Jap. J. Ornithol. 36: 71-78, 1987

Breeding Success of the Grey-faced indicus

Yukihiko KOJIMA*

Laboratory of Sociology, Faculty of Science, Osaka City University, Sugimoto, Sumiyoshi-ku,Osaka 558

サ シ バButastur indicusの 繁 殖 成 功 率

小 島 幸 彦*

大阪市立大学理学部動物社会学研究室

The Grey-faced Buzzard Eagle Butastur indicus is a medium-sized hawk, which breeds in Ussuriland, South Manchuria, North China and Japan, and winters in Indo- China, the Philippines and North Celebes (BROWN & AMADON1968). It is a common summer resident in Japan, widely distributed except in northern districts (WBSJ 1980). The Grey-faced Buzzard Eagle, however, has received little investigation, and only recently basic information on the biology of the became available (KOJIMA 1982). In this paper, I describe the timing and success of breeding by Grey-faced Buzzard in an area of central Japan. I also discuss the possible relationships between breeding success and food supply, by comparing with those of some other raptors, in which responses in breeding performance to density changes of their main prey have been intensively studied (see review by NEWTON 1979).

STUDY AREA

This study was conducted in mountain forests near Kawachi-nagano City (34°27'N, 135°34'E), Osaka Pref., during 1977-1980. An initial study area of 2,800 ha in 1977 was enlarged to 11,000 ha in 1978-1980. The topography is generally flat to gently rolling, except for a steep ridge lying along the southern boundary of the area. Altitude ranges from 150 m to 900 m above sea level. About half of the region is covered by Japanese cryptomeria Cryptomeria Japonica- hinoki Chainaecyparis obtusa plantations, and pine forests, mainly of Japanese black pine Pinus thunbergii and red pine P. densiflora. The rest of the area consists of clear-felled lands, deciduous forests, rice fields, orchards, and human housing. Further description of a representative area can be found in KOJIMA (1982).

METHODS

Data on nesting success were from 8, 16, 25, and 17 nests in 1977-1980, respec- tively. Some pairs re-used their old nests in successive years, and a total of 53 nests

*Present address:Muikamachi Eirinsho Yuzawa Seihinjigyosho, Yuzawa 1920-2, Yuzawa-cha, Uonuma-gun, Niigata 949-61.現 所 属:六 日 町 営 林 署 湯 沢 製 品 業 業 所(〒949-61新 潟 県 南 魚 沼 郡 湯 沢 町 大 字 湯 沢 1920-2). 72 Yukihiko KOJIMA [Jap. VoL36J. Ornithol. N Vol.36 No.2/3 o. 2/3 was detected. Nest contents were inspected directly or with a telescope (40*) or binoculars (10*) from nearby points overlooking the nests. Nests were regarded as active if they contained one or more eggs. For nests detected in the nestling stages, clutch and initial brood sizes were of course unknown, and only the final brood size (number of nestlings near fledging at age 30 days or more) was determined.. Several nests were found after fledging. These were easily located because young just fledged often came back to the nest to obtain food which the parents brought; the young also roosted at or near the nest. In several nests the laying date of the first egg was known exactly. However, in other nests it was estimated by comparing the nestling weight against the age- weight curve derived for known-age young, and from the stage of feather develop- ment (KOJIMA, unpublished). A 32-day incubation period (see later) was used for backdating from hatching dates to estimate laying dates. Nests in which at least one young fledged were classed as successful. The fledging age, when young first left the nest tree, was determined at some nests by continued observations at the appropriate stage, but in most nests it was determined only within a range of a few days.

