Responses of Birds in Rice Fields to Winter Flooding

27 Mar Responses of Birds in Rice Fields to Winter Flooding

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Abstract

茨城県霞ヶ浦沿岸の水田地帯において，冬期（12～2月）に約0.4 haの水田2か所を湛水管理し，日中および夜間の鳥類の利用およびその潜在的な餌となる小動物の現存量を調査した．また，自然に水の溜まった水田，年間を通して湛水されている蓮田，対照区としての非湛水田においても比較調査を行ない，冬期湛水による鳥類や餌動物への効果について明らかにした．乾燥した田には水鳥の生息は全く見られなかったが，湛水田にはタゲリVanellus vanellusやタシギ類Galli go spp.を中心とした水鳥が飛来し，採餌場所として利用された．これは主に，湛水によって水田表層や土壌中に増加したイトミミズ類，ユスリカを除く昆虫類，ヒル類などによるものと考えられた．また，未耕起の湛水田にタシギ類がより多かったことから，隠れ場所（刈り株や二番穂）の存在も重要であると考えられた．蓮田はシギ・チドリ類に加え，魚食性のサギ類やカワセミAlcedo atthisなどにも利用され，ポンプによる給水のため魚類や両生類幼生の生息しない湛水田との違いが現れた．湛水管理は陸鳥にも有意な個体数の違いをもたらし，ヒバリAlauda arvensisとホオジロEmberiza cioidesは非湛水田に多かったが，タヒバリAnthus spinolettaは湛水田に集まり，ツグミTurdus umanni，ハクセキレイMotacilla alba，セグロセキレイM. grandisは蓮田で密度が高い傾向にあった．これらの結果から，冬期の乾田地帯に湛水田を設置することは，水鳥・陸鳥ともに多様な種が選好する新たな採餌場所を供給し，鳥類の越冬環境としての水田の機能を高める効果があると考えられた．

English Abstract

Two segments of rice fields (0.4 ha each) in the coastal rice-producing region of Lake Kasumigaura, central Japan, were experimentally flooded from December to February (non-cultivation season), and bird abundance and macroinvertebrate availability in the fields were compared with those in unflooded rice fields and continuously flooded lotus fields. No waterbirds were recorded in the unflooded fields, but the experimentally flooded fields attracted Northern Lapwing Vanellus vanellus and snipe Galli go spp., probably due to higher numbers of Tubificidae, Insecta (except Chironomidae) and Hirudinoidea in the soil and the water on the fields. Snipe were more abundant in unplowed flooded fields than plowed ones, indicating the importance of leaving rice stubble and post-harvest shoots as shelter for these secretive foragers. The flooded rice fields did not attract fish-eating waterbirds such as egrets Egretta spp., herons Ardea spp. or kingfishers Alcedo atthis, which occasio lly appeared in the lotus fields. This was, probably because water pumped on to the experimentally flooded fields provided few fish or tadpoles. Landbird abundance also differed significantly among the field types. Unflooded fields supported more Skylarks Alauda arvensis and Siberian Meadow Buntings Emberiza cioides than others, whereas flooded fields were preferred by Water Pipits Anthus spinoletta. Lotus fields attracted Dusky Thrushes Turdus umanni, White Wagtails Motacilla alba and Japanese Wagtails M. grandis. The results indicate that flooding fields in a dry rice-producing region in winter provides many water- and landbird species with suitable habitats, and contributes to increasing the diversity of the wintering bird community.