Successive Nesting and Autumnal Breeding in Harris' Hawks

Abstract

-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-