolfenden and Fitzpatrick 1984, Faaborg 1986). Because survival probabilities and stress are difficult to measure, most fieldworkers have concentrated on obtaining estimates of reproductive success. Data on reproductive success show that, in general, large groups produce more fledglings than smaller groups of the same species (Brown 1978; Emlen 1978, 1984). There are exceptions, however, and in many cases this positive relationship is not significant. Koenig (1981) compared productivity measures for 15 species and found that per-capita reproductive success tended to decrease with increased group

Successive Nesting and Autumnal Breeding in Harris' Hawks

-I monitored 24-29 breeding groups of Harris' Hawks (Parabuteo unicinctus) in southeastern New Mexico during a 40-month period. Each year 29-70% of these groups made a second breeding attempt in either late summer or autumn, often following successful spring breeding. The interbrood period was positively correlated with the size of the first brood. Clutches were initiated between 17 February and 1 November. The mean number of hawks fledged from autumn nests was less (0.94) than that produced from spring (1.30) and summer (1.36) nests. The composition of prey remains found at nests in different seasons was similar. The number of second nests initiated was correlated with the number of lagomorphs but not with precipitation. Harris' Hawks did not time nesting to coincide with peak population levels of their principal prey; rather, this species has a flexible and prolonged breeding season that may have evolved in part to minimize the impact of stochastic climatic conditions that increase the probability of nest failure. Received 21 February 1986, accepted 27 August 1986. AVIAN breeding seasons typically coincide with circumstances such as abundant food, decreased competition, minimal predation, or benign climatic conditions (ultimate factors) that result in the successful recruitment of young into the population (Immelmann 1971). Birds breed in different months in different regions of the world, but each species usually lays its eggs at about the same date each year. The initiation of reproduction is triggered by the proximate cues that most reliably predict the onset of key ultimate factors (Perrins 1970, Immelmann 1971). For most temperate-region birds, photoperiod is the most important proximate cue used to time the breeding seasons (Murton and Westwood 1977). In the tropics, where photoperiod varies little, or in unpredictable desert environments, precipitation is regarded as an important cue in the initiation of reproduction (see Brown and Britton 1980). In both temperate and tropical regions, food supply is probably the most important ultimate factor in the determination of avian breeding seasons (Lack 1968, Perrins 1970, Immelmann 1971). At northern latitudes, therefore, breeding seasons typically coincide with the annual spring and summer pulse of productivity. A few species such as the Red Crossbill (Loxia curvirostra; Tordoff and Dawson 1965), Tricolored Blackbird (Agelaius tricolor; Orians 1960, Payne 1969), and Pinyon Jay (Gymnorhinus cyanocephalus; Ligon 1971) also breed in the autumn. Birds of prey normally raise only one brood in a year (Newton 1977, 1979), although some of the smaller raptors, such as the American Kestrel (Falco sparverius) and the Black-shouldered Kite (Elanus caeruleus), occasionally rear two broods in one year (Pickwell 1930, Stahlecker and Griese 1977, Toland 1985). Of the larger species, the Common Barn-Owl (Tyto alba) breeds almost year-round, primarily in tropical environments (Otteni et al. 1972, Bunn et al. 1982, Lenton 1984). De Vries (1975) recorded three successful second-brood attempts by the equatorial Galapagos Hawk (Buteo galapagoensis). Harris' Hawks (Parabuteo unicinctus) in temperate North America are known to nest in the autumn in Texas (LeSassier and Williams 1959, Brannon 1980), Arizona (Radke and Klimosewski 1977, Ellis and Whaley 1979), and New Mexico (Pache 1974). Mader (1975) and Whaley (1986) documented that Harris' Hawks laid eggs as early as January and fledged young as late as October. In addition, Mader (1975) observed five cases in which breeding groups with one or more banded adults renested after successfully rearing a first brood. Harris' Hawks, therefore, are unusual among large Temperate Zone raptors in having a prolonged breeding season and in rearing successive broods in one year. Here I report year-round observations on the reproductive efforts of breeding groups of Harris' Hawks in a New Mexico population over a 40-month period, and describe the temporal 85 The Auk 104: 85-96. January 1987 This content downloaded from 207.46.13.86 on Sat, 15 Oct 2016 04:36:21 UTC All use subject to http://about.jstor.org/terms 86 JAMES C. BEDNARZ [Auk, Vol. 104 patterns of breeding and successive nesting attempts. I then relate these patterns to fluctuations in available prey and precipitation to identify potential proximate and ultimate fac-

Extra Birds and Helpers at the Nests of Richardson's Merlin

Extra Birds and Helpers at the Nests of Richardson's Merlin

The guild structure of a community of predatory vertebrates in central Chile.

