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).

The distribution of burrowing owls roughly correlates with the distribution of prairie dogs and ground squirrels of the genera Cynomys, Citellus, and Spermophilus (Coulombe,1971; Martin 1973b). In some areas, 1 burrowing owls depend heavily on the abandoned burrows of other animals for

nest sites to the point that the availability of burrows may become a

limiting factor (Coulombe11971). Both active and abandoned prairie dog (Cynomys spp.) towns often support a colony of burrowing owls (Butts,1971;

Marti,1969). However, nest density is not directly related to the size of the dog town~ and many active dog towns support no owl colony at all (Butts,

1971). Ground squirrel holes are also preferred by the owls as nest sites.

The burrows of Beechey ground squirrels (Citellus beecheyi), Richardson ground squirrels (Citellus ·richardsoni), roundtail ground squirrels

(Citellus tereticaudus) , and rock squirrels ( Spermophilus variegatus) have all been used frequently for nesting by burrowing owls (Coulombe 1971; 1 9

Thomsen,1971; Wheeler, Douglas, and Evans 1 194l; Grant,1965; Martin 11973b). Burrowing owls have also been known to use the abandoned burrows of the. following animals: badger ( Tax idea taxus), marmot ( Marmo ta spp. ) , skunk (Mephitis spp., Spilogale spp.), armadillo (Dasypus spp.), muskrat

(Ondatra zibethica), banner-tail kangaroo rat (Dipodornys spectabilis), coyote (Canis latrans), swift fox(Vulpes velox), and tortoise (Gopherus spp.) (Bent,1938; Butts 1971; Grant,1965; James and Seabloom.11968; Platt> 1 1971; Scott 1940; Zarn,1974). 1 Burrowing owls often nest together in colonies of varying size

(Butts,1971; Grant,1965; Rhoads,1892). The largest reported colony was found by Rhoads (1892) in Florida and it consisted of several hundred pairs. It may be that colonies are less common now than previously (Bent,

1938; Grant,1965).

Thomsen (1971) found that in Caifornia, owls typically searched for possible nest sites during the evenings and probably also at night but she could not determine which sex took the initiative. Martin (1973b) found, though, that in New !',exico, returning males selected the same bur.rows as they had in the previous breeding season. Zarn {1974) reports that a burrow may be reused but not necessarily by the same pair. Some nest material is brought to several different holes until gradually one hole is chosen as the nest site (Thomsen,1971). Burrowing owls normally have from l to 10 accessory burrows usually located within 100 m of the main burrow that are used frequently by all members of the family group (Coulombe>l971;

Grant,1965; James and ScabloomJ1968; Martin,1973b; Scott,1940).

There is some controversy in the literature over whether burrowing owls can dig their own burrows without having at least a "start" provided for them by some other burrowing animal. Martin (1973b) and Thomsen (1971) 10 both believe that burrowing owls occasionally dig their own burrows.

But Thomsen (1971) also points out that burrowing owls are almost always

associ~ted with burrowing mammals and this suggests at least a certain

amount of dependence. It is clear, though, that these owls are at least

capable of modifying their burrows extensively by digging. with their feet,

beaks, and wings (GrantJ1965; Kennard 1915; Martin 1973b; Thomsen 1971). 1 1 1 Most of the burrow modification is done by the male (Martin 1973b).

The dimensions of the burrow have been described by several

investigators. Coulombe (1971) found that burrows in the Imperial Valley

of California typically had entrances of about 20 cm in diameter (maximum

80 cm). Most of the tunnels descended at approximately a 15° angle and

all had left or right turns within l m of the entrance. He also found that

86 out of 104 of the holes were located "between firm, eroded sandstone and

a softer layer of silt belo,1. 11 Butts (1971) has described burrows in

Oklahoma as having tunnels measuring 15 cm x 12 cm {just large enough for

the owl to fit through) and either circular or oblong nest chambers approx­

imately 25 cm wide and 10 to 15 cm high. Many of the burrows had a second

tunnel running from the nest chamber but it was usually partially plugged.

The nest chamber was at an average depth of 65 cm (range 43 to 100 cm) and

at an average distance of 150 cm (range 105 to 210 cm) from the entrance.

Kennard (1915) and Rhoads . (1892) have described the dimensions of typical

burrows in Florida and Martin ( 19.73b) has done the same for burrows in New

Mexico.

Commonly, the burrows are lined with dried cow or horse dung, some­ times up to 7.5 cm deep (Bent,1938; Dutts,1971; Martin 1973b; PlatS1971; 1

Rhoads1 1892; Scot~l940). Martin (1973b) has suggested that the manure lining serves as an insulator as well as possibly helping the owls to ll disguise their scent from predators. If the manure is removed, it is replaced within one day (Martin11973b). Burrowing owls also appear to be junk collectors. Gum wrappers, rags, corn cobs, a woolen mitten, a piece of calico, cow hair, a pronghorn Si:.le (Antilocapra americana), and a coyote hide are j'ust a few examples of some - ----11 of the odd debris found in or about burrows (Bent_1 1938; Thornsen,1971). The reason for this behavior is not clear although some of the material is undoubtedly used for lining the burrow.

Field Observations

Most of the burrows I studied were located in road banks and in ditch banks along irrigation canals. Some of the burrows, however, were located in more or less natural sagebrush-rabbitbrush areas (see·Description of Study Area). One burrow was found in a culvert under a dirt road.

I am not sure of the origin of most of the owl burrows I studied.

I never saw any ground squirrels but I often saw burrowing owls kicking sand out of the~r burrows. Consequently. I believe that the owls were probably modifying badger holes and/or possibly digging their own burrows.

I observed only two owl colonies and they were very small. Three family groups lived within 100 m of each other along an irrigation canal ditch bank and two other groups had burrows in a highway road bank about

25 m from each other. I made no measurements of the burrows themselves. They did not appear to be facing in any particular direction. Some of the burrows were heavily lined with dried cow(?) dung, some were lightly lined, and some had no dung lining at all (at least near the burrow entrance). At some of the burrows, I found a number of odd objects including aluminum foil, corn 12 cobs~ broken glass, a cl9th rag. a tuft of reddish (horse?) hair, and a wing from a mallard (Anas platyrhynchos). 13 DAILY ACTIVITY PATTERH

Literature Summary

Burrowing owls have been described as having diurnal, nocturnal,

and crepuscular activity periods. Marti (1969), Platt (1971), and Scott

(1940} have all noted that these owls are active at all hours of daylight.

