Breeding Behaviour of the Brown Goshawk Accipiter Fasciatus by T
Total Page：16
File Type：pdf, Size：1020Kb
A V !:> T.RI'I..L.lt\N BIRD WATCHER AUSTRALIAN BIRD WATCHER 1988, 12, 258-267 Breeding Behaviour of the Brown Goshawk Accipiter fasciatus by T. AUMANN, Lot 1 Hansens Creek Road, Hoddles Creek, Victoria 3139 Summary Brown Goshawks Accipiter fasciatus were studied near Melbourne, Victoria during the 1980-83 breeding seasons. Pre-laying displays including conspicuous perching, unilateral and mutual soaring and pair pursuit are described, as are courtship feeding and copulation. Both sexes were involved in nest construction. A territory of200-300 m diameter around the nest was defended against conspecifics and certain other raptors. Nests were also defended against human visitors and avian predators, however the manner and intensity of defence was variable. Females played the main role in nest and territory defence, although it was not determined which sex initially established the territory. Males provided all food for themselves and their mates during the late pre-laying and incubation periods. Egg coverage was near-constant from about the time of clutch completion to hatching. Although males usually covered the eggs while their mates fed, )90% of incubation was undertaken by females. Female attachment to the nest remained high in the days after hatching, declining from 89% to 10% of observation time from the first to the third week of the nestling period. Males continued to provide all food in the early nestling period, the stage of female foraging recommencement varying from nest to nest. Rates of prey delivery declined from approximately 4-5 to 3-4 meals/day during the nestling period. Various other aspects of adult and young behaviour are also described. Of 55 breeding females, two were in first-year plumage and two were identified as second-year birds. All 23 breeding males appeared to be in full adult plumage. No evidence was found for polygamy or polygyny. At one nest a male (grey morph) Grey Goshawk A. novaehollandiae replaced a male Brown Goshawk as the mate of a female Brown Goshawk at about the time of laying. Introduction The secretive accipiters breed at lower densities than most birds, and many nest in tall trees in remote vegetation, Pairs are difficult to locate, particularly early in the reproductive cycle. Breeding biology is well known only for the European Sparrowhawk A. nisus, Northern Goshawk A. gentilis, Cooper's Hawk A. cooperi and the Sharp-shinned Hawk A. striatus. A few African species have also been closely observed at the nest in limited parts of their range. The breeding biology of the Australian accipiters is known only in general terms. Previous studies have been restricted to a single year, to one or a few nests, and usually the nests concerned have been discovered after the hatching of the young (Tonge 1925; Slater 1961; Hough 1972; Cupper & Cupper 1981; Metcalf 1982; Hollands 1984; Metcalf & Metcalf 1986). Methods During 1980-83, breeding Brown Goshawks were studied near Macclesfield (37 °54 'S, 145 °30 'E), 50 km east of Melbourne, Victoria. Although approximately 65 % of the area concerned has been cleared for pasture, shade trees remain in most paddocks, and tall eucalypts (Eucalyptus spp.) and wattles (Acacia spp.) occur beside fences and watercourses and in stands of up to 25 hectares. Nests and their environs were watched from hides mounted on trees 15-40 m from the nest tree, and occasionally from a car parked 20-50 m from the nest tree. Individual observation sessions ranged in length from 2.5-12.5 hrs, total observation time being approximately 50, 55, 130 and 70 hrs for the pre-laying, incubation, nestling and post-fledging periods respectively. Data presented here were largely derived from 12 nests, although all pairs located were watched to some extent. To minimise disturbance, climbing was restricted to the nestling period at four territories. The nests concerned were visited 3-4 times in Y91.to ~ (0) DECEMBER 1988 Brown Goshawk., Breeding Behaviour 259 one or more years, ten nestlings being fitted with stainless steel bands supplied by the Australian Bird & Bat Banding Scheme. To the extent possible, the age of breeding Brown Goshawks was estimated using the criteria of Aumann (1988). The nomenclature and phonetic transcription of vocalisations follow Cramp & Simmons (1980). An attempt was made to estimate territory size through observation of occupied territories from vantage points. Results Discounting failed nests where replacements were subsequently used in the same season, but including those replacements, 62 active Brown Goshawk nests were found: 11, 20, 18 and 13 in 1980, 1981, 1982 and 1983 respectively. Within any breeding season, all matings appeared monogamous. However, because most breeders could not be recognised as individuals, polygamy or polygyny may have occurred. Mate