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カラスの冬ねぐらに近い九州大学農学部附属農場篠栗農場果樹園において，カラス被害を防止する最も安価で簡便な方法と思われる釣り糸を用いた防烏線の設置方法とその効果を20年間にわたって追究した．カラスによる食害はリンゴ，ビワ，ブドウ，イチジク，カキで特に甚大で，ナシ（‘幸水’など）やモモで甚大，スモモやウンシュウミカン（興津早生）ではやや軽く，キウイフルーツ，ウメおよびクリではなかった．また，カンキツや...ナシでは糖度が高くて食味が良い品種ほどカラス害が甚大であり，糖含量が低くて酸含量が高い品種（レモンやユズなど）では被害はほとんどなかった．長さ7mの2本の竹製支柱間に1m間隔で釣り糸を平行に張り，撒き餌（興津早生；7×20m）の上に1.5mの高さで支柱を水平に設置した場合，及び二つの短い辺（7m）の中心にそれぞれ支柱を垂直に設置した場合，いずれも高い（98％）食害防止効果があった．そこで，設置が容易なこの垂直・平行張りを基本として18年間圃場試験を行い，改良を加えた．平棚栽培のブドウやナシでは棚面からの釣り糸の間隔を30，30，30，50，50，100，100，100cmとし，釣り糸線で囲む1区画を10×100mまたは20×20mとすることにより食害を回避できた．リンゴ（倭化栽培），ビワなどの並木植えの果樹では釣り糸間隔を30～100cm，1区画を縦10m，横20～100mにして防烏線で取り囲むことによりほぼ完全に回避できた．また，スモモでは園（20a）の周りに同様にして釣り糸線を100cm間隔で設置し，中心部に爆音機を作動させることにより，カラス害を回避できた．ウンシュウミカンでは，釣り糸線の間隔を100cm，釣り糸線で囲む1区画を30×30mまたは20×50mとすることにより食害を防止できた．20年の試験期間中に12羽のカラスが防烏線に捕捉されたことを考慮すると，釣り糸の防烏線が示した高いカラス害回避効果は物理的隔離のみによるものではなく，釣り糸線がカラスに見え難いため，カラスが警戒して採餌を断念する一種の撃退効果によるものと結論された。
Method for setting fishing lines considered to be the most convenient and cheapest tool for big-sized bird control and their effect on the protection of fruits from crow pecking were investigated for 20 years in the Sasaguri orchard of the Kyushu University Farm adjacent to the winter roost of crow. Crows were the most troublesome in such fruit crops as apple, Japanese pair 'Kosui', loquat, grape, peach, Japanese plum, fig and Japanese persimmon, followed by early ripening satuma mandarin. However, they did not peck the fruits of kiwifruit, mume and walnut. In citrus and Japanese pear cultivars, the higher the sugar content in the fruits, the heavier the pecking damage occurred, whereas little damage was found among the cultivars with low sugar and high acid content such as lemon and yuzu. Fishing lines were run parallel to each other between two 7 m bamboo poles at intervals of 100 cm. One set of the two poles was horizontally placed at 1m height on the scattered food ('Okitsu wase' satsuma ; 7 × 20m), and another set of the two poles was vertically placed on the center of next scattered food parallel to the long side. Both the horizontal and vertical systems exhibited great effect (98 %) on the prevention of pecking by crow. On the basis of this result, field trials to adjust appropriately the intervals between fishing lines and between the poles were performed for 18 years using the vertical system being easy for handling. In grape and Japanese pear trees trained on the horizontal trellis, it was the best that intervals between the fishing lines above the trellis were 30, 30, 30, 50, 50, 100, 100 and 100 cm in order from the trellis to terminal of the 7 m poles and that one plot surrounding by the fishing lines was 10 × 100m or 20 × 20. In standing trees such as apple and loquat planted in a line, the intervals of fishing lines were 30-100m and the plot was 10 × 20-100m. In the Japanese plum orchard (20a), mature fruits were protected from crow pecking by surrounding with fishing lines run at 100cm intervals and by setting an automatic noise cannon on the center of the orchard. In satsuma mandarin, the intervals were 100m and the plot was 30 × 30m or 20 × 50m. During the present study carried out for 20 years, 12 crows were caught in the fish lines. It was suggested from these results that the effect of fishing lines on the prevention of crow pecking in fruit trees is not only due to physical separation of fruits from crows, but also due to their wariness making them to give up to eat fruits because of their difficulty to identify the fishing lines by their eyes.続きを見る
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