Ellsworth, Ethan A. – Postfledgling behavior and dispersal of juvenile Western Screech-Owls: Patterns of movement and the effects of gender and social dominance
CHAPTER 1: EFFECTS OF SOCIAL STATUS ON THE DISPERSAL BEHAVIOR OF JUVENILE WESTERN SCREECH-OWLS
Variation in the timing and distance of dispersal movements by juvenile birds may result from differences in competitive ability. On one hand, low-ranking juveniles may initiate dispersal before dominants if dominants aggressively force subordinate siblings from natal areas. Conversely, if vacant territories are limited and early arriving birds are more successful in acquiring territories, selection would probably operate on young to disperse as early as possible. In this case, dominant individuals with priority of access to resources will mature more quickly and disperse before subordinate siblings. Dominance status also could affect the distance traveled by dispersing juveniles. If long distance dispersal is more costly than short distance dispersal, dominant juveniles should take advantage of their status and disperse shorter distances than subordinates. Alternately, if dominants are in better condition than subordinates and there are advantages to long distance dispersal, dominants should disperse farther than subordinates.
I examined effects of social rank on the timing and distance of dispersal movements by juvenile Western Screech-Owls, Otus kennicottii. (N = 25) from seven broods. Based on observations of aggressive interactions made using video cameras attached to nest boxes, I assembled dominance matrices and assigned dominance ranks to juveniles within each brood. I radio-tracked juveniles throughout the postfledging period to determine the order in which they dispersed and located juveniles after dispersal to determine postfledging dispersal distances.
Social dominance influenced the timing of dispersal in juvenile Western Screech-Owlsbut gender did not. In six of seven broods, the most dominant juvenile dispersed before its siblings. Moreover, in five of seven broods the least dominant individual was the last individual to disperse. These data are consistent with the notion that dominant juveniles matured sooner than subordinates and early maturation enabled them to disperse before less developed siblings. In contrast, social status did not affect postfledging dispersal distance. Instead, variation in dispersal distance appeared to be related to gender, with females dispersing fewer than males.
CHAPTER 2: DISPERSAL IN WESTERN SCREECH-OWLS, OTUS KENNICOTTII
This study examined the timing, duration, distance, and direction of dispersal movements by juvenile Western Screech-Owls, Otus kennicottii, in southwestern Idaho. In 1994 and 1995 young owls were radio-tracked throughout the postfledging period on the natal area until they dispersed. They were subsequently located for as long as 12 weeks after dispersal. Young owls typically dispersed in mid-July (x = 13 July; range: 28 June to 18 August) after spending an average of 60.0 ± 2.4 days in the natal area after fledging. The mean number of days between dispersal of the first and last juveniles in a brood was 8.5 (N = II). Males dispersed slightly before females, but the difference was not significant. Young from larger broods remained on the natal area longer than those from smaller broods, which suggests that it takes young in large broods longer to reach a particular skill level or body condition suitable for dispersal. Thirty-one of 35 juvenile screech-owls were found after dispersal from natal areas. These juveniles dispersed an average of 10,6 ±1.8 kilometers from the nest site to overwintering sites. Females (14.7 ±3.0 km, N = 15) dispersed farther than males (5.5 ± 1.3 km, N = 16), which fits the general pattern among birds. However, only one of the owls in this study was followed until it bred, so these distances are not natal dispersal distances. Finally, dispersal was directed along a northwest-southeast axis that mirrored the orientation of the Snake River and corresponding riparian woodland habitat in the study area.
CHAPTER 3: ROOSTING BEHAVIOR OF WESTERN SCREECH-OWL FAMILY GROUPS DURING THE POSTFLEDGING PERIOD: EFFECTS OF SOCIAL STATUS, AGE, AND GENDER
The aim of this study was twofold: 1) to describe the roosting behavior of Western Screech-Owls, Otus kennicottii, during the postfledging period along the Snake River in southwestern Idaho in 1994 and 1995, and, 2) to investigate the influence of social status, age, and gender on the roosting behavior of screech-owls. I used radio-telemetry to locate 2,784 diurnal roost sites of 47 juvenile and 15 adult Western Screech-Owls from 15 families throughout the postfledging period prior to dispersal. I video-taped interactions among nestling owls in seven broods to examine social interactions and determine social hierarchies within screech-owl families. Over half of all roosts (58.1 %) were in willows, Salix spp; screech-owls used an additional seven tree and six shrub species, and infrequently roosted on the ground (1.2 %), in cliffs (0.5%), and in dead trees. Screech-owls roosted in vegetation that provided good cover and concealment (tangle roosts) most frequently (57.8%), followed by roosts on open limbs (29.1%), and next to the trunk (12.3%). Unlike in winter, no screech-owls used tree cavities during thepostfledging period. The mean roost site was 2.5 ± 0.1 m high in a tree or shrub 5.5 ± 0.1 m tall with a dbh of 13.7 ± 0.2 cm. Based on a random sample of potential roost trees (N = 100 in each natal area), screech-owls used shorter trees with a smaller dbh than available trees. On average, screech-owls roosted 119.1 ± 3.0 ill from their nest sites, and 36.0 ± 1.7 m from the previous day’s roost. There was a trend for subordinate young to roost farther from the nest, from adults, from siblings, and from the previous days roost than did dominant young. I observed some differences in roosting behavior between adults and juveniles, particularly one to two weeks after juveniles left the nest box when juveniles had limited flight skills, and during the last few days before young dispersed. Roosting behavior of juvenile males and females was similar. However, adult males and females appeared to assume different roles in protecting and provisioning young. Adult females roosted closer to the young on average than did adult males, and adult females did not make as many long-distance movements outside of core roosting areas.
Ellsworth, E. and J.R. Belthoff. 1999. Effects of social status on the dispersal behaviour of juvenile Western Screech-Owls. Animal Behaviour 57:883-892.