The trophic ecology of eleven predator species (Falconiforms: Buteo polyosoma, Elanus leucurus, Falco sparverius, Geranoaetus melanoleucus, Parabuteo unicinctus; Strigiforms: Athene cunicularia, Bubo virginianus, Tyto alba; Carnivores: Dusicyon culpaeus; Snakes: Philodryas chamissonis, Tachymenis peruviana) in two nearby localities of central Chile is analyzed. The localities exhibit the typical climate (hot-dry summers, coldrainy winters), and vegetation (chaparral), of mediterranean ecosystems. Densities of the staple prey (small mammals) were estimated by seasonal trapping during two years in both open and dense patches of chaparral.The trophic parameters examined are: 1) proportion of diurnal, crepuscular, or nocturnal prey found in the predators' diet; 2) relationship between abundance of different mammalian prey in the predators' diet, and in both open and densely vegetated habitat patches; 3) mean weight and variance of weight of small mammal prey consumed; 4) average weight of the predators; 5) food-niche breadth of the predators; 6) relationship between average weight of predators and mean weight of mammalian prey taken, its variance, and food-niche breadth; 7) overlap in food-niche between all the predator species; 8) guild packing of the predators. Parameters 1) and 2) are used to assess the importance of temporal and habitat segregation of the predators, respectively; parameters 3), 4), 5), and 6) provide information on the possibilities of partitioning the prey resources among the predators; parameters 1), 2), 7) and 8) are used to investigate the organization of the community in terms of guilds.Three niche dimensions seem to be important in determining the structure of the predator community: 1) hunting activity period (diurno-crepuscular, nocturno-crepuscular), 2) hunting habitat (open, or both open and dense patches), and 3) mean prey size taken. Segregation along these three axes results in generally low food niche overlaps (<54% in 47 of the 55 pairwise comparisons) among the predators, but it is not possible to determine whether this was produced by competitive interactions or stochastic differences. Three guilds (niche overlap >90% in pair-wise comparisons) can be recognized: a) the carnivorous-insectivorous guild formed by the diurnal raptors A. cunicularia and F. sparverius, which tend to hunt in open habitat patches; b) the herpetophagous guild formed by the diurnal snakes P. chamissonis and T. peruviana, which presumably hunt in open habitat patches; c) the carnivorous guild (highly specialized in the capture of two rodent species) formed by the diurnal raptors B. polyosoma, G. melanoleucus, P. unicinctus, and the carnivore D. culpaeus, which hunt in open habitat patches. The diurnal raptor E. leucurus is not clearly associated with any guild, and the only two nocturnal raptors in the community (B. virginianus and T. alba) exhibit marked differences in their trophic ecology.

Geographical Ecology of the Common Species of Buteo and Parabuteo Wintering in North America

North American hawks of the subfamily Buteoninae have received considerable attention, mostly with regard to their food habits and breeding biologies (e.g., Errington and Breckenridge 1938, Fitch et al. 1946, Smith and Murphy 1973). Twelve species occur in the New World north of Mexico but only five are widely distributed in winter (A.O.U. 1957). These are the Harris' Hawk (Parabuteo unicinctus), Ferruginous Hawk (Buteo regalis), Red-shouldered Hawk (B. lineatus), Red-tailed Hawk (B. jamaicensis), and Rough-legged Hawk (B. lagopus). Previously published maps based upon Christmas count data indicate that only the last three of these species are at all common over wide areas (Bystrak 1974). A number of workers have examined the relationships between locally sympatric populations of two or more Buteo species (e.g., Stewart 1949, Orians and Kuhlman 1956, Weller 1964, Schnell 1968). In this study we have compiled data from the 1969-70, 197071, and 1971-72 Christmas counts to determine the overall abundance pattern of this genus in the United States and southern Canada. We could then quantify the contributions made by the individual species to that pattern on different parts of the continent. We compared these calculations with certain environmental variables. Results show the degree to which the species' abundance patterns are complementary, and reveal something about the environmental gradients involved.

Predation by a White-Tailed Hawk and a Harris Hawk on a Wild Turkey Poult

On 6 May 1970, I observed a White-tailed Hawk (Buteo albicaudatus) and a Harris Hawk (Parabuteo unicinctus) attack and kill a six-day-old wild Turkey poult (Meleagris gallopavo intermedia). The incident occurred at 11:35 CDT on King Ranch, Inc., Kleberg County, Texas. The events prior to and after the incident are as follows. While observing a Turkey hen with poults, I noticed a White-tailed Hawk diving for the ground. It spread its wings and extended its feet just before landing on the Turkey poult. Two or three seconds after the White-tailed Hawk hit the poult, a Harris Hawk also landed on the poult. The hawks were facing each other on the ground with their feet grasping the young bird. The Turkey hen left her hiding place, some 25 m away, and flew directly toward the two hawks. As she approached, the hawks released the poult and took to the air with the Turkey hen in close pursuit. After chasing the hawks for some 20 m and flying to a height of approximately 45 m, at one time almost straight up, the hen glided to the ground and promptly returned to her previous hiding place. I believe this incident is of scientific interest for the following reasons: (1) it increases our knowledge of the White-tailed Hawk, a relatively unstudied species; (2) it illustrates an instance of direct competition between two species of raptors; and (3) it adds to the knowledge of predation on very young wild Turkeys.