AUDITORY PREY LOCATION IN A PAUSE-TRAVEL PREDATOR - SEARCH HEIGHT, SEARCH TIME, AND ATTACK RANGE OF TENGMALMS OWLS (AEGOLIUS-FUNEREUS)

We studied the nocturnal hunting and diurnal roosting behavior of 17 radio-equipped Tengmalm's owls (Aegolius funereus), 12 males and 5 females, in coniferous forest during their nesting season. The owls perched lower when hunting than when roosting, probably because hunting perches were selected to minimize the predator-prey distance or to obtain unobstructed access to the ground-dwelling small mammal prey, whereas roosting perches were selected to minimize the probability of being detected by an avian predator. There was no difference between perching heights associated with giving up and prey attack, nor were there any differences between perching heights, perching times, and attack distances associated with successful and unsuccessful attacks. There were no sexual differences in perching height during hunting or roosting. However, giving-up times tended to be longer for females than for their mates, which is expected because females are larger than males, and the relative cost of flight increases with body mass. The instantaneous attack rate was independent of perching time. The owls gave up their perches at a constant rate and independently of the amount of time already spent on the perch in an exponentially decaying pattern. The owls perched longer, however, before launching an attack than before giving up, probably in order to observe detected prey until the right moment for an attack. Attack distance was independent of both perching height and perching time. Perching time was inversely related to perching height, which fits the theoretical expectation that the search area will decrease with increasing height in birds that locate prey auditorily.