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FEEDING ECOLOGY AND SELECTED OTHER ASPECTS OF THE BEHAVIOR AND ECOLOGY Of THE BURROWING- OWL ( Speotyto cunicularia) by Jeffrey Thomas Climpson Submitted in partial fulfillment of the requirements for the degree of Master of Science (non-thesis) Washington State University Department of Zoology August 1, 1977 ECOLOGICAL SERVICES U.S. ASH & WIL!>L!FE SERVICES SEP 2 2 1977 B' RECEIVED ~-;1 '@\' OLYMPIA . .1 TABLE or CONTENTS INTRODUCTION •• . 1 DESCRIPTION or STUDY AREA . 2 PROCEDURE . 3 SPECIES DESCRIPTION . 4 DISTRIBUTION. 7 BURROW DESCRIPTION. 8 DAILY ACTIVITY PATTERN •• . 13 FEEDING ECOLOGY . 16 Food Habits . 16 Food Abundance and Caching . 27 Seasonal Variation in Diet . 30 Hunting Behavior . 32 Eating and Drinking Behavior . 35 Pellet Description and Formation . 37 Defecation Behavior •• . 39 VOCALIZATIONS . 40 PREDATORS . 45 BEHAVIORAL RESPONSES TO PREDATORS . 47 OTHER INTERSPECIES INTERACTIONS . 51 PRODUCTIVITY. ...... -· ................. 53 MORTALITY . 56 MIGRATION AND DISPERSAL . 61 LITERATURE CITED •• . 64 l INTRODUCTION Durine the summer of 1976, I studied a population of burrowing owls (Speotyto cunicularia) predominately located in Grant County, Washington. Much of what I observed is described in this paper along with a summary of pertinent information on burrowing owls already in the literature. Most of this paper deals with the feeding ecology of these owls although there are also sections on productivity, mortality, mirration and dispersal, and miscellaneous aspects of general behavior. I have not included sections on breeding biology, physiology, and many other aspects of the general behavior and ecology of burrowing owls mostly in an effort to scale down the size of the paper,but also because l did not have the opportunity to observe the owls enough in the appropriate situations. The discussion of each subject category is divided into two parts, a section entitled Literature Summary and a section entitled Field Observations. I briefly outline most of the literature on a particular topic in the Literature Summary and then follow with a report of what I observed in my study in the Field Observations section. 2 DESCRIPTION or STUDY AREA . My study area was located in east-central Washington, encompassing approximately 325 square miles of Grant County. Most of my observations were made within the area bounded to the north by U.S. Route 90, to the south by State Route 26, to the west by the town of George, and to the east by Potholes Reservoir. The climate for Grant County is semi-arid (less than 25 cm annual precipitation) with most of the precipitation occurring in the winter months. The construction of the Columbia Basin Irrigation Project has allowed much of the land to be converted to agriculture with irrigation canals traversing much of the county. Though annual crops cover most of the study area, there are also large patches of a more "natural" Artemesia tridentata - Bromus tectorum - Chrysothamnus spp. habitat. The terrain is generally flat~ interrupted only by some low hills in the southern portion of the study area and scattered sand dunes. 3 PROCEDURE I began my field observations on 8 June 1976 and continued them through much of the summer (except for two weeks in late July), ending my study for the most part on 19 August 1976. I returned to the study area just once more in mid-September for only one day. I spent a total of 34 days in the field, averaging 3 to 4 days per week. The first several weeks of the study were largely devoted to locating the owls. I eventually found 29 burrows. Once I had located enoueh birds, I then began to observe the owls for longer periods of time, concentrating my efforts on 5 pairs in particular. Typically, I watched the birds with 8.5 x 44 binoculars and a 20-45x zoom spotting scope from a car parked 50 to 100 m down the road from the burrow. The car was needed as a blind. Without it, the owls spent too much of their time watching me and this was, of course, unsatisfactory. I observed the birds most often when they were most active, i.e., from 0500 to 0830 and from 1900 to 2130. However, I observed them frequently at other times of the day as well, especially while collecting owl pellets (which I regularly did one day each week). 