RESULTS

1) Breeding chronology Grey-faced Buzzard Eagles were present in the breeding areas for about six months; the first (usually males) appeared in early April, and by early October the last ones disappeared. Immediately after arriving, males started aerial displays such as soaring with calls, wing-flapped circling display, and diving (see KOJIMA 1982). Females arrived in the males' territories usually a few days after the males. About a week before the first egg, pairs began to add twigs to previously used nests or to build new nests. Most pairs started incubation between late April and early May. Females usually laid two or three eggs at 2-day intervals. The hatching interval between the first and second eggs was always less than a day, because full time incubation began one day before or immediately after the laying of the second egg. The incubation period (including the laying day but not the hatching day) was 32-33 days for the first egg and 31 days for the second and later eggs, respectively. In most nests the first egg hatched between late May and early June, with a mean date of 31 May (range=18 May-15 June, n=53). Most young fledged in the first half of July, with a mean date of 5 July (range=23 June-20 July, n=46). The nestling period (for the first young) lasted an average of 36 days (range=32-39 days, n=46). Thus, the breeding cycle from onset of territoriality until fledging of young was com- pleted in about three months. 2) Laying date The laying date of the first egg was estimated in 53 clutches. In most nests laying started in late April or early May (Fig. 1); the mean date was 26, 29, 30, and 30 April in 1977-1980, respectively (all years combined: 29 April). Dates were earlier in 1977 than in other years; but annual differences were statistically significant only between 1977 and 1980 (Mann-Whitney U-test: 1977-78, U=24.0, NS; 1977-79, U=46.5, NS; 1977-80, U=24.0, P=0.05; 1978-79, U=94.5, NS; 1978-80, U=51.5, NS; 1979-80, U=130.0, NS). Thus laying dates varied little between years. December 1987] Breeding Success of the Grey-faced Buzzard Eagle 73

Fig. 1. Annual variation in the laying date of the first egg. Vertical line shows mean, open bar SD, and horizontal line range.

Fig. 2. Seasonal trends in mean clutch size, all years combined (n=25). Mean±SD are shown.

3) Clutch size The clutch size was known in 28 nests; ten nests (36%) contained two eggs, sixteen (57%) three eggs, and two (7%) four eggs. The clutch size averaged 2.7 in 1977 (n=3), 2.7 in 1978 (n=6), 2.8 in 1979 (n=11), and 2.6 in 1980 (n=8), respectively (all years combined: 2.7+0.6 SD, n=28). Differences in clutch sizes between years were not statistically significant (Student's t-test: 1977-78, t=0,00, NS; 1977-79, t=0.39, NS; 1977-80, t=0.12, NS; 1978-79, t=0.44, NS; 1978-80, t=0.12, NS; 1979-80, t=0.73, NS). The clutch size varied little with advance in laying date until 5 May (Fig. 2); the later samples were too small to show the seasonal decline.

4) Brood size The initial brood size per nest which produced at least one hatchling averaged 2.7 in 1977 (n=3), 2.8 in 1978 (n=5), 2.6 in 1979 (n=10), and 2.1 in 1980 (n=7), respectively (all years combined: 2.5±0.9SD, n=25). The smaller size in 1980 was due to a greater partial egg loss during incubation (see later); in this year 19% of eggs were lost from clutches, compared with 6% in 1979 and none in 1977 and 1978. Differences in initial brood sizes between years were not statistically singificant (1977- 78, t=0.24, NS; 1977-79, t=0.13, NS; 1977-80, t=0.78, NS; 1978-79, t=0.43, NS; 1978-80, t=1.14, NS; 1979-80, t=0.99, NS). The final brood size averaged 2.4±0.7SD (n=46) in all years combined. No young died around the time of fledging, and all that had survived till then left the nest 74 YukihikoKOJMA [Jap.J. Ornithol.Vol.36no.2/3

Fig. 3.. Seasonal trends in mean final brood size, all years combined (n=46). Mean±SD are shown.

successfully. Differences in final brood sizes were not significant between years (1977-78, t=0.60, NS; 1977-79, t=0.62, NS; 1977-80, t=0.10, NS; 1978-79, t=0.11, NS; 1978-80, t=0.80, NS; 1979-80, t=0.86, NS). Final brood sizes varied very little with advance in laying date until 5 May (Fig. 3); the later samples were too small to show the seasonal decline. The average number of fledglings per active nest was 1.9±1.1SD (n=58) in all years combined (Table 1). The smaller values in 1979 (1.8) and 1980 (1.6) were due to a higher proportion of nest failures (Table 2); though the differences between years were not significant (1977-78, t=0.03, NS; 1977-79, t=0.93, NS; 1977-80, t=1.39, NS; 1978- 79, t=1.05, NS; 1978-80, t=1.54, NS; 1979-80, t=0.61, NS).