Nocturnal hunting has been observed by Thomsen (1971} although Marti (1969)

claims that burrowing owls are poorly adapted for night hunting. Coulombe

(1971} points out that burrowing owls are often seen flying through the path

of car headlights. Usually, though, burrowing owls are described as being

chiefly crepuscular (especially when hunting} (CoulombeJ197l; Grant,.1965;

Marti,1974; Martin1 1973b; Thomsen,1971). Grant (1965} found that foraging activity was highest from 0400 to

0700 and from 1900 to 2300. Martin (1973b} recorded the greatest amount of

hunting activity for the owls he studied to be in the periods from sunset to

2400 and from 2400 to dawn. Marti (1974} noted three peaks of hunting

activity, a five hour period centered at sunrise, a two hour period just

before midday, and a five hour period centered at sunset. Thomsen (1971)

observed the owls she studied spending most of the day near their burrows.

From noon until 1600, she saw very little activity. In late afternoon, the

owls came out to sit until evening when they began to hunt. "Adults with

young to feed returned to the burrow at nir,ht" (Thomsen,1971). In a

laboratory experiment, Coulombe (1971} found that activity was hiehest 4 to

5 hours after the onset of·the photoperiod.

A number of investigators have noted seasonal changes in the activity patterns of burrowing owls (Zarnil974). For example, Coulombe (1971) has 14 described the following sequence for owls living in the Imperial Valley of

California. In winter, both members of a pair of owls are usually observed together at the burrow entrance in early.morning and late afternoon. During the day, one of the owls is usually below ground and the other is stationed at the burrow entrance. In March as the breeding season begins, both owls are seldom seen together at the burrow entrance. When incubating, the female remains in the burrow for most of the day. The male only occasionally enters t he burrow at this time. In early summer, the .male still rarely" enters the burrow, but instead spends most of its time acting as sentry.

The female is usually very near the burrow. Juveniles may be outside the burrow during the morning and afternoon but they only rarely come out at midday. By the end of the breedine season when the air temperature is hottest, both adults and young tend to spend more time inside the burrow at midday.

The activity pattern of°' burrowing owl also appears to be related 11 to temperature. Coulombe (1971) observed that as the weather cools , the owls become increasingly more active in daylight. In a laboratory exper~ irnent, he was able also to show that at higher temperatures, the owls become more nocturnal. On the contrary> Thomsen (1971) noted that the owls she studied in California were less active in the daylight and more active at night in September than earlier in the season. Dy February, activity was beginning earlier in the evening and ending later in the morning. Ross and

Smith (1970) found that in Texas, the number of owls sighted increased as the averaee monthly temperature increased. Zarn (1974) has pointed out that nocturnal hunting activity might be expected to occur more in the winter since thE) owls are forced to shift from a summer diet of insects and rodents to a diet made up almost exclusively of rodents (many of which are 15 night-active) .

The sleeping patterns of the burrowing owl have also been studied.

In a l~boratory setting, Berger and Walker (1972) found that burrowing owls slept approximately 60% of the time over a 24~hour period. The eyes may be open or closed depending on the type of sleep. Some of the owls slept more in the day while others slept more at night.

Field Observations

The owls I studied were most active from 0500 to 0700 and from

1900 to 2200. I was not able to determine how active they were at night.

During the daytime, the male was usually at a sentry perch and the female was usually either in the burrow or just outside it. The activity level was rather low at this time. Once the young were old enough to sit out­ side of the burrow~ they regularly did so in the mornings and evenings and sometimes at midday.

The owls hunted mostly in the evenings and occasionally in the mornings. The extent to which the owls hunted at night (2300 to 0500) was not investigated. 16

FOOD HABITS

Literature Summary

The food habits of burrowing owls are fairly well-known. Table

1 and Table 2 summarize the literature on this subject. Nearly all of these studies depended on pellet analysis for determiniQg the diet of the owls. Pellet analysis is a fairly reliable method for identifying the food habits of these owls but there can be bias from several sources (see section on Pellet Description and Formation) . Generally, it appears that burrowing owls are rather opportunistic in their feeding habits (Longhurst,

1942) although they do tend to catch mostly beetles (Order Coleoptera) and small rodents. In terms of numbers taken, invertebrates (especially insects) predominate in the diet over vertebrates (mostly rodents). Marti (1969) determined that for a population of owls in Colorado, the mean prey weight was 6 g with 85 . 9% of the prey weighing less than S g . However, Marti (1974) also found that the majority of the prey biomass taken was mammalian.

Occasionally, burrowing owls seem to concentrate on a particular type of prey, feeding almost exclusively on it. Neff (1941), for example, found L~,~~ al\with the wings of 64 nestling black terns (Chlidonias nir,er) in it.

Burrowing owls may also occasionally feed on carrion (Marti.1974; Thomsen>

1971; and others) .

Field Observations

Although I collected close to 1000 pellets, I have not yet analyzed them. From a cursory examination of a few of the pellets, though, it appears that the food habits of the owls I studied probably fit t he general pattern TABLE 1, , Invertebrate prey of the burrowing owl (Speotyto cunicularia).

CLASS ORDER FAMILY COMMON NAME SOURCE* FOR FAMILY

Arachnida Seorpionida a,b,f,l,m

Araneida k Crustacea Isopoda s

Decapoda a,d,h,k,n

Chilopoda a

Insecta Odonata a Calopterygidae broad-winged damselflies b

Orthoptera d,q,s,t Acrididae short-horned grasshoppers a,b,e,f,i,j,k, l,p

Gryllacrididae camel crickets a,c,h,l,n,r,s

Gryllidae crickets a,k

Dermaptera d,l Forficulidae common earwigs t

Hemiptera Scutelleridae shield-backed bugs j .... Pentatomidae stink bugs b ..J

- continued next page - TABLE l. continued

CLASS ORDER FAMI LY COMMON NAME SOURCEi: FOR FAMILY

Coleoptera b Cicindelidae tiger beetles e,k,l,p

Carabidae ground beetles b,e,f,g,h,i,j, k,l,r

Histeridae hister beetles p

Hydrophilidae water scavenger beetles e,g,m,p

Silphidae carrion beetles b,e,g,i,j,k,p

Staphylinidae rove beetles j

Dermestidae dermestid beetles g,j

Elateridae click ·beetles b,e,l,p,t

Buprestidae metallic ~ood-boring beetles b,e

Coccinellidae ladybird beetles g

Tenebrionidae darkling beetl es b,d,g,h,i,j,k, l,p,t

Lucanidae stag beetles b,p

Scarabaeidae scarab beetles e,f,g,h,i,j,k, l,p

~ Chrysomelidae l eaf beetles l,p a,

- continued next page - TABLE 1. continued

CLASS ORDER FAMILY COMMON NAME SOURCE* FOR FAMILY

Curculionidae snout beetles b,g,h,j,k,l,p,t

Lepidoptera a,h,j,l,p Danaidae milkweed butterflies s,t

Diptera k,p,s

Hymenoptera h Mutillidae velvet ants j

Formicidae ants b,j,k,l,p,t

Vespidae vespid wasps e,k

Apidae digger, carpenter, bumble, & honey bees s

* a - Bent, 1938 k - Marti, 1974; Colorado b - Bond, 1942; Nevada . l - Maser et al., 1971; Oregon c - Carson, 1951; California m - Neff, 1941; California d - Coulombe, 1971; California n - Robertson, 1929; California e - Errington and Ben~ett, 1935 ; Iowa o - Robinson, 1954; Kansas f - Glover, 1953; Arizona p - Scott, 1940; Iowa g - Grant, 1965; Minnesota, N. Dak., S. Oak. q - Sperry, 1941; Colorado, Kansas, h - Hamilton, 1941; Colorado Montana, Washington i - James and Seabloom, 1968; North Dakota r - Stoner, 1932a; California j - Longhurst, 1942; Colorado s - Thomsen, 1971; California t - this study, 1977; Washington .... 1.0 TABLE 2. Vertebrate prey of the burrowing owl (Speotyto cunicularia).

