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北台灣黑鳶的換羽及成幼鳥比例的探討
Black Kite (Milvus migrans formosanus) Molting and Adult/Juvenile Proportion in Northern Taiwan
摘要
本文利用數位相機與望遠鏡頭拍照及陰影修圖法，研究2004年8月~2006年2月造訪基隆港覓食之台灣黑鳶（Milvus migrans formosanus）的換羽過程及成烏與幼鳥比例。黑鳶幼鳥於出生隔年3月下旬開始首次換羽，可看到新的初級飛羽P1及P2正在成長中，舊的P3及P4被擠出而伸長突出。初級飛羽只有一個換羽波鋒，由最內側P1開始到最外側P10的順序緩慢進行。在同一時間進行換羽的初級飛羽不超過兩根以上，因而不致對飛行覓食有重大影響。次級飛羽的換羽則於P1～P4完成時，由S1及S4（或S5）以兩波鋒的方式開始，而且更換的數目有限，對飛行並未產生太大影響。尾羽的換羽時間略同次級飛羽，進行對稱性換羽，一次更換可達8根，通常T6最先而T5最後完成。翼上初級大覆羽、次級大覆羽及尾上覆羽均在第一時間就跟飛羽進行換羽，而翼下各覆羽則稍後於P6、P7換羽時才進行換新。新覆羽的淡色羽緣均明顯比幼鳥原覆羽狹窄，使得幼烏相對於成鳥的分辨容易許多。胸腹部及頭部的淡色縱紋在換羽後亦變較窄。相對於幼鳥，成鳥直到5月才開始換羽，過程幾乎與幼鳥相似。全部換羽完成約需6個月，於9月下旬至10月中旬完成。除了初齡幼鳥，黑鳶每年都完整換羽一次。利用電腦陰影修圖後比較翼下斑紋，也就是「翼紋法」，可成功辨認成鳥及幼鳥的個體，以此方法所得出現在基隆港的黑鳶總數為傳統計算使用之「最多隻數法」的2.7倍。此結果顯示，傳統的最多隻數法未顯現出烏的真正族群數量，有嚴重低估情形，需乘上一定係數（例如2 .7）才行。統計結果顯示，北部黑鳶成鳥／幼鳥的數量比例為1. 5，由此估算的黑鳶年死亡率高達30% 。
並列摘要
Black Kites (Milvus migrans formosanus) foraging at Keelung Harbor in August 2004 to February 2006 were studied by digital tele scopic photography and ”Shadow/Highlight software”. The juveniles started molting in late March, the first molting witnessed was newlygrowing P1 and P2 with simultaneous extruding of old P3 and P4 primaries. Molting progressed in wave form with single frontier from the innermost P1 to outermost P10. The molt progressed slowly and the number of primaries under molt at one time was limited to two probably not to affect flying. After completion of Pl-P4 molt, the seco ndary began molting but now with two frontiers starting from S1 and S4 (or S5). Again, the number under molting in one time appeared limited . The tail feathers started molting at the time when the secondary molt. In contrast to the primaries, up to 8 tail feathers might molt simultaneously in symmetric manner. T6 was the first while T5 the last to molt. The upper coverts, including greater primary, greater secondary and tail coverts, started molting together with the flying feathers, whereas the under greater coverts molted later than the upper coverts. The pale tip-band of all new coverts became thinner than the respective juvenile ones. These changes make the post-molt adults easily distinguishable from the juveniles. the pale streak on breast and belly also became thinner after molt. The adult started to molt in May in a manner similar to juveniles. Entire molting took six months and completed in late September to October. It is concluded that Black Kite executes complete annual molting every year except the first year fledglings.Digital imaging provides beneficial wing-print (like finger-print) and makes recognition of individual possible for Black Kite. The population ratio or adult to juvenile, recorded between the post-molting and pre-breeding period (September to December), was only 1.5 indicating that the annual mortality rate is as high as 30%. The total numbers of juveniles and adult were always greater than the traditional counting i.e., the highest number of birds appearing at the same time in a day, by 2.7 times. This implies that the traditional ”highest number counting” greatly underestimated the Black Kite population.