THE SPACING OF BLACK EAGLES IN THE MATOPOS, RHODESIA

Abstract

Gargeit, V. 1975. The spacing of Black Eagles in the Matopos, Rhodesia. Ostrich 46:1-44. Ten adjacent pairs of Black Eagles Aquila verreauxi in the high density population in the Matopos were studied from September 1970 to the end of April 1973. 1 458 hours were spent observing the behaviour of all Black Eagles in the area encompassed by the study pairs and some of their neighbours. All perches were plotted and all flights recorded. Ground level boundaries were estimated; common boundaries were fixed at half the distance between neighbours' low level flights. Territory size was calculated and a habitat division made between open areas and koppie areas (rocky outcrops). The latter were where the eagles main prey, hyrax Procavidae, lived and were seen. Territory size was not constant, the largest territories had the greatest area of open grassland probably in order to encompass a sufficient koppie area. Koppie areas varied less in size than open areas. Territory shape in the airspace appeared to be in the form of an inverted truncated cone. This resulted in an overlapping airspace above ground level boundaries. Eyries were not all centrally placed and boundaries were partly determined by the position of neighbours' eyries. All perches were within terrestrial boundaries and only once were two pairs observed to use the same perch. Paired birds were territorial, single birds were not. Territories were maintained throughout the year by the use of conspicuous perches, by flights, pendulum and undulating displays and by calling. Neighbours were recognised and their territory respected. Aggressive encounters between neighbours were rare. Territories were defended by both male and female from intrusion by single adult, sub-adult and juvenile Black Eagles. Single adults were the most persistent intruders and one caused a breeding failure. Chases continued beyond territorial boundaries. Neighbours tolerated such intrusions and sometimes took over a chase. Single birds did not congregate together and were often seen in empty areas. Single adult Black Eagles must acquire a territory and a mate before they can breed. There was an influx of single birds in the first six months of each year. The dispersal of single non-territorial birds and the sightings of window-marked juveniles indicated considerable movement within and out of the Matopos. Parts of a territory vacated in 1971 were incorporated into the territories of two neighbouring pairs. A new territory was established in 1972 in part of an empty area and the new pair appropriated a portion of two neighbours' territories. A boundary adjustment was made in 1972 between two neighbouring pairs. The breeding performance of the study pairs could not be correlated with territory size but was affected by territory changes. The relative importance of different factors in maintaining the stable Black Eagle population is not known, but there is no shortage of nest sites.