CLASS ORDER FAMILY COMMON NAME GENUS COMM ON NAME SOURCE* FOR FAMILY FOR GENUS (if appropriate) (if appropriate)

Osteichthyes a Amphibia

Urodela a Salamandridae Pacific newts Triturus r

Anura a,e,g Bufonidae true toads Bufo d,s Pelobatidae spadefoot toads Scaphiopus h;q Hylidae tree frogs Hyla d Ranidae true frogs Rana h,p Reptilia

Squamata m Iguanidae lizards Phyrnosoma horned lizards k See lop orous spiny lizards l Colubridae colubrids Pituo2his (young) gopher snakes b Sonora ground snakes d Thamnophis garter snakes k "'0 TABLE 2. continued

CLASS ORDER FAMILY COMMON NAME GENUS COMMON NAME SOURCE* FOR FAMILY FOR GEN US (if appropriate) ( if appropriate)

Aves

Anseriformes Anatidae swans,geese, and ducks Anas (carrion) dabbling ducks k,t

Galliformes Phasianidae quail, pheasant, and partridges Phasianus pheasants g,l,s

Charadri- iformes s Scolopacidae sandpipers Calidris a,r Laridae gulls & terns Chlidonias m Larus s

Columbi- formes Columbidae pigeons & doves Columba (carrion) s Zenaidura s Strigiformes Strigidae typical owls

SZeoti to "-> carrion?) burrowing owls d,g,k,o,s ....

- continued next page - TABLE 2. continued

CLASS ORDER FAMILY COMMON NAME GENUS COMMON NAME SOURCE* FOR FAMILY FOR GENUS (if appropriate) (if appropriate)

Caprimulg- iformes Caprimulgidae goat suckers Chordeiles nighthawks a

Passeriformes g,k,l Tyrannidae flycatchers Tl_rannus h

Alaudidae larks Eremophila a,e,j,k

Laniidae shrikes Lanius t

Vireonidae vireos Vireo a

Parulidae wood warblers Geothl~ is e

Icteridae blackbirds & orioles Dolichonyx bobolink a Sturnella meadowlarks a,p,s Agelaius d,m,s Eu~us s Fringillidae grosbeaks, finches, sparrows, and I\.) longspurs a,i,p I\.)

- continued next page - TABLE 2. continued

CLASS ORDER FAMILY COMMON NAME GENUS COMMON NAME SOURCE:': FOR FAMI LY FOR GENUS (if appropriate) (if appropriate)

Pheuticus a Calamoseiza k Passerculus j Ammodramus a SE_izella e

unident. egg shell p Mammalia

Insectivora Soricidae shrews a Sorex j Blarina short-tailed shrews p Chiroptera a Vespertilion- idae plainnose bats Lasiurus hairy-tailed bats s Antrozous pale. bats r

Lagamorpha Leporidae hares , rabbits, & pikas t (carrion) Sflvila~us often young) cottontails a,b,k,l,p ~rion) hares & jack- rabbits k,s "'w

- continued next page - TABLE 2. continued

CLASS ORDER FAMILY COMMON NAME GENUS COMMON NAME SOURCE* FOR FAM I LY FOR GENUS (if appropr iate) (if appropriate)

Rodentia Sciuridae squirrels S ermo hilus (Citellus (carrion?) ground squirrels a,d,e,i,j,p Cynomys (carrion) prairie dogs h

Geomyidae pocket gophers Thomom~ a;b,d,j,l, m,s

Heteromyidae heteromyids Perognathus pocket mice d,f,i,j,k,l DiEodomys kangaroo rats f,i,j,l

Cricetidae New World mice a,p Reithrodont- omys harvest mice b,e,j ,k,l,p Peromyscus white-footed mice b,d,e,g,i, j,k,l,p Oni'.chomys grasshopper mice j Hicrotus meadow voles e,g,h,i,j, k,l,p,s Lagurus sagebrush vole l Synaptomys bog lemmings p

Muridae Old World mice & rats "-> Rattus rats s .:

- continued next page - TABLE 2. continued

CLASS ORDER FAMILY COMMON NAME GENUS COMMON NAME SOURCE:': FOR FAMILY FOR GENUS (if appropriate) (if appropriate)

-Mus house mice d,e,k,p,s Zapodidae jumping mice Zapus e Carnivora Felidae cats Felis (house cat) k (carrion) 1'; a - Bent, 1938 k - Marti, 1974; Colorado b - Bond, 1942; Nevada 1 - Maser et al., 1971; Oregon c - Carson, 1951; California m - Neff, 1941; California d - · Coulombe, 1971; California n - Robertson, 1929; California e - Errington and Bennett, 1935; Iowa o - Robinson, 1954; Kansas f - Glover, 1953; Arizona p - Scott, 1940; Iowa g - Grant, 1965; Minnesota, N. Oak., S. Oak. q - Sperry, 1941; Colorado, Kansas, h - Hamilton, 1941; Colorado Montana, Washington

i - James and Seabloom 9 1968; North Dakota r - Stoner, 1932a; .California j - Longhurst, 1942; Colorado s - Thomsen , 1971; California t - this study, 1977; Washington

I\) <.n 26 described in the l i terature. I noted at least two instances of feeding on carrion; a mallard wing was found at one burrow and a leg from a large mammal (probably Lepus californicus) was found at another. 27

FOOD ABUNDANCE AND CACIIItJG

Literature Summary

It appears from the literature that burrowing owls have little trouble finding enough to eat. Grant (1965) points out that in prime habitat in Minnesota, insect, batrachian, and rodent prey are plentiful.

If the owls were having trouble findine prey, one would expect for them to have much larger hunting ranges than they normally have. Burrowing owls seldom hunt further than a few hundred meters from their burrow. Coulombe

(1971) concluded that food is not limiting for burrowing owls in the

Imperial Valley of California. Thomsen (1971) felt that death by starvation contributed very little to the overall mortality rate for the resident population of owls at the Oakland Municipal Airport in California.

