2.4. Radio tracking of goshawks

In 1990-1995 goshawks were trapped in cages baited with live pidgeon Columba livia, provided by the university zoo. Trappings started in October-November and continued until the end of January. Usually there were 5-10 cages operating, excluding the weekends and latter half of December. The amount of hawks caught (including retrapped individuals) during 1990-95 was 38. Each trapped hawk was sexed, aged, weighed, wing length (straightened) measured and ringed. In a pilot study in winter 1990-91, 10 hawks were trapped but only 3 of them had been radio-tagged. The proper study began in October 1991. All hawks caught, excluding 2 individuals, were radio-tagged (Biotrack, TW-2 or TW-3 modifications) weighing 15 g. Tags with activity sensors were attached to central rectrics of hawks (Kenward 1978). The number of tagged hawks in 1991-1995 was 26, which was distributed as follows: 1991-92 5 males and 2 females, 1992-1993 3 males and 6 females, 1993-1994 1 male and 5 females, and 1994-1995 1 male and 4 females. Of these, 16 were accepted for final range and habitat analysis. Radio-tagged hawks were located by triangulation using portable radio receivers (RX-81, Televilt and CE-12, Custom Electronics of Urbana, Illinois) equipped with 3 or 5 element Yagi antenna. Three or more bearings were required for a reliable estimate of the hawks position. During tracking sessions each hawk was located 1-3 times per day and then finally at the night roost site. A low tracking frequency probably assured the independence of the locations (Kenward 1987). The whole data consisted of 331 day-time locations and 299 night-roost locations. An activity sensor on the radio-tag revealed when the hawk was sitting or flying, from the difference in pulse rates. An irregular pulse rate was detected when a hawk was eating prey. Using this cue it was possible to find foraging places and recognise the prey species. During 1990-1995 40 such occurences were located. Additional winter prey were identified from pellets that hawks left in cages and also from a stomach of one radio-tagged hawk just killed by an eagle owl Bubo bubo. Preys the size of red squirrel Sciurus vulgaris and brown rat Rattus norwegicus could be recognised because their foraging lasted long enough to find the feeding site.