Trefkans bij het inventariseren van Ransuilen Asio otus in de broedtijd

In a wood of 50 ha near the town of Assen I studied the vocal behaviour of Long-eared Owls during 1987-92. Over the years I visited the study area 117 times at dusk and located 7, 8, 9, 14, 7 and 2 territories respectively. Male song was mostly heard from January through March, begging calls of females peaked from Febuary through April (Table 1). The probability of recording a territory during an average visit on a suitable (calm) evening was only 21%. Calling frequency was temperature-related (Figure 1), with no vocal activity at temperatures below -2°C, a significant increase in vocalizations between -2°C and +3°C, and a constant level of vocal activity at temperatures >3°C. Begging fledglings were heard from May through August, but mainly in June (Table 1). Recording probability was high for begging young; all pairs with fledglings were located with a visit-frequency of once per three weeks in May-August; paying more visits was not worthwhile. The above analysis shows that the following well-spaced sequence of visits is most profitable when (nearly) all territories should be located: 1 at the end of January, 3 in February, 2 in March, 1 at the end of May, 1 mid-June and one at the start of July.