Danilson, Christopher D. – Post-fledging ecology of Great Horned Owls in south central Saskatchewan. 1998.
In 1993 and 1994 I studied 26 fledgling great horned owls (Bubo virginianus) from 23 nests in south central Saskatchewan to quantify their post-fledging habits. By radio-tracking 26 fledgling owls at night and locating diurnal roosts, I documented changes in juvenile and adult behavior throughout the post-fledging period. I relate these changes to survival and dispersal and discuss them relative to parent-offspring conflict theory.
I identify three distinct phases of the post-fledging period (early, middle, and late) that were characterized by behavioral changes of adult and juvenile owls. During the early post-fledging period (0 – 5 wks post-fledging), juvenile owls were relatively sedentary and relied on adult owls for defense and food. During the middle post-fledging period (6 – 1 0 weeks post-fledging), juvenile owls became more mobile, begged for food more in response to decreased adult provisioning, and spent less time with parent and sibling owls. From 11 weeks post-fledging to when they dispersed (the late post-fledging period), juvenile owls became increasingly mobile and independent while adults rarely provisioned or roosted with them.
In 1993 and 1994, great homed owls produced an average of 2.22 young per pair, and the mean date of fledging was 5 June (range: 18 May – 24 June). Siblings, on average, fledged three days after the first young left the nest (range: 0 – 15 d). Nestling mortality was low (2 of 54 nestlings), however, at least 11 owls died prior to dispersal from their post-fledging areas. Primary causes of mortality were starvation, and trauma to head and body, which is characteristic of great horned owl predation.
Up to six weeks after fledging, adult owls vigorously defended offspring by vocalizing, bill-clapping, and making approaching flights, but the intensity and frequency of defense ceased after this time. Parental provisioning and diurnal and nocturnal food begging peaked during the middle post-fledging period, however by 10 weeks post-fledging parents rarely fed their offspring and begging decreased significantly. Radio-tagged owls roosted together with adult owls and sibling owls less frequently as the post-fledging period progressed. While associations among radio-tagged owls and their parents at diurnal roosts became less frequent during the early post-fledging period, roost associations among siblings continued well into the middle post-fledging period.
Mean post-fledging period of 12 owls known to disperse from their post-fledging areas was 111 days (Range: 90 – 137 d). Five radio-tagged owls made temporary movements, roosting away from post-fledging areas for one or more days, which coincided with the dispersal of other radio-tagged owls. The distance of these movements ranged from 2.3 to 20.6 km. No dispersing radio-tagged owls were relocated within 30 km of the study area boundary. One radio-tagged owl was recovered, more than 150 km from the study area three years after it had fledged.
Post-fledging range size and the farthest distance ventured from the nest increased as the post-fledging period progressed. Mean post-fledging range size was 193.2 + 100.1 ha (range: 46.6 – 401.9 ha) and owls ventured, on average, up to 1.7 km from their nests.
Parental investment in offspring decreased soon after the young fledged, and parent-offspring conflict peaked at the end of the middle post-fledging period when parents discontinued provisioning and avoided offspring. At this time, offspring spent less time begging, moved greater distances, and became more solitary. I did not observe juvenile owls capturing vertebrate prey, but based on observations of hunting attempts and lack of parent-offspring interactions, I believe they fed themselves before they dispersed. Decreasing food supply, rather than renewed courtship of adults or direct aggression by adults toward offspring, was associated with dispersal by juveniles.