There is some evidence that burrowing owls waste food. Grant (1965) frequently found the "dessicated remains of prey which had been merely picked at after being killed." He also made a rough estimate of per diem consumption of prey by an owl family of two adults and two young . By recording the number and type of prey taken and multiplying by an averag.e prey weight factor, he estimated that the total weight of the food taken by the owls in one day was 300 to 350 g . In aggregate, the four members of the family group weighed about 640 g. He then concluded that it is unlikely that the owls could eat half of their own weight daily especially since a captive juvenile was kept well-fed on 40 g of food per day . Marti (1973) kept an adult owl which consumed an average of 26.4 g of laboratory mice or

15.9% of its body weight daily. Of course, active owls require much more food than captive ones do. However, if one assumes that Grant's figures are roughly correct and if 011e. allows for the probability that part of the prey .28

may not be nutritionally useful to the owls, it seems likely that there is

considerable wastage. Wastage of food, however, is not unique to burrowing

owls a~ong raptors. For example, Brown and Watson (1964) estimated that

golden eagles (Aquila chrysaetos) in Scotland wasted about 20% of the

ptarmigan and red grouse, 40% of the red deer, and SO% of the lambs taken

as food. Dr. Don Johnson (course notes, Zooloey 501, Raptor Ecology) has

concluded that it appears for prey too large to be swallowed whole, wastage

is proportional to the size of the prey. And as Grant (1965) has pointed

out, burrowing owls seldom swallow prey, especially large prey

(e.g. rodents), whole.

There are conflicting reports in the literature on whether burrowing

owls maintain food caches. Bent (1938) mentions a report by C. E. McBee -fu1.t"d of numerous rodentsJ\in the burrows (one burrow had 28) of owls living near

Kiona, Washington. Thomsen (1971) felt that the burrow is used to store

food on occasion. She also quotes· a report by G. S. Agersborg that burrowing owls may store food in bad weather. Grant (1965) noted the presence of small food depots usually located within 100 feet of the burrow.

These depots normally contained just one food item and they were most often used by the sentry males (see section on Species Description). Martin

(1973b), however, did not observe typical s torage behavior although he noted that sometimes the owls made a kill at midday but did not eat it until evening. Butts 0976) studied a .wintering population of owls in Oklahoma and found no evidence of either extensive food caches or any sort of hiber­ nation or torpor. But he did think that in bad weather (particularly a blizzard) burrowing owJs are probably capable of fastinF, for at least three days. field Observati ons

Wh ile I made no measurements of food abundance, I did observe

that the owls seldom hunted more than several hundred meters from the main

burrow and seldom were the birds gone for very l~ng on a hunting foray.

It could be argued that territorial behavior is restricting the hunting area close to the main burrow rather than food abundance. But this seems unlikely for the Grant County population as the population density is not particularly high and there is very little overlap in home ranges for most of the owls. Thus, it seems reasonable to conclude that prey was plentiful.

However, I saw very little indication that food was wasted.

Since I did not dig up any of the burrows, I cannot say whether extensive food caches were maintained in the burrow proper. I saw little evidence of food caches away from the burrow, however. Occasionally, I did find a dead rodent near a burrow that was either partially eaten or hardly touched at all. Once, I saw an adult attempting to feed pieces of a rodent to several juveniles. But when the young owls showed very little interest, the adult carried the rodent into the burrow. 30 SEASONAL VARIATION IN DIET

Literature Summary

Many investigators have noted seasonal changes in the diet of

burrowing owls, the most common observation being an increase in the

numbers and diversity of insects taken in summer over that taken in

winter (Errington and BennettJ1935; James and Seabloom 1968; Maser, Hammer, 1

and Anderson11971; Ross and Smith,1970). Of course, this might be expected as there are normally fewer insects available in winter in colder climates.

But there are many other examples of seasonal changes in diet which are not

so readily explained. Thomsen {1971) found that the consumption of meadow

voles {Microtus californicus) and toads increased during the spring and

summer. The increase in the number of toads taken in warmer months might

be expected for the same reason that more insects are taken but what is the

reason(s) for the increased consumption of meadow voles? And Coulombe

(1971) found that for the California owl population he studied, earwigs

(Order Dermaptera) were the major food in winter and in early summer only.

There are many possible explanations for the seasonal variability

in diet characteristic of many burrowing owl populations. Ultimately, most likely these changes in diet merely reflect the relative availability of various prey items (Errington and Bennett,1935) since burrowing owls appear to be somewhat opportunistic in their hunting patterns (Longhurst,

1942). But there are a number of proximate factors affecting the relative availability of certain prey species. Seasonal changes in vegetation undoubtedly affect the vulnerability of many species to predation (Marti,

1974). Activity patterns of the prey and predator may overlap only at 31

certain times of the year (Marti, 1974). Many insects have very short

life cycles; thus, they may be abundant for short periods of time and then

virtuai1y disappear from the fauna when between generations (Scott, 1940;

Dr. William I. Turner, WSU, personal communication). Some vertebrate prey hibernate and thus are unavailable to the owls periodically (Marti, 1974).

Weather may also have an effect on the type of prey available.

Thomsen (1971) found a high number of birds in the diet of the owl popu­ lation she studied after a week of wet weather in mid-April. Marti {1974) a.ho hasApointed out that "longer daylight hours in summer together with young to feed may require owls to forage loneer and thus make some prey available that are not at other times." Finally, Errington and Bennett (1935) thought that an increase in the number of insects taken in late summer may result from the fact that juvenile owls are just then learning to forage on their own and they probably find insect prey easier to catch than mammalian prey.

Field Observations

I did not have the opportunity to investigate whether the diet of the Grant County owl population varies seasonally; it most likely does, however. It probably varies within a season as well. I suspect that adult owls take more rodents when they must provide food for the young than at other times over the course of. the summer. It would seem to be more efficient to feed all of the young from the carcass of one rodent than to make several foraeing trips to catch enoueh insects of equivalent biomass. 32 HUNTING BEHAVIOR

Literature Summary

Thomsen (1971) has categorized the huntine techniques employed by

burrowing owls into four basic types: ground foraging, observation foraging,

hovering, and flycatching. In ground foraging, the owls run across the

ground in pursuit of the prey item much in the fashion that a robin (Turdus

migratorius) does. Occasionally, they may make short flights, land, and

then continue running. Coulombe (1971) and Marti (1974) have also

observed this behavior. Maser et al. (1971) have pointed out that the

occurrence of plant roots in the pellets is further evidence of ground

foraging. Thomsen (1971) found that in California ground foraging was the

only hunting method used in winter. Observation foraging involves flying

(rarely more than 100 m) from a perch to the ground to catch the prey,

and then returning to the same or a different perch (Grant,1965; Marti,

1974; Marti, 1969; Martin, 1973b; Thomsen, 1971).

