A study of Greater Spotted Eagles (GSE)(Aquila clanga), was carried out by direct observation from 20 April to 19 August 1997 in the Biebrza National Park in north-east Poland. This study provided information on home range, flight behavior, hunting methods, hunting territory, prey composition, and interaction with other species. Behavior was recorded at 5 min intervals, and was plotted on map quadrants 200 x 200 m in size. Observations were carried out daily from 0800 to 1800 H. Since the first pair (M1 and F1) selected for study did not attempt to breed, attention was shifted to a second pair (M2 and F2), whose young fledged on 16 August. Sexes were determined at copulation and, thereafter, by moulting pattems. Both males (M1 and M2) hunted chiefly on the wing (soaring in search of prey and stooping to the ground) and were regularly observable (40% of the time). Females (F1 and F2) were seldom observed hunting. Flight activity lasted between 0.5 and 7.5 H/day, largely between 0900 and 1700 H, with a peak between 1000 and 1400 H. Accordingly, the major proportion of prey was also recorded between these hours. Between 1400 and 1500 H, there was usually a period of repose. The start of hunting by F2 (on 12 July when the eaglet was about 3-4 weeks old) led to a clear decline in M2's flying activity. The territorial flights of M2 (undulating display flights), however, increased. The breeding and hunting territories marked out were defended against members of the same species, the closely related Lesser Spotted Eagle (LSE) (Aquila pomarina) and other large birds of prey. Male GSEs in the Biebrza valley probably have clearly defined territories that they defend. Up to mid-July the male's hunting success was 34%. The success of hunting on the wing declined to below 20% during the day. Since prey continued to be carried to the eyrie, this clearly indicated a strategic change in favor of still-hunting or hunting on foot. For the most part, M2 arrived with mice (65%, likely Microtus), and frogs (19%, Rana spp.) at the eyrie. F2, so far as it could be observed, showed a preference for frogs. Based on the estimated weight of the observed dietary needs, as compared with the presumed needs of the young eagle based on the literature, an attempt was made to determine the completeness of observations of arrivals with prey at the nest for the total observation period. This led to the conclusion that 2/3 of the arrivals with prey was likely observed. The two pairs (1 and 2) of GSE defended home ranges of 15 km² and 19 km², respectively. These values correspond to those given in the literature. However, studies of LSEs by conventional telemetry have revealed clearly larger home ranges than those assessed from direct observation. The breeding territories of GSEs in Biebrza valley displayed a variety in landscape structure. There were clearly defined areas for hunting, for conflicts with other large birds of prey, and areas where undulating display flights were performed. The hunting grounds of M2 clearly shifted after 12 July. This could have resulted from avoidance of increased in human activity in the meadows 62 of the old hunting territory, which had been mown by this date; as well, large numbers of other raptors and White Storks (Ciconia ciconia) in search of prey began to appear after this date. In this, GSEs studied clearly differed from the closely related LSE. We recognized a "contact call", a "territorial call" connected with the undulating display flights and a special, quite distinct "waming call" or "alarm call" which was audible when other large birds of prey were around.

The Greater Spotted Eagle (Aquila clanga) is one of 24 European bird species regarded as globally vulnerable (Collar et al. 1994). Formerly common in several east European countries, less than 1 000 pairs may have survived in these areas, mainly in the European part of Russia; the global breeding population has been estimated at less than 2500 pairs (Tucker and Heath 1994, Meyburg et al. 1997). This unfortunate situation is even more serious since our knowledge of the biology and ecological requirements of this species is quite limited, severely hindering protection activities. In the westem part of its range, it occurs with the Lesser Spotted Eagle, (A. pomarina), a species considered so similar, that together they were fonnerly considered as a single species or semi-species (Zhezherin 1969, Meyburg 1974, Meyburg 1994, Bergmanis 1996, Seibold et al. 1996). Up to now, little remains known of the differences between the ecological requirements of both species.

There is no explanation as to why the breeding range of the Greater Spotted Eagle (GSE) is limited westwards by eastern Poland although the similar-looking Lesser Spotted Eagle (LSE) occurs further to the west as far as east Germany. On the other hand, the breeding area of the LSE is restricted in the east by an indistinct (or inadequately known) border situated near Moscow although the area of its sister species stretches far to the east through Siberia extending to the Pacific Ocean (Meyburg 1994).

We tried to contribute to the solution of these questions by direct long-time observation of two pairs of GSE nesting in Biebrza National Park in eastern Poland.