Working for birds in Africa

Ecology, voice and territorial competition of Fraser's and Akun Eagle Owls

p 147-156

Observations are presented from many encounters with two eagle owl species, mainly in Congo and Cameroon (1989-2001). The ranges of nominate Fraser's Eagle Owl Bubo poensis
and Akun Eagle Owl B. leucostictus are fairly similar, except that the former extends to much higher altitudes. These large owls are partly separated by their ecological requirements (B. poensis preferring secondary situations, B. leucostictus old secondary or primary forest with small openings) and diet (B. poensis taking a range of insects and small vertebrates, B. leucostictus mainly or exclusively insects). However, they occasionally come into contact, especially in open-canopy semi-evergreen forest. The most frequent vocalisations of B. poensis are guttural trills (or purring rattles), short or long (several seconds), given singly or in duet; the form vosseleri of Tanzania produces identical trills. High-pitched, far-carrying modulated whistles are heard persistently at some times of year and circumstantial evidence suggests they are produced by immatures; they have no known aggressive or reactive purpose. The most characteristic call of B. leucostictus, always produced by single birds (apparently males), is a series of low, soft grunts roh, roh, . . . ; short, hoarse ascending whistles and short, slow trills are also given, singly or in duet. In northern Congo and south-east Cameroon at least, this duet has a distinctive dialectal variant, with the second bird producing a slow (staccato) trill ending in two exclamatory notes (kokokoko, ka-kah!). In Ghana however, duets heard were hard to distinguish from those of B. poensis (with short trills). Long whistles are also occasionally produced by B. leucostictus, but are somewhat lower pitched, hoarser and more 'sinister' than those of B. poensis; they tend to recall those of Shelley's Eagle Owl B. shelleyi. Tape playback of the long guttural trill of B. poensis (never produced by B. leucostictus) elicits strong reactions in B. leucostictus too, but species-specific vocal characters are preserved in interspecific reactions. It appears that both eagle owls may defend interspecific territories where they come into contact; distances measured between eagle owls of different species being 1 km or more. Interspecific countersinging is known in several other pairs of congeneric species in the African tropics.

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