Hovering at a height of 8 to 30 mis another hunting method occasionally used by the owls, especially if the cover is heavy (Butts,

1971; Coulombe, 1971; Grant, 1965; Marti, 1974; Martin, 1973b; Thomsen,

1971). Maser~ al. (1971) did not observe hovering in their Oregon study.

Thomsen (1971) felt that hovering was used when the owls were hunting meadow voles. She also noted that 98% of the hove~lng was done by males and that it occurred most often in the evenings. Marti (197ti), however, found no apparent relationship between the method of hunting employed and the time of day.

The final hunting method used is flycatching. This technique is 33 rarely used but involves either a sharp climb or a level flight from a perch

until the prey is caught with the talons (Grant, 1965; Marti, 1974; Robert­

son; 1929; Thomsen, 1971). Only insects are captured with all four techniques (Marti, 1974).

Most of the foraging is done in the mornings and evenings (see

se~tion on Daily Activity Pattern). Foraging trips are seldom greater than

200 min distance although juveniles may range further than adults (Butts,

1971). Virtually all prey is captured with the talons ( Graht, 1965; Martin,

1973b; Scott, 1940). A small prey item is hopped upon and crushed with the

bill whereas a large prey item is a~tacked in typical raptor fashion

(Martin, 1973b). The prey is often decapitated or torn in two (Grant, 1965). Small prey is carried with the feet up under the tail whereas heavier prey

is usually carried with the feet in a dangling position. Sometimes, small

prey is transferred in flight from the feet to the beak (Grant, 1965; Martin, 1973b• Robertson, 1929).

Marti (1974) has studied the particular adaptations that burrowing owls have for hunting. They apparently do not see nearly as well in the dark as do barn owls (Tyto ~), great horned owls (Bubo virginianus), and long-eared owls (Asio ~). However, they can capture prey in total dark­ ness by hearing. The talons and feet are rather weak in comparison to long-eared, barn, and great horned owls; it took a f orce of only 500 g to open the clenched talons of a burrowing owl whereas it took 1350 g, 3000 g, and 13,000 g respectively for the other three owls. Marti (1974) refers to

G. E. Goslow's conclus i on that burrowing owls "are adapted to pin prey to the ground rather than scoop it up in hawk-like fashion." finally, the flight feathers of burr owing owls are not quite as developed for muffling flight as are those of a number of other owls. 34

Field Observations

I was able to observe three of the four hunting methods described

by Thomsen (1971) . The only technique I did not see was flycatching.

Ground and observation foraging often took place near the burrow. Several times I observed a combined observation foraging - hovering technique

used for foraging. This method can be described as follows. The owl hovers at a height of 30 to 50 m, then suddenly dips to a height of 10 to

15 m.followed by a climb back up to the original hovering altitude. This sequence may be repeated several times . Occasionally, the owl dips to a ~~ height of l to 2 m above the vegetation and then,;apidly at this height for perhaps 50 to 100 in before climbing to hover again.

Most of the foraging was done in the mornings and evenings , but especially from 1900 to 2200. I was not able to determine how much the birds were hunting at nir,ht (2200 to 0500).

I attempted to get a close look at prey killing behavior by tethering a large (approximately 4 cm long) beetle (species unknown) in the vicinity of the burrow but the owls either stared at it, ignored it, or avoided it. The reason for such a reaction is not clear although it may be that I chose a species of beetle that is not normally prey for the owls .

Usually, prey was carried in to the burrow area in the talons.

On one occasion, though , an adult flew to the burrow with an insect in its beak which it then eave to a juvenile. Huntine forays were seldom greater than 500 min distance from the main burrow. 3$

EATING AND DRINKING BEHAVIOR

Literature Summary

The observations made by Grant (1965) ar_e typical of much of the literature on the eating behavior of burrowing owls. Quite often, the prey item, especially if it is large, is decapitated before it is eaten. No large prey (e.g. a rodent) is swallowed whole . Sometimes, the owl will raise its head and stand erect, looking down at the prey before taking the first bite. Often, only the soft parts of batrachians are eaten. Insects may be torn in two and trodden upon repeatedly before they are consumed.

They may also be carried live or dead to a perch where they are held up in the talons of one foot and picked at with the bill much in the fashion that a human eats chicken. After feeding , the bird may wipe its bill on the perch or on some nearby vegetation. The feet may also be picked clean of food particles.

Martin (1973b) has described the typical juvenile response to an adult approaching the burrow with food. Young owls give the Rasp call

(see section on Vocalizations) and run out to meet the adult. Usually , the juvenile that arrives first to greet the adult receives the food item. The other juveniles normally do not try to take the food item away ·from the first chick, Instead , they continue to Rasp, and this stimulates the . adults to resume foragine.

It appears likely that burrowing owls require some free water to survive. Coulombe (1971) observed "captive owls drinking water, particularly at ambient air temperatures above 30°C." Najve owls drank water even when they were kept at room temperature. He also documented an increase in the 36 frequency of drinking by captive owls with increasinr, ambient air tempera­ ture. There are very few reports in the literature of drinking by a wild burrowing owl. Coulombe (1971) watched an adult drink for several minutes from a drainage canal.

Field Observations

The owls I studied exhibited much of the eating behavior described above. Decapitation was common. I often found rodent heads around the burrows and on one occasion I found the head of a shrike (Lanius). At one burrow, I found the dessicated remains of several batrachians with apparently only some of the soft parts eaten. Insects were sometimes eaten "chicken style". Occasionally, I observed an owl repeatedly walking over and stomping on a food item. The function of this behavior •is not clear unless it is merely an effort to kill a prey item that is not quite dead. Sometimes, the owls wiped their bills on some nearby object and picked at their feet.

The feeding behavior of the juvenile owls I studied approached closely that described by Martin (1973b). I also found that occasionally the younR owls walked over and pounced on the prey item before eatine it.

Usually, the juvenile that first got control of the food item was left in peace to consume it. On one occasion, however, a juvenile crouched and spread its wings over the prey as. if to guard it from the other birds.

Several times, I watched an adult strip pieces of meat from a rodent carcass and pass them beak-to-beak to whichever juvenile was iu the best position. The young did not always take food when it was offered.

I never observed any of the owls I studied to drink although standing water was readily available to most of the birds. 37

PELLET DESCRIPTION AND FORMATION

Literature Summary

Zarn (1974) summarizes the literature on burrowing owl pellets

by describing them as follows:

•. pellets are cylindrical in shape, with blunt , rounded ends. They measure 30-40 mm in length, 15 mm in diameter, and weigh slightly over one gram when dry. Color ranges from gray to brown, and in the warmer months the pellets may be quite fragile and con­ sist almost entirely of insect parts. Since the owls pick at their food as they eat, badly crushed and broken prey remains characterize b~rrowing owl pellets. ecpt>cidlly Sand, dirt, and stone appear frequently in the pelletsAfrom January

through April when the owls are escavating burrows (Martin, 1971;

Thomsen, 1971). Pellets are regurgitated in the typical raptor manner.

