江﨑 保男

The rodents' predation intensity and discrimination ability toward the predispersal beechnuts (Fagus crenata) were investigated using a tree tower in a beech forest, central Japan in 1999 and 2000. In this stand, using seed traps, the densities of fallen viable nuts were 35.1 m(-2) in 1999 and 8.4 m(-2) in 2000. The vertebrate-damaged nuts had fallen 5.6 and 2.2 m(-2) in 1999 and 2000, respectively. Yet, the crop of viable nuts in 1999 was not so rich as that in a mast year. In 1999, predispersal predation by rodents was recognized at 16-19 m above ground through the bagging experiment. In 2000, there were no predispersal predation and yet we captured Apodemus argenteus three times and Glirulus japonicus frequently on the tree. Judging from the facts of their feeding behaviors and the tooth scars left on the cupules and nuts, Apodemus argenteus might have been more responsible for predation to the predispersal beechnuts rather than Glirulus japonicus. Apodemus argenteus population seemed to be abundant on the ground in both years. If the main agent of predispersal predator were Apodemus argenteus, their number shifted to the canopy would be much larger in 1999 than in 2000 according as the crop of viable nuts. In an additional experiment, rodents preferred intact cupules to insect-damaged cupules on the tree, suggesting that they discriminated the quality of the predispersal nuts, even in the cupule stage, through olfactory and/or visual senses. Thus, predispersal nut predation by rodents was prevalent during the limited period in autumn.

樹洞営巣性鳥類の樹洞をめぐる種内•種間の相互関係について,自然樹洞の研究を中心にレビューした.樹洞営巣性鳥類の研究では巣箱が積極的に使われてきた.巣箱を利用することには,巣箱の中を容易に観察できるために繁殖成功を正確に測ることができる点や巣箱とその中身の追加•除去などにより操作実験が可能となる点など幾つかの利点がある.しかし,繁殖密度や種構成などが容易に変化してしまうなど不利な点もある.van Balen et al. (1982)以降,自然樹洞での樹洞営巣性鳥類に関する研究が少なからず行われるようになってきた.そこで,自然樹洞に関する研究について,利用可能な樹洞数と樹洞営巣性鳥類の占有率,頻繁な樹洞の再利用,営巣場所選択における競争と捕食の重要性を概説した.最後に,森林管理や保全の観点から興味深い "Nest Webs" の考え方について簡単に紹介した.

1)京都府南部の丘陵地域に約40km²の調査地をもうけ,オオタカの目視調査を1年間おこなった.丘陵部はアカマツが混じりコナラを主とする山林,平地部は農耕地と市街地であった.<br>2)オオタカは周年にわたって出現した.調査地中心部の約5km²の孤立林で1つがいが営巣し,繁殖に成功した.孤立林の外側には広い山林あるいは農耕地•市街地が虫食い状にまじる山林が存在したが,これらの場所には営巣つがいが存在しなかったと考えられる.<br>3)翼羽の欠損によりつがいのオスは個体識別ができたが,繁殖期にはこのオス以外のオスは確認できなかった.繁殖期を前半と後半に区分すると,オスの行動圏の大きさはそれぞれ6.8km²と10.8km²であった.<br>4)7月から8月にかけて出現した幼鳥は調査地内で巣立った個体であると推測されたが,巣立ち直後の7月には巣の付近でのみ目撃され,8月にはかなりよく動き回るようになり活動域が季節とともに拡大する様子がみられた.<br>5)オオタカの出現パターンやその他の状況証拠から,孤立林を含む調査地中心部は繁殖期にはつがいによって独占使用されていて,行動圏の大きさは5-10km²であったと考えられる.一方,非繁殖期には同じ地域が複数の成鳥オスや亜成鳥を含むオオタカによって非排他的に共同利用されていたとみられる.<br>6)孤立林は繁殖期のみならず周年をとおしてオオタカの生息の好適地であったと考えられるが,尾根にかこまれた比較的安全な巣場所を有することにくわえて,比較的大きな孤立林であること,かつ狩り場として適当な農地につつく林縁部を有することなどがオオタカに好まれる理由ではないかと推察される.

1)京都府南部の丘陵地域に約40km²の調査地をもうけ,オオタカの目視調査を1年間おこなった.丘陵部はアカマツが混じりコナラを主とする山林,平地部は農耕地と市街地であった.<br>2)オオタカは周年にわたって出現した.調査地中心部の約5km²の孤立林で1つがいが営巣し,繁殖に成功した.孤立林の外側には広い山林あるいは農耕地•市街地が虫食い状にまじる山林が存在したが,これらの場所には営巣つがいが存在しなかったと考えられる.<br>3)翼羽の欠損によりつがいのオスは個体識別ができたが,繁殖期にはこのオス以外のオスは確認できなかった.繁殖期を前半と後半に区分すると,オスの行動圏の大きさはそれぞれ6.8km²と10.8km²であった.<br>4)7月から8月にかけて出現した幼鳥は調査地内で巣立った個体であると推測されたが,巣立ち直後の7月には巣の付近でのみ目撃され,8月にはかなりよく動き回るようになり活動域が季節とともに拡大する様子がみられた.<br>5)オオタカの出現パターンやその他の状況証拠から,孤立林を含む調査地中心部は繁殖期にはつがいによって独占使用されていて,行動圏の大きさは5-10km²であったと考えられる.一方,非繁殖期には同じ地域が複数の成鳥オスや亜成鳥を含むオオタカによって非排他的に共同利用されていたとみられる.<br>6)孤立林は繁殖期のみならず周年をとおしてオオタカの生息の好適地であったと考えられるが,尾根にかこまれた比較的安全な巣場所を有することにくわえて,比較的大きな孤立林であること,かつ狩り場として適当な農地につつく林縁部を有することなどがオオタカに好まれる理由ではないかと推察される.

