Time and Energy Budgets of the Red-Tailed Hawk in Southern California

-Time and energy budgets were constructed for three nesting pairs of Red-tailed Hawks (Buteo jamaicensis) in the Santa Monica Mountains, Los Angeles County, California, during the 1976 breeding season. Male and female energy expenditure was equal and constant throughout the breeding season at about 109 and 102 kilocalories per kilogram per 24-hour day, respectively. Paired males and females displayed similar patterns in amount of time spent soaring. This increased the visual effect of two birds with respect to defense of territory and protection. Males were more active and spent more time in territorial defense. For both sexes, territorial activity was most intense during courtship and just prior to fledging the young. This suggests a territory maintained for reproductive rather than foraging purposes. The availability of nest sites was likely limiting. Although many descriptive natural history studies exist for Red-tailed Hawks (eg. Austing, 1964; Craighead and Craghead, 1956; Wiley, 1975) there are few quantitative studies of their activity patterns of other behavior. In this study, I measured time budgets and derived energy budgets for freeliving Red-tailed Hawks. I asked the question, does foraging or territory maintenance by the male equal the energetic expense of clutch formation by the female? Because of the energy required for clutch production by the female, a large difference in energy expenditure between the male and female would lead to a difference in division of labor, leading to both sexes expending about the same amount of energy. Red-tailed Hawks are wellsuited for determining required time and energy budgets because they remain conspicuous when perched or active and remain close to the nest during the breeding season. Energy-consuming activity (excluding maintenance costs) is essentially limited to various flight activities and to clutch formation for the female. STUDY AREA.-Century Ranch, the 15 ha study area, comprises the southern half of Malibu Creek State Park (300 m average elevation) in the central portion of the Santa Monica Mountains, 50 km northwest of Los Angeles. The area includes chaparral-covered foothills, steep north-facing canyons, and rocky buttes and valleys with introduced annual grasses. Coast live oak (Quercus agrifolia) and valley oak (Quercus lobata) savanna and chaparral are the dominant vegetation types in which Red-tailed Hawks and their territories occurred. A more detailed description of the area is given by the California State Parks and Recreation Department (State of CaliforniaResource Agency, 1976). The overgrazed valley grasslands support a large Beechy Ground Squirrel (Spermophilus beecheyi) population and a variety of reptiles which constitute a substantial food source for resident raptors. METHODS.-Nest sites were located prior to the 1976 breeding season and three pairs with approximately equal-sized territories (2.5 km2) were selected. Individuals were identified by unique plumage markings, such as missing feathers or variations in color. Females were easily distinguishable by their larger size and copulation posture. Observations were made between 0800 and 1800 PST, two to three days per week during February through June, 1976. Each nesting pair was observed alternately for 2.5-3 hrs such that This content downloaded from 157.55.39.192 on Thu, 25 Aug 2016 04:23:47 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist an entire day could be tallied for each pair at the end of every fourth visit. All three pairs were visited during each full day in the field. Whole days were reconstructed only from observations in which the weather conditions were sufficiently similar to eliminate weather-induced variation. A total of six days was tallied for each pair during 1976. The amount of time spent in the following activities was recorded: 1) Territorial defense. Aggressive behavior, caused by the presence of an intruder; soaring punctuated by dives, stoops and screams. 2) Courtship. Undulating flights by either male or female (Craighead and Craighead 1956). 3) Nest building. Flight associated with the successful incorporation of nesting material into the nest. 4) Foraging. Flight activity, predominantly soaring, associated with the procuren:ent of food. In 1976, all soaring activity not recorded as either territoriality, courtship or nest building was tallied as foraging. 5) Perching. All instances when a hawk was either seen or assumed to be perched. When a hawk was out of sight, it was considered perched. This is a reasonable assumption as Red-tailed Hawks are obvious when soaring and I knew the location of favored perches both in and out of view. The Red-tails were out of sight approximately 10% of the time. Basal metabolic rate (BMR) was calculated following Lasiewski and Dawson (1967). Basal metabolic rate closely approximates the energy utilization of a sleeping bird during the night (King, 1974). Energy expenditure during the night was calculated by multiplying BMR (expressed on an hourly basis) by the average duration of darkness during the breeding season (12 hr). Diurnal energy expenditure for non-flight activity in day-active birds which are quiescent but awake is approximately 1.7 X BMR (King, 1974; Gessaman, 1973). Daytime energy expenditure for non-flight activity was estimated by multiplying 12 hours by 1.7 X hourly BMR. Time spent soaring was multiplied by the additional energy required to soar per unit time and added to the diurnal and nocturnal energy expenditures for both the male and the female of each nest site per day to yield the relative energetic expenditures. I measured the length and moment arm of the deep portion of the pectoralis muscle, the wingspan and the spanwise center of pressure of a wing on each of three Red-tailed Hawk carcasses and calculated energy expenditure for soaring according to Pennycuick (1972). Male and female Red-tailed Hawk weights were taken from Craighead and Craighead (1956). These weights are similar to those collected by Bloom (unpublished) in California (averages of 0.957 kg and 1.250 kg for males and females respectively). Energy of clutch formation for the female was expressed as a percentage of BMR (Ricklefs, 1974). Because ambient temperature during the active portion of the day was rarely more extreme than 20° to 35°C and because evening temperatures were rarely lower than 5°C, birds were assumed to be within their range of thermal neutrality (roughly O°C to 35°C; Calder, 1974; Kendiegh, 1969). Flapping flight was restricted to occasional short bursts during some soaring bouts. Because of its infrequent occurrence, short duration, and corresponding minimal costs, I assumed its influence was negligible in construction of the overall energy budget. RESULTS.-Pair A fledged two chicks and pair C fledged one chick. Pair B was unsuccessful probably due to the disappearance of the male in midspring. Males and females of each pair displayed similar patterns in amount of flight activity throughout the breeding season (paired two-tailed t-test on differences score t = 1.87, 15 df, p < .05). However, the male was 10 to 50% more active than the female until the clutch hatched (Fig. 1, top, paired two-tailed t-test on difference scores, t = 2.85, 8 df, p < .05) after which female flight time in two pairs equaled, and later surpassed male activity by 10 to 30%. This trend was, however, not statistically significant. Both the male and female decreased flight activity during egg incubation and early chick brooding (two-tailed t-test on pooled male, female scores, t = 2.71, 22 df, p < .05) presumably in order to incubate; however, it is the female and only rarely the male, who actually broods (pers. obs.). Flight activity was therefore highest for both the male and female during courtship and 150 vol. 29, No. 2 This content downloaded from 157.55.39.192 on Thu, 25 Aug 2016 04:23:47 UTC All use subject to http://about.jstor.