They are ejected almost anywhere in and about the burrow area but rarely

more than 100 m from the burrow (Grant, 1965; James and Seabloom, 1968).

Grant (1965) observed owls regurgitating pellets in late afternoon and in

early morning. A captive bird he kept cast a pellet regularly 8 to 10

hours after feedinP, ,

The analysis of burrowing owl pellets is not a perfectly reliable

method for determining the food habits of this species. Glading, Tillotson,

and Selleck (1943) have determined through laboratory trials that the

pellet reliability is only interm~diate when compared to the results for

the other hawks and owls they studied. There are a number of reasons

why. Thomsen (1971) points out that often an owl will decapitate a large

food item and then feed on it intermittently rather than consuming it entirely at one sitting. This makes it difficult to be sure of the exact number of a certain type of prey taken. Two or more owls may eat from the 38

same kill (ffl2 1974). Often, only the soft parts of some types of prey, IP especially batrachians, are eaten and consequently they do not appear in the pellets -----{Grant, 1965; Longhurst, 1942; Thomsen, 1971). Because burrowing owls pick at their food, their pellets are frequently quite

fragmentary and thus difficult to analyze (Glading et al., 1943; Thomsen,

1971; Zarn, 1974). Finally, pellets·composed of insect remains disintegrate

much faster than those composed of mostly vertebrate remains (Coulombe,

1971; Grant, 1965). Marti (1974), for example, found that in Colorado,

pellets containing insect remains fell apart as soon as they .dried whereas

pellets containing mostly vertebrate remains lasted up to 2 months.

Field Observations

I collected close to 1000 pellets during the course of my study

and nearly all of them match the description outlined by Zarn (1974).

Only once did I observe an owl reeurgitate a pellet and it was cast in

typical raptor fashion. Pellets were cast almost anywhere around the burrow

but particularly at favorite perches. Those pellets composed of insect remains were quite fraeile, often falling apart in my hand, whereas those composed of mostly vertebrate ramains were compacted and easily handled. 39 DEFECATION BEHAVIOR

Literature Summary

There is very little mention of defecation behavior in the litera­

ture. Grant (1965) noted that adults defecated at perches somewhat away

from the main burrow. These perches were often also fa~orite places for

casting pellets. Also, according to Grant (1965), young birds old enough

to be outside the burrow defecate at first on the main burrow mound. Then,

as they get older, they apparently learn to defecate in the grass a few

inches or feet away from the main burrow mound. Grant (1965) may be in

error, however in attributing this apparent change in location for defe~

cation as a change in behavior with age per se since the older a juvenile

owl is, the farther it wanders from the burrow. Thus, the owls may simply defecate wherever they happen to be at the time.

Field Observations

I agree with Grant (1965) that burrowing owls commonly defecate at favorite perches but the birds I studied still defecated quite a bit around the main burrow area as well. On numerous occasions, an owl defecated just before I chased it from from a particular perch. 40

VOCALIZATIONS

Literature Summary

A number of investigators have described the various vocalizations of the burrowing owl as best they could without using elaborate recording equipment (Berft)l938; Coulombe, 1971; Grant, 1965; Thomsen, 1971).

Ma~tin (1973a), however, is the first researcher to complete a detailed spectrographic analysis of burrowing owl vocalizations. The spectrograph is, of course, a much more discerning instrument than the human ear. For this reason, Martin (1973a) is the accepted authority and I have condensed his results and comments into Table 3.

According to Martin (1973a), burrowing owls exhibit at least 17 different vocal displays; the adults give 13 of these, the young give 3, and both give l. But "when the similarities between the calls are consid­ ered, it appears the owls may have only 9 basic calls, with variations on these producing the other 8." The 9 basic vocalizations are:

Primary Song (Coo coo), Female Copulation Warble, Female Defense Warble,Rattle

Tweeter, Chuck, Rasp, Juvenile Eep, and Snap. The ?rimary Song (Coo coo), Tweeter l and Female Copulation Warble are all associated with pair formation and copulation. The Primary Song (Coo coo) is also used in territorial defense as are the Femule Defense Warble and the Rattle. The

Chuck serves as a warning call. The Rasp is associated with feedine although it may also indicate distress. The Juvenile Eep call is chiefly an alarm call. Finally, the Snap (from snapping the mandibles together) serves as a protest or threat display.

Because of the similarity in sound between the Rattlesnake Rasp call TABLE 3. Vocalizations of the burrowing owl (Speotyto cunicularia). From Martin (1973).*

VOCALIZATION DESCRIPTION and FUNCTION

Primary Song (Coo coo) It functions in pair formation, precopulatory behavior, and territorial defense. Only males were seen giving it. The song consists of two notes of almost equal frequency, with the second note being longer than the first.

Smack This call is given by female owls during copulation. It consists of one to many down­ slur notes.

Tweeter This multinoted call is -given by the male near termination of copulation. It is not always given, but when it is, _it always follows the male's song during copulation. The notes of the call consist of a multitude of long up-and-down slurs.

Song During Copulation Males give one or two primary songs during copulation. The call is two-noted and has the same characteristics as the primary song.

Rasp This call is given primarily by the adult fe­ male. She gives it when distressed, when a predator is near the burrow, upon receipt of food from the male, while presenting food to the young, as the male sings, and when begging for food. The call often appeared to stim­ ulate the male to begin foraging. Males occasionally gave the call when presenting food to the female or young.

Eep This call is given by the female as the male sings. It may grade into the rasp call, and therefore, is thought to be associated with food begging or precopulatory behavior...... s:

J. 1973. A spectrographic analysis of w Martin, D. continued next page burrowing owl vocalizations. Auk 9O:56ij-578. TABLE 3. continued

Female Defense Warble This call is given by the female while defending her burrow against conspecific females. It appears to have a very irregular pattern and is relatively high in frequency.

Female Copulation Warble Infrequently given during copulation.

Male Warble Males may add an undulation to the end of their sond during copulation. This produces a warbling sound. The call has the same characteristics as the primary song, but at the end undulates.

Rattle· The female gave this call when played recordings of the primary song in the male's absence. In most cases the males quickly return to their burrows and assume a defensive posture.

Chuck It is a single-noted, low-level warning call given by both sexes; it may be associated with the owl's bowing display. It consists of a gradual upward slur followed by a sharp up-slur and then a down- slur.

