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King, R. Andrew (Andy) – Postfledging dispersal and behavioral ecology of Burrowing Owls in southwestern Idaho. 1996.

burrowing owl in grassGeneral Introduction

This thesis consists of three chapters describing my investigation of the behavioral ecology of burrowing owls (Speotyto cunicularia) in southwestern Idaho. The purpose of my field research was to (1) determine how an environmental factor, such as food availability, could influence the behavior and dispersal movements of young owls, (2) develop a descriptive model of the post-fledging dispersal of juvenile burrowing owls, and (3) characterize habitat variables associated with burrowing owl nest sites and roost sites. Information contained in this thesis should be particularly useful to those interested in the proximate causes of dispersal or the effects of food availability on dispersal behavior, and to those involved and/or concerned with the management and conservation of burrowing owls.

Overview of Chapters One, Two, and Three

In Chapter One I present the findings of a food supplementation experiment that was designed to examine how post-fledging dispersal movements of juvenile burrowing owls differ in relation to food availability. I provided treatment families with dead whole laboratory mice (Mus musculus) and day-old poultry chicks (Gallus gallus domesticus) daily. I compared the timing of dispersal movements, the distance traveled, and the post-fledging behavior of supplementally-fed juveniles to juveniles that received no food supplements (controls). I monitored the daily movements of treatment and control juveniles by using radiotelemetry until they departed the study area or radio contact was lost.

Supplementally-fed juveniles exhibited significantly increased growth (body mass and tarsus length), delayed dispersal, and shorter dispersal distances than control juveniles. Therefore, juveniles of this species did not appear to have an urgent need to disperse when they had easy access to an ample and reliable food source. They also appeared to minimize the costs associated with dispersal by moving to nearby areas containing an adequate food supply. At the end of Chapter One, I discuss the possible functional role of post-fledging dispersal in burrowing owls.

Chapter Two focuses on the post-fledging dispersal of juvenile burrowing owls. I defined post-fledging dispersal as a permanent movement away from the natal area prior to fall migration. I describe the timing, distance, and direction of dispersal movements of 13 radio-tagged juveniles. I also describe the use of “satellite” burrows, which I defined as any burrow other than the nest burrow at which a juvenile was observed during daytime observations. The timing, causes and extent of mortality of juvenile and adult owls also are summarized and discussed. At the end of this chapter, I develop a descriptive model of juvenile dispersal in western burrowing owls, which describes the dispersal process that a “typical” juvenile undergoes once it leaves its nest.

In general, juvenile burrowing owls fledged (i.e., began flying) at 30 days post-hatching, but they remained on their natal areas for approximately four more weeks before permanently dispersing beyond 300 m. After dispersing, most juveniles continued moving farther away from their natal areas, either by themselves or with other juveniles (usually siblings). Prior to fall migration each juvenile had used an average of five satellite burrows that were typically within a 500-m radius of the natal burrow. The mean orientation of juvenile dispersal movements was directed towards the southeast, which may correspond to a southerly migration route. Most juveniles had departed the study area by mid-September. Juvenile mortality was highest during the post-fledging period and often was the result of human activities or structures.

Chapter Three describes habitat use and range sizes of burrowing owls in southwestern Idaho. Burrowing owls (n = 34 pairs) typically nested in open grasslands dominated by cheatgrass (Bromus tectorum) and tumble mustard (Sisymbrium altissimum). Nest sites in the study area and other parts of southern Idaho often bordered irrigated agricultural fields (Gleason 1978, Rich 1986). Before the invasion of cheatgrass and subsequent wildfires the study area contained expansive shrublands, dominated by big sagebrush (Artemesia tridentata ssp. Wyomingensis). Elevated perches appeared to be important features of burrowing owl nest sites. The most commonly used perches were fenceposts, dead sagebrush trunks (killed by fire), and rock piles. Nest burrow entrances were significantly oriented in a southeasterly direction, which may provide roosting owls with thermoregulatory benefits in the early spring. The home ranges of five adult burrowing owls and the post-fledging ranges of 11 radio-tagged juveniles were estimated and possible causes of variation were addressed. Juveniles (n = 13) usually roosted in open grasslands during the daytime, but three juveniles also used areas with scattered or dense sagebrush after dispersing.

I conclude that the large-scale conversion of sagebrush communities into fire-prone grasslands and irrigated agricultural fields may benefit burrowing owls in the study area. However, more studies are needed to confirm or refute this notion. Lastly, I suggest some environmental factors and relationships that appear to influence the behavioral ecology of burrowing owls in southern Idaho (e.g., spring precipitation levels and vegetative cover, vole abundance and irrigated agricultural fields, and wildfires and post-fire rehabilitation efforts).

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