蛇鵰（Spilornis cheela）是台灣非常普遍且相當適應人類開墾環境的森林性猛禽。牠性不畏人，在晴天時很好飛且好鳴，在求偶期有非常活躍的空中活動，經常成小群共飛，可在一地同時見到多隻，這些特性都有利於調查其數量。我們選定新北市觀音山作為調查地點，因為這是一座範圍不大的獨立丘陵，已知終年留棲的蛇鵰數量不少，而範圍內道路發達，視野良好，很適合目視調查。我們在觀音山區選定6 個定點，各有不同的視野涵蓋範圍，但整體範圍可涵蓋觀音山90% 的空域。2007 及2008 年春季，我們在蛇鵰最愛飛的求偶期3~4 月間，共進行了8 天次的族群調查，每次調查自09:00~15:00 共執行6 小時，由6 個定點同時進行，每隔10 分鐘同步計算所見的蛇鵰數量。結果8 次調查最多同時見到36 隻蛇鵰，這就是觀音山蛇鵰族群的最低數量。假設我們見到的是整個族群量的60~70%，可推測觀音山約有57~66 隻蛇鵰，密度約為2.3~2.6 隻/ km2。以一日內的調查隻數變化來看，10:00~12:00 之間是蛇鵰飛行的高峰期，也是調查數量的黃金時間。在調查日期的選擇上，應把握3 月份，因此時母鳥尚未產卵，可避免母鳥抱卵不飛造成的調查偏差。

Crested Serpent Eagle (Spilornis cheela, CSE) is a common forest raptor in Taiwan. It adapts well to fragmented forests and shows little fear to human environment. In spring, CSEs frequently form small groups and call loudly when flying over wooded hills during warm days. These
characteristics make them non-secretive and easy to observe. In spring of 2007 and of 2008, I led a team of field observers to survey the CSE population in Guanyinshan where is an isolated volcano hill near Taipei City. We conducted field surveys from March to April on good weather
days that ensure maximum visibility. The survey period I chose is the courtship season for CSE that leads to a lot of airborne activities. 6 field teams were assigned in different sites to cover 90% of the Guanyinshan aerial territory. During each survey day, the teams observed from 9 am to 3pm. Every observer would look for CSEs once every 10 minutes on a synchronous basis. For each 6-hours survey day, 37 synchronous counts were accomplished. In total, the teams made 296
synchronous counts on 8 different days. For the results, higher aerial activities and counts were recorded between 10 am and 12 noon in each survey. The highest CSE count obtained in one synchronous count was 36 eagles. Considering the surveys covered an area of 25 square kilometers and assume that 60~70% of local CSE population were counted; it would be reasonable to conclude that the total population was between 57 to 66 individuals which translates to 2.3 to 2.6 individuals per square kilometer. The data also revealed that March had more airborne activities
than April, probably due to the reduced activities of incubating females in April.