Chatter It is a series of shortened chucks given by both sexes. It may consist of from 3 to at least 15 notes, with 5-7 notes being most common. A greater number of notes indicates a higher level of agonistic behavior by the owls.

Scream It indicates the highest degree of threat or agonistic behavior the owls express antl it is .,: given by both sexes. It may be given by N itself, within the chatter call, or most commonly, by replacing the first note of the chatter call. continued on next page TABLE 3. continued

Juvenile Eep This call, given only by distressed young 2-4 weeks old when handled or trapped, appears to serve as a low-intensity aJ...rm call, but may also function as a hunger call. It is given at a low intensity and can be heard only a few feet. The eep call up-slurs, then down-slurs.

Juvenile Rasp This is a hunger call given by the young. It appears to stimulate the male to begin foraging. It is also given upon receiving food.

Juvenile Rattlesnake Rasp This call is given by the young when severely distressed, as when cornered by a predator. It serves as a threat display. It is widely stated that when given from inside the burrow, it closely resembles a rattling rattlesnake. The female also is supposed to give this call.

Snap It is given by the adults and the juveniles. It is produced by snapping the .mandibles together and appears to be common among most owls. It serves as a low-level protest or threat display.

-'="w 44

of a juvenile burrowing owl and the rattling sound produced by rattle­

snakes. It has been proposed that this call is used occasionally by juvenile owls as a predator defense mechanism (Bent, 1938; Coulombe, 1971; Grant, 1965). Martin (1973a) has determined the two sounds to be very

similar spectrographically.

field Observations

Of the 17 vocal displays outlined by Martin (1973a), I believe

I heard only six of them: Primary Song (Coo coo), adult Rasp, Chuck,

Chatter, Scream, and Juvenile Rasp. I began my study too late to hear many of the calls associated with pair formation and copulation. I heard the Primary Song on only three occasions. Oddly enough, they were heard

in August (at three different burrows) after the breeding season was over.

The adult Rasp and the Juvenile Rasp were frequently heard especially when the adults were feeding the young. I agree with Thomsen (1971) that these two calls sound very similar to radio static. The Chuck and the

Chatter were the two most frequently heard calls; these were commonly given whenever I approached the owls closely. The Scream was heard just once. It was dark at t he time but the Scream call may have been given in response to a domestic cat that I saw in the area several minutes earlier. 45

PREDATORS

Literature Summary

The possibility of a predatory attack i s undoubtedly a problem for a small, ground-dwelling owl. The deposition of eggs in a ground-level burrow and the tendency of young owls to loaf outside the burrow most certainly make both the eggs and the juveniles especially susceptible to predation. Virtually any large predatory mammal, bird or reptile that is found in burrowing owl habitat could pose a threat. The extent to which predation contributes to raortality in the burrowing owl has not been investigated. However, Table 4 summarizes the literature and my field observations on those animals either known or suspected to prey on burrowing owls. A larp,e animal is suspected of being a predator of burrowing owls if the owls react to it as if it is one (see section on

Behavioral Responses to Predators).

Field Observations

All of the predators in Table 1i attributed to this study were suspected predators. That is, I never saw any actual predation by any of these animals, I only saw the owls react to them as if they might be predators. The striped skunk (Mephitis mephitis) observation was not visual but was based instead on the odor found around one of the burrows on one occasion. The reaction to a great horned owl was induced by placing a styrofoam dummy great horned owl near several of the burrows. Finally, 0.. I foundAcarnivore scat (possibly coyote) near a burrow on four different occasions. 46

TABLE 4. Some known and suspected predators of burrowing owls (Speotyto cunicularia).

11 PREDATOR REFERENCES ' gopher snake (Pituophis cateniferJ a large snakes a,g unidentified gull h turkey vulture (Cathartes aura) h golden eagle (Aquila chrysaetos) g ferruginous hawk (Buteo regalis) g red-tailed hawk (Buteo jamaicensis)

Swainson's hawk (Buteo swainsoni) g marsh hawk (Circus cyaneus) b,g,h prairie falcon (Falco mexicanus) b,f,g barn owl (Tyto alba) g great horned owl (Bubo virginianus) b,c,g,h badger (Taxidea taxus) a,g bobcat (Lynx rufus) g coyote (Canis latrans) g long-tailed weasel (Mustela frea&ta) d,e,g,h striped skunk (Mephitis mephitis) a,e,g,h domestic cat a,g,h domestic dog a,e,g human a,b,e, g,h

* a - Coulombe (1971) e - Thomsen (1971) b - Grant (1965) f - Webster (1944) c - Peeters (1963) g - Zarn (1974) d - Robertson (1929) h - this study (1976) 47 BEHAVIORAL RESPONSES TO PREDATORS

Literature Summary

Burrowing owls react to the presence of predators in a number of

ways. The type of display exhibited often depends on the level of the

threat. A potential predator that is not approaching closer to the

burrow area may be only carefully watched or even ignored. Should the

predator begin to move toward the burrow, the owls frequently exhibit a

curious bobbing or bowing behavior (Coulombe, 1971; Grant, 1965; Martin,

1973b; Thomsen, 1971) while also giving the Chuck call (see section on

Vocalizations). Martin (1973b} has proposed that this behavior arises from a conflict between the desire to hide and the desire to flee.

Instead of bobbing, the owls may crouch low to the ground in an effort to hide from the predator (Coulombe, 1971; Martin, 1973b; Platt,

1971; Scott, 1940, Thomsen, 1971) or engage in a displacement activity such as preening (Grant, 1965), Crouchine is an especially common reaction to the presence of raptors (Thomsen, 1971).

Frequently in response to a predator, the birds simply give a warning note (Chuck call} and run into the burrow (Martin, 1973b). As a variation, one adult, usually the male, remains outside the burrow and flys to a nearby perch apparently in an attempt to decoy the predator away from the burrow (Coulombe, 1971).

Sometimes, burrowing owls, especially males, will dive at or mob the predator while at the same time giving the Chatter call (Grant, 1965;

Rhoads, 1892; Thomsen, 1971). An extreme threat may cause the owls to give the Scream call (Martin, 1973b). 48

Burrowing owls also may respond to an extreme threat by adoptinr.

a defensive posture like that typical of many other owls (Coulombe, 1971;

Grant,.1965; Thomsen, 1971}. This posture is assumed by fluffing the feathers and opening and rotating the wings forward. At the same time, the bird crouches and then begins to weave back and forth. The Scream call or the Snap call may be given as well (Coulombe, 1971).

Another common reaction to the presence of a predator is flight

(Martin, 1973b; Thomsen, 1971). Before taking off, however, burrowing owls typically turn their backs toward the predator without taking their eyes off of it. Martin (1973b) has proposed that this back-turning behavior is an intent.ion movement for flight that also points the owl in the best direction for a quick escape.