1997年5月から7月にかけての繁殖期に，長野県下高井郡木島平村カヤの平地区(35°50'N，138°30'E)のブナ原生林(5.7ha，以下，原生林という)とカラマツ人工林(7.7ha，以下，人工林)で鳥類群集を調査した．原生林と人工林でそれぞれ21種と22種の鳥類を記録した．二つの林の群集の類似度は0.86であった．群集の多様度(H')は人工林よりも原生林で高かった．どちらの林でもヒガラParus aterの個体数が最も多かった．原生林では，ヒガラのほかにも樹洞営巣性鳥類(シジュウカラ P. major，ニュウナイスズメ Passer rutilans，キビタキ Ficedula narcissina，ゴジュウカラ Sitta europaea)が個体数の上位を占めた．一方，人工林では，ヒガラと開けた生息環境を好むニュウナイスズメ，ビンズイ Anthus hodgsoniやアオジ Emberiza spodocephalaの個体数が多かった．密生したササ藪を好むクロジ E. variabilisの生息密度は原生林が人工林の3倍に達した．おそらく，二つの林の鳥類群集の違いは植生構造の違いや樹洞の多さの違いによるものと思われた．

Traditional rice paddy cultivation in Japan has continued to supply wading birds with good foraging habitats. We studied habitat segregation among species of herons and egrets throughout one year in a pond-paddy complex area in Harima, Hyogo Prefecture. The principal foraging habitats diffed between diurnal egrets: arable and grassland for cattle egrets Bubulcus ibis, emergent vegetation at the pond-shore for intermediate egrets Egretta intermedia, pond open water for great egrets Egretta alba, and highly diverse habitats for little egrets Egretta garzetta. Smaller percentages of grey herons Ardea cinerea and night herons Nycticorax nycticorax were recorded as foraging during the daytime, reflecting their dusk/nocturnal activity patterns. The foraging habitats of each diurnal species were essentially the same throughout the study period, with partial seasonal changes some of which occurrcd commonly among species. The diversity of foraging habitats of little egrets was a result of their seasonal movement between habitats, which was probably related to agricultural water-level control. The seasonal change of foraging habitats by little egrets caused serious habitat overlaps with other species, some of which were recognized to be superficial. The overlap between little egrets and great egrets on pond open water in autumn, however, was considered to be a true overlap due to temporarily rich food availability.

Settlement of male great reed warblers in the breeding ground was highly asynchronous at Lake Biwa in Japan. It took over 1 month from the appearance of the first male to a saturation in number of males. In resource-defense-polygyny, males are expected to try to defend as large an area as possible in the optimal habitat. In fact, a small number of the earliest settling males divided up the breeding ground almost completely as territories among themselves and these were later reduced in size with the addition of later settlers. The reduction was not due to a seasonal decline of aggressiveness on the part of the owners but to a higher level of intrusion pressure by later arriving males. The neighbor-neighbor relationship, once established, was rather stable. Home range overlap was small and territorial contacts were few between neighbors. Territorial boundaries seem not to shift despite the addition of new males as long as the neighbors were the same. The stable relationships between neighbors are expected to help the earlier resident to maintain his large and good territories throughout the breeding season and hence polygyny is favored in this species.

A flock of Oriental White Storks appeared in Yonaguni Island, the westernmost Japan in November 1993. The flock size at the time was eleven. The communal roost that was formed in a lowland valley with paddy field seems to have been shifted to another paddy field area during a short period in March 1994, probably due to human disturbance at the former place. Another important habitat for storks was pasture where they foraged. The flock of eleven storks were never recorded after December, but it is highly probable that the eleven birds overwintered in the island, since roosting of a single bird apart from the communal roost with ten birds was suggested for a night in February. Large flocks at the communal roost were recorded last on 19 March and thereafter one or two storks were witnessed until the summer of the year.

A flock of Oriental White Storks appeared in Yonaguni Island, the westernmost Japan in November 1993. The flock size at the time was eleven. The communal roost that was formed in a lowland valley with paddy field seems to have been shifted to another paddy field area during a short period in March 1994, probably due to human disturbance at the former place. Another important habitat for storks was pasture where they foraged. The flock of eleven storks were never recorded after December, but it is highly probable that the eleven birds overwintered in the island, since roosting of a single bird apart from the communal roost with ten birds was suggested for a night in February. Large flocks at the communal roost were recorded last on 19 March and thereafter one or two storks were witnessed until the summer of the year.

Foraging habitat selection of breeding great reed warblers was studied at a shore of Lake Biwa. The foraging grounds of parent warblers during the nesting period were not restricted to the breeding territory of the reed marsh, their nestling habitat. The paddy field outside the reed marsh was used communally by them throughout the breeding season. Females with early stage nestlings did not visit the paddy field whereas when nestlings were older than 3 days, more than half of their total food was collected there. Females with nests adjacent to the paddy field tended to exploit the paddy field more often than those with nests distant from it. Monogamously mated females tended to exploit the paddy field more often than polygynously mated females. Food collected in the paddy field was larger than that in the reed marsh and parent birds were prepared to travel longer distances to exploit the rich source of food in the paddy field. The importance of the communal foraging ground outside the reed marsh as it background of the polygynous mating system of this species is discussed.