4 SPECIES DESCRIPTION Literature Summary The burrowing owl (Speotyto cunicul aria) is a rather unusual member of .the Orde.r Strigiformes in North America. Al thoygh gt ossly r a~ling oth~i:... Nart l:i --Amet ·lcan owl s , J. t al so differs from t hem in several key "ways. Specifically, it is the only North American owl that inhabits burrows in the ground, fully exhibits "normal" or nonreversed sexual dimorphism (Earhart and Johnson,1970), and has practically denuded tarsi (Coues, 1903). There are two subspecies of the burrowing owl in North America,~• 5:.. hypugaea or the Western burrowing owl and S. c . floridana or the Florida burrowing owl (Bent,1938) . mLAch t:i Zarn (1974) has summarizedAthe literature describing the burrowing owl ' s physical appearance as follows: Adult color consists of dull brown, barred and spotted with buff and white dorsally , with white barring on the wings and tail. The underparts are buffy, barred with brown. Burrowing owls have yellow eyes and compact , rounded heads,lacking ear tufts. White markings on the chin and over the eyes are exhibited in courtship and territorial displays, most often by the male. Short, hairlike feathers cover the l ong, slender tarsi of the burrowing owl and grade into sparse bristles on t he gray-colored feet. The tail is short (75-90 mm ) • Earhart and Johnson (1970) report that wing length averages 168.7 mm for males and 165.8 mm for females. Burrowing owls also exhibit non reversed sexual dimorphism for body weieht; males average 158.6 (range 120-228) g and females average 150.6 (range 129-185) g (Earhart and Johnson,1970). Males outweighed females by an average of 7. 9 gin a New Me xico study (Martin,1973b) but only 4 gin a California study 5 (Thomsen,1971). Burrowing owls are sexually dimorphic in several other ways as well. Only females develop a brood patch during the breeding-season and so it is assumed that only females incubate (Howell,1964; Martin, 1973b) •. Feather coloration is also a useful criterion for sexing indi vidual birds, especially in summer before the postnuptial molt. Generally, males are lighter in color on the head, back, wings, and tail than females and they are not as heavily barred on the breast. Apparently, this color ation difference arises . from fading and wear of the plumage in males as a result of spending more time outside the burrow than females (see below) (Martin>l973b). Color dimorphism, however, is not an entirely reliable m~thod for sexing burrowing owls because of the great amount of individual variation in both sexes (Grant,1965; Thornsen 1971). And after 1 the postnuptial molt in late summer, the sexes are virtually identical in coloration (Martin11973b). Consequently, most observers have depended more on behavioral dimorphism to sex birds in the field (Grant,1965; Martin 1973b; Thomsen 1971). 1 1 Behaviorally, males are easily separated from females. During the breeding season, the male s pends almost alt of its time outside the burrow acting as a sentry while the female remains in or just outside the burrow, especially when incubating. The male also does mos t of the foraging for the f amily group until the young are 3 to 4 weeks old (Grant, 1965; Martin,l973b) . Finally, burrowing owls may also be sexed from some of their vocal izations (see section on Vocalizations). Juvenil es that are old enough (2 weeks) to stand at the burrow entrance appear buffy brown and they have a characteristic wing stripe across the middle secondary coverts (Bent 1938; Thomsen 1971 ). After t he 1 1 6 post-juvenal molt in late summer, they are virtually indistinguishable from adults (Grant>l965; Thomsen,1971). Field Observations I was able to more or less sex the birds· I studied in the field by using both coloration and behavioral differences in combination. Normally, I could not be reasonably sure of the sex if I depended on either coloration or behavioral cues alone. Juveniles were at first easily dist.inguishable from adults as they had a rather distinctive dark, solid brown plumage dorsally along with buffy wing patches. But by mid-August, they looked very similar to the adults and I .found it virtually impossible to tell them apart. 7 DISTRIBUTION Literature Summary The Western burrowing owl (Speotyto cunicularia. hypugaea) breeds from southern Canada to Central and South America as far south as Argentina and Chile, west to Washington, Oregon, and California, and east to Manitoba, Minnesota, Iowa, Nebraska, Oklahoma, and Texas (Bent 1938). It is a 1 year-round resident in some parts of its breeding range and a summer resident only in others, regularly migrating to wintering areas (see section on Migration and Dispersal). The Florida burrowing owl (S. c. floridana) has a range mostly restricted to Florida and the Bahamas and it is non-migratory (Bendire,1892). 8 BURROW DESCRIPTION Literature Summary The burrows of uurrowing owls are usually found on flat or gently rolling terrain (Grant,1965; James and Seablocm,1968; Scott 19t• O). Open, 1 dry areas located either in grasslands, prairie, or desert are preferred habitat (Martin,l973b). Burrowing owls also commonly nest in grazed pas tures and in alfalfa fields (Butts,1971; Martin>l973b; Scott11940). They appear not to be disturbed greatly by human activity as they have been known to establish colonies at airports and golf courses (Coulombe~l971; Martin11973b; Thomsen>l971) as well as along irrigation canals (Coulombe, 1971; this study).

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