5) Breeding success Over the four breeding seasons, almost all nests (97%) contained eggs (Table 1); however, in two inaccessible nests (3%) I could not determine whether eggs had been laid, because they had failed when found. Most active nests (91%) produced at least one hatchling, but in four nests (6%) no young hatched. In two nests (3%) it was not determined whether hatchlings were produced or not. Both hatching success and fledging success were determined from 28 nests in which the clutch size was known. Out of 76 eggs from these full clutches, 63 (83%) hatched and from these 49 (78%) young fledged successfully. Nest success (the proportion of successful nests) was 79% in all years combined (Table 1). Nest success was slightly lower in 1979 (72%) and 1980 (71%) than in the other two years; but the differences were not statistically significant (x2-test: 1977-78, x2=1.09, NS; 1977-79, x2=2.84, NS; 1977-80, x2=2.94, NS; 1978-79, x2=1.37, NS; 1978-80, x2=1.41, NS; 1979-80, x2=0.01, NS).

6) Causes of nest failure Of 66 nests examined, 14 (21%) failed to produce fledglings (Table 2). In 11 nests (79%) the cause of failure was known with reasonable certainty, but in three inacces- sible nests neither the stage of failure nor the cause was confirmed. Of these nest failures, three (21%) occurred at the egg stage, and eight (57%) at the nestling stage (Table 2). Four nests with eggs and/or nestlings fell to the ground when dead branches sup- porting the nest broke, destroying the nest contents. Such nest collapse accounted for 29% of all complete failures (6% of all nests, Table 2). One nest in 1978 con- December 1987] BreedingSuccess of the Grey-facedBuzzard Eagle 75

Table 1. Breeding success of the Grey-faced Buzzard Eagle in 1977-1980.

Nests which contained one or more eggs. 2 Nests in which at least one young fledged. Figures in parentheses indicate the number of fledglings/nests. Nests in which the real number of fledglings was unknown are excluded; one nest (1977), four nests (1978), and one nest (1979) were found after fledging.

Table 2. Causes of complete nest failure.

1 Including four nests (two in 1979 and two in 1980) which contained both hatchlings and eggs when the complete failure occurred. taining three young fell late in the nestling period, but all these young survived and flew successfully. All collapsed nests were in dead or dying pine trees; of 53 nests found in the study area, 21 (40%) were in such pines. Other clutches and broods disappeared from the nests without trace between nest visits, probably due to predation or human intervention. Such losses in four nests accounted for 29% of all failures (6% of all nests, Table 2). In three nests full broods disappeared, leaving the remains of down, feathers, and broken legs at and below the nest, suggesting that some climbing mammalian predators, such as martens, were responsible. Such predation accounted for 21% of all failures (5% of all nests, Table 2). Three eggs (4% of all eggs) in three nests (5% of all nests) did not hatch, probably because of embryo deaths. Egg-cracking occurred only once accidentally by the parent . Within a few days the cracked egg was taken away by the hen, but other unhatched eggs remained in the nest and were incubated for longer than usual. Partial nestling loss occurred in two successful nests (3% of all nests); two nestlings in the size-4 and size-3 broods were found dead in the early and mid-stages, respec- tively. In the former brood the dead young disappeared from the nest within a few 76 Yukihiko KOJIMA [lap.J. Vol.36Ornithol.Vol.36 No. 2/3 No.2/3

days, and in the latter it was fed by the hen to the remaining young. In both cases the youngest nestling was repeatedly attacked by its siblings, and possibly it died from starvation because it was unable to compete for food with its larger nest mates. Thus, the death of young seemed to be associated with asynchronous hatching and sibling aggression; the hatching of the first and second eggs was almost synchronous, but later eggs hatched two or more days later. As a result, the nestling that hatched last had considerable disadvantage in competitive ability compared with its older siblings. No other partial losses were recorded in this study.