replacement was noted in only one case. A grey morph Grey Goshawk A. novaehollandiae replaced a male Brown Goshawk in pairing with a female Brown Goshawk at about the time of laying in 1982. Other intraseason mate replacement may have occurred. Most breeding goshawks could be aged only as 'first year' or 'adult'; female age was known at 55 nests and male age at 23 nests. Two first-year females were found incubating (one in 1980 and one in 1981). The remainder were adults; of these, two were recognised to be second-year birds. All breeding attempts by first- or second year females were successful. No first-year males were found breeding, and no breeding males were identified as second-year birds. Pre-laying period Brown Goshawks of both sexes were seen infrequently at Macclesfield in early and mid winter. By late winter, both sexes were often found on territories, and I was unable to attribute initial territory establishment to either sex. Many displays occurred in late winter and early spring, elements of courtship and pair bond formation/maintenance probably integrated with the advertisement of territory occupancy. Both sexes perched for long ()20 min) periods on high exposed branches in late winter/early spring. Positions used were mostly (100 m from the nest site ultimately used. Commonly recorded from 0700 to 0930 h, conspicuous perching also occurred in the late afternoon and evening. Perches used were generally well lit by the sun. Although usually silent, one or both of a perching pair sometimes called loudly. In two instances, females used a relatively slow, loud ee. .ee . .ee . .. approximately every 30 s for 30-40 min. Duetting was usually sustained using this call for 1-5 min. As for all parallel calls, the male voice was higher pitched than that of the female. Duetting also occurred with one or both goshawks of a pair at concealed perches within or close to the nest woodland. Soaring displays were observed in the 4-5 weeks before egg laying, and following intruder exclusion throughout the breeding season. Pairs circled over the territory in the pre-laying period, separating and drawing together repeatedly. Physical contacts in these displays were fleeting and light. Mutual soaring flights often reached great heights, culminating in lateral drifts or gradual slanting descents to a position sometimes )2 km from the territory. Alternatively, the male sometimes followed his mate back to the nest woodland in a slow or accelerating slanting descent on closed wings. Such mutual descents culminated in rapid pair pursuit through the territory. During female unilateral soaring males often watched from a perch in the territory below, sometimes AUSTRALIAN 260 AUMANN BIRD WATCHER calling a relatively slow ee.. ee.. ee... for the duration of the flight. On four occasions a soaring female drifted to a position over the nest, lowered first one wingtip then the other, then immediately drifted off. On ten occasions in late winter/early spring a mixed-sex group of 3-4 goshawks soared over an area between traditional territories. These interactions involved high circling, shallow diving, little or no physical contact and no overt antagonism, and they ended with the birds involved drifting apart. In contrast to pair pursuit, much slower following displays were also observed at territories during the pre-laying period. These were of the type described by Debus (1980). In a variant the pair flew side by side 1-3 m apart along and slightly above the margin of the nest woodland. Courtship (supplementary) feeding was observed 11 times, and on at least seven of these occasions it preceded copulation. Four such events occurred )2 weeks before egg laying. Typically, the male flew in with prey in his claws to either the nest or (more often) a nearby perch. As he arrived he food called ee-oo.. ee-oo.. ee-oo ... , the first syllable drawn out, the second sharp and accentuated. The syllables became less distinct if the female was slow to respond. Sometimes males called rapidly e ..e .. e ... as they approached the territory with prey, before food calling. Food was either taken from the male in a foot-to-foot pass, via a drop, or most commonly from the branch where the male perched on arrival. One specific transfer perch was favoured at most territories, 15-120 m from the nest, and 2-15 m above ground. In response to the food call the female flew low (sometimes (1 m above the ground) and directly to the male, calling a loud, slow ee. ee.. ee ... as she neared him. The male either remained momentarily at the food transfer perch as the female collected the food, or flew to a nearby perch just before female arrival. Some 10-90 s later he flew to a different perch (often (5 m from the nest, and once to the nest) closely followed by his mate. Copulations lasting 5-15 s followed, generally as described for accipiters elsewhere (Cramp & Simmons 1980).