Factors affecting the vocal behaviour of eagle owls Bubo bubo: Effects of sex and territorial status

We performed a playback experiment in order to assess the efficiency of two methods for detecting Eagle Owls Bubo bubo and to assess factors affecting detection rates. On each experimental visit we recorded the number and sex of the responding owls, the calling rate and the corresponding type of voice during three periods: spontaneous calls (SC; 30 min), elicited calls (PB, by broadcasting male territorial hoots; 30 min) and post-challenge period (PPP; 15 min). We conducted a survey of 17 territories in Spain, and found 10 unpaired males, six paired males and seven paired females (one male attended two females). Paired males delivered territorial hoots during the SC period and continued hooting during the PB period, while most unpaired males became silent in the PB period, and the calling rate of those that continued on hooting was lower than that for paired males. When hearing the intruding male’s voice, females increased their call rate. Most puzzling, females responded to male playbacks with mating-begging calls, not with territorial or alarm calls. During the PPP period, paired males delivered significantly more territorial calls than unpaired males. Thus, mating status may have an effect on male Eagle Owl response to male playback. We suggest that: (1) the exclusive use of playbacks may bias surveys towards paired owls, (2) the combined use of recording spontaneous calling owls and the broadcast of male territorial voices may improve survey methods by revealing floaters, and (3) paired males tried to defend aggressively both mate and territory and unpaired males hooted primarily to attract females, but they tried to avoid territorial disputes with other males.