DISCUSSION Within species, much of annual variations in breeding success can be associated with annual variations in food supply (see review by NEWTON1979). This has been evident for example in rodent-eating species, such as the Kestrel Falco tinnunculus (CAVE 1968), Rough-legged Buzzard lagopus (HAGEN1969), and Hen Harrier Circus cyaneus (HAMERSTROM1979). Food supply during spring can also influence the laying date as reported in some other raptors, such as the B. buteo (MEBS 1964) and Kestrel (CAVE 1968), in which laying dates were earlier in good food years than in poor ones. In the Grey-faced Buzzard Eagle, the present study showed no significant differences in laying date, clutch and brood sizes, and nest success between years (Fig. 1, Table 1). The results, therefore, suggest that food supply was stable during the study period. The stability of breeding success may be associated partly with exploitation of diverse prey types, probably because such diversity can reduce the chances of total food supply changing greatly; Grey-faced Buzzard Eagles are versatile hunters, catching small snakes, lizards, frogs, insects, and smal mammals (see BROWN& AMADON1968). Seasonal trends in breeding success have been noted in some other raptors, such as the Common Buzzard (MEBS1964), Kestrel (CAVE1968), and Sparrowhawk Accipiter nisus (NEWTON& MARQUISS1984); pairs laying earlier in the season bred more success- fully than those laying later. Seasonal declines in breeding success are produced mainly by a decline in clutch size. In the Grey-faced Buzzard Eagle, by contrast, the clutch and final brood sizes did not decline significantly with advance in laying date, although this result was based on small samples (Figs. 2, 3). A complicating factor may be involved in this; age of breeding birds can influence both the laying date and the clutch size as found in the Kestrel (CAVE1968) and Sparrowhawk (NEW- TONet al. 1981). However, in the present study, it was unclear whether those were age-linked because breeding birds were not marked individually and not aged. In the Grey-faced Buzzard Eagle, partial egg and nestling losses had only slight effects on productivity. The rare occurrence of unhatched eggs may imply a good food condition around the study area; egg addling in other raptors has been linked with poor food conditions, acting through the nutritional state of the female (NEWTON1979). In the Common Buzz ard, addled eggs accounted for 15% of eggs in poor food years, compared with 5% in good ones (MEBS1964). Also in the Sparrowhawk, addled eggs more frequently occurred in poor food habitats than in good ones (NEWTON1976). While asynchronous hatching can be regarded as an adaptation to an unpredictable food supply, sibling aggression can be regarded as a further adaptation to ensure the rapid reduction of the brood to a number that parents can feed in times of food- shortage (LACK 1954, NEWTON1979, FUJIOKA1985). In the Grey-faced Buzzard Eagle, aggression between nest mates seldom resultedd in a death; only two cases of December Breeding Success of the Grey-faced Buzzard Eagle 77 1987 ] such mortality occurred. This gave another indication that food resources were favourable during the study period. However, a quantitative study on food abund- ance is needed to clarify the correlation between breeding rate and food supply.

ACKNOWLEDGEMENTS

I would like to thank Dr. S. YAMAGISHI for his encouragement, advice and criticism throughout this study. Special thanks to Dr. I. NEWTON for his many useful comments on the manuscript. Dr. K. UEDA also offered useful comments on the manuscript. Dr. M. FUJIOKA helped with obtaining literature.

SUMMARY

1) . The breeding of the Grey-faced Buzzard Eagle Butastur indices was studied in a mountainious region of central Japan during 1977-1980. 2) Mean laying and hatching dates of the first egg were 29 April and 31 May, respectively. The clutch size averaged 2.7 with a range of 2-4. Hatching success was 83%. 3) The initial and final brood sizes averaged 2.5 and 2.4, respectively. The average number of fledglings per active nest was 1.9. Mean fledging date of the first young was 5 July. The length of the nestling period averaged 36 days. Fledging success was 78%. 4) Of 66 nests examined, at least 64 (97%) contained eggs, 60 (91%) contained hatchlings, and 52 (79%) produced fledglings. 5) 21% of nests failed to produce fledglings. Predation and nest collapse caused by breakage of dead branches supporting the nest were the most important factors. 6) There were no apparent differences in laying date, clutch and brood sizes, and nest success between years. This suggests a stable food supply during the study period. The rare occurrence of unhatched eggs and of nestling mortality through sibling aggression may indicate a favourable food condition in the study area.

摘 要

1)1977年 か ら1980年 ま で,大阪 府 河 内長 野 市 近 郊 の山 間部 に お い て,サ シバButastur indicus の 繁 殖 成 功 率 に つ い て 調 査 した. 2)第1卵 の平 均 産 卵 日 と孵化 日は， そ れ ぞ れ4月29日 と5月31日 であ った.一 腹 卵 数 は平 均 2.7卵 で,孵化 率 は83%で あ った. 3)1巣 当 りの初 期 雛 数 と終 期 雛 数 は,そ れ ぞ れ2.5羽 と2.4羽 で あ った .繁 殖 が 失 敗 した 巣 も含 め,産 卵 が 行 な わ れ た 巣 の巣 立 ち 雛 数 は 平 均1．9羽 であ った.第1子 の平 均 巣 立 ち 日 は7月5日 で, 育 雛 期 間 は36日 間 で あ った.巣 立 ち 率 は78%で あ った. 4)調 査 した66巣 の うち,.少 な くと も64巣(97%)で 産 卵 が行 なわ れ,60巣(91%)で 少 な くと も雛1羽 が孵化 し,52巣(79%)で 雛 が 巣 立 った. 5)21%の 巣 で は,1羽 も雛 が巣 立 たず 繁 殖 は失 敗 した.そ の原 因 は 主 に 捕 食 と落 巣(巣 を 支 え て い た枝 が折 れ た こ とに よる)で あ った. 6)平 均 産 卵 日,一 腹 卵 数,:雛 数 と繁 殖 成 功 率 に 関 して 顕著 な年 変 化 が 見 られ な か った こ とか ら,

調 査 期 間 に お い て は 食 物供 給 量 は安 定 して い た と推 察 され る.ま た 未孵化 卵 と 雛 間 競 争 に よ る雛 の死 亡 が ほ とん どな か った こ とか ら,調 査地 の食 物 条 件 は 良好 で あ った と思 わ れ る.

LITERATURE CITED

BROWN, L. H., & D. AMADON, 1968. Eagles, hawks and falcons of the world. London, Country Life Books. 78 Yukihiko K0OJIMA [Jap. J.l. Ornithol.Vol.36 No. 2/3 No.2/3

CAVE, A. J., 1968. The breeding of the Kestrel, Faclo tinnuculus L., in the reclaimed area Oostelipk Flevoland. Netherlands J. Zool.18: 313-407. UJIOKA,M., 1985. Sibling competition and siblicide in asynchronously-hatching broods of the cattle egret Bubulcus ibis. Anim. Behav. 33: 1228-1242. HAGEN,Y., 1969. Norwegian studies on the reproduction of birds of prey and owls in relation to micro-rodent population fluctuations. Fauna 22: 73-126. HAMERSTROM,F., 1979. Effect of prey on predator: voles and harriers. Auk 96: 370-374. KOJIMA,Y., 1982. Territory and territorial behaviour of the Grey-faced Buzzard-eagle Butastur indicus. Tori 30: 117-147. (In Japanese with English summary.) LACK,D., 1954. The natural regulation of animal numbers. Oxford, Clarendon Press. Mass, Th., 1964. Zur.Biologie and Populationsdynamik des Mausebussards (Buteo buteo). J. Ornithol. 105: 247-306. NEWTON,I., 1976. Breeding of Sparrowhawks (Accipiter nisus) in different environments. J. anim. Ecal. 45: 831-849. 1979. Population ecology of raptors. London, T. & A. D. Poyser. NEWTON,I., & M. MARQUISS,1984. Seasonal trend in the breeding performance of Sparrow- hawks. J. amin. Ecol. 53: 809-829. NEWTON,I., M. MARQUISS,& D. Moss, 1981. Age and breeding in Sparrowbawks. J. anim. Ecol. 50: 839-853. WILDBIRD SOCIETYOF JAPAN(WBSJ), 1980. The breeding bird survey in Japan 1978. Tokyo, Wild Bird Society of Japan. (In Japanese.)

(Received 10 July 1987)