Juvenile burrowing owls inside the burrow may react to a hunting predator by giving the Rattlesnake Rasp call. This call has been described as soun

Several investigators have suggested that this is an example of vocal mimicry (Coulombe, 1971; Grant, 1965; Martin, 1973a}. See the section on Vocalizations for a more detailed discussion of this behavior.

Martin (1973b) has observed some seasonal variation (apparently related to the breeding cycle) in the predator displays given by burrowing owls in New Mexico. He reported the following sequence •

• • • between October and February, Burrowinr, Owls usually crouch low to the r.round, run into a burrow, or fly away quietly when approached by a predator. In the spring (March-May) the owls usually give a warning call, then run into a burrow. When the call is given by either the male or the female, both usually run into a burrow, or the male may remain outside. From June until the young are independent, if the warning call is given by either parent, all young run into the burrow, with the female usually following the young. Males 49 usually remain out unless a direct attack is in progress, If the attack is by another raptor, the male usually retreats into a burrow; if by terres­ trial predator, the male·.will usually mob the predator.

Field Observations

I observed most of the behavioral reactions to predators that are described in the literature. Passing raptors and gulls were usually watched carefully. Bobbing, crouching, and chattering were common responses to my presence. I frequently watched a family group run into a burrow when a warning note (Chuck call) was given by the sentry male.

Even a passing truck was occasionally enough to elicit this response.

I also often saw adults flying away from the burrow to favorite perches in an apparent decoy maneuver, On one occasion, an adult hovered· over and dived at a passing lone-tailed weasel (Mustela frenata).

I was able to observe the typical defense posture to extreme threat on three occasions. I saw both adult and young owls roll their wings forward and give Rasp calls, again in response to passing weasels.

The Scream call was heard only once, probably in response to a domestic cat although it was too dark at the time to tell for sure.

Finally, I also observed the flight response of a burrowing owl to an approaching predator, Back-turning behavior was commonly seen before flight. I never heard the.Rattlesnake Rasp call that is described in the literature as resembling a rattlesnake rattle.

In an effort to get a closer look at predator displays, I placed a styrofoam great horned owl model on a perch near each of three. different burrows. In r esponse, the owls (both adults and juveniles) would some­ times ignore it and at other times watch it carefully. Occasionally, 50 both adults and juveniles would hover over the model and chatter at it as if they were threatened. Once , the fake owl fell off the post I had pl_aced it on. This sudden movement caused some "rubberneckine" from two of the burrowini owls and one of them also began to hover over and chatter at the model . 51 OTHER INTERSPECIES INTERACTIONS

Literature Summary

Burrowing owls prey chiefly on insects and small rodents and only

rarely take birds. Many small birds, however, still respond to the

presence of a burrowing owl by mobbing it. The following are some of

the species of birds reported to harass or mob burrowing owls:

American kestrel (Falco sparverius), American avocet (Recurvirostra

americana), black tern (Chlidonias niger), barn swallow (Hirundo rustica),

cliff swallow (Petrochelidon pyrrhonota), mockingbird (Mimus polyglottos),

robin (Turdus migratorius), western meadowlark (Sturnella neglecta),

red-winged blackbird ( Ap,elaius phoeniceus), common grackle (Quiscalus

quiscula), chestnut-collared longspur (Calcarius ornatus), western

kingbird (Tyrannus verticalis), eastern kingbird (Tyrannus tyrannu~), and

common flicker (Colaptes auratus). Thomsen (1971) noted that only two

(western meadowlark and red-winged blackbird) of the seven species of

birds she saw harassing burrowing owls were also known to have been taken -H-o"l)~ (rarely) as prey. Some small birds ,. apparently take very little notice of burrowing owls. Coulombe (1971) observed no interaction between nesting

owls and nearby breeding colonies of yellow-headed (Xanthocephalus

xanthocephalus), tri-colored (Agelaius tricolor), and red-winged blackbirds.

Burrowing owls normally ignore larger birds (except raptors) such

as ducks, shorebirds, and pheasants (Grant, 1965; Thomsen, 1971) and it

appears the reverse is also true. However, the owls do respond to killdeer (Charadrius vociferus) screams as an alert to potential danger

(Thomsen, 1971) . 52 Mammals squirrel- sized or larger that are not predators mostly ignore burrowing owls and vice versa. Running jackrabbits (Lepus spp.) are so~etimes interpreted by the owls as a sign of potential danger

(Thomsen, 1971). Very little interaction has been observed between burrowing owls and several species of ground squirrels that normally associate with the owls (Coulombe, 1971; Grant , 1965; Thomsen, 1971 ).

This lack of interaction may be of adaptive value to the owls since they often depend on these squirrels for burrows or it may simply be a case of the squirrel~ being too large a prey for the owls to handle.

Field Observations

An.e.stern kingbird, some starlings (Sturnus vulgaris), a barn swallow, some unidenti fied swallows, and an unidentified blackbird were the only species I observed harassing the owls . But several species of birds includinp, a rinr,-necked pheasant (Phasianus colchicus), a robin, a mourning dove (Zenaidura macroura) , a loggerhead shrike (Lanius ludovicianus), western kingbirds, western meadowlarks, and an unidentified blackbird were observed to perch or walk near the owls , sometimes within several meters. The owls seemed to pay very little attention to these birds and vice versa. Sometimes though, i .t appeared that the small birds were

"trying'' to perch as close as possible to the owls. On one occasion, a

Nuttall's cottontail (Svlvilagus nuttali) passed within 5 m of several owls but it was i~nored. Low flyinp, Canada geese- (Branta canadensis) were also r.erirally ignored. A 53

PRODUCTIVITY

Literature Summary

Most of the field studies on burrowing owls include at least some

estimate of productivity. The most commonly used productivty parameters

are clutch size, brood size, fledging success, per cent .of the population

nesting successfully, and extent of renesting.

Clutch size is not easily determined for burrowing owls without

destroying the/burrow as well. Consequently, there are few recent studies

that include any information on clutch size. Generally though, the clutch

size ranges from 6 to 11 (Bent, 1938). Murray (1976) has documented a

weak trend of increasing average clutch size with increasing latitude,

She has also dtermined that clutch size is larger toward the center of " the United States than toward the coasts.

Normally, brood size is not determined for burrowing owls until

juvenile owls emerge from the burrow at about 2 weeks of age. Brood

size mostly ranges from 3 to 6 (Coulombe, 1971; Grant, 1965; Scott, 1940;

Thomsen, 1971), The relatively large brood size for burrowing owls "may

be an adaptation to a high rate of predation on .the young and/or to

scarcity (in many areas) of nest sites" (Coulombe, 1971).

Fledging success for three different populations of burrowing owls was estimated to be 89%, 94,9% (4.9 young per pair), and 73% (Butts,

1971; Martin, 1973b; Thomsen, 1971).

The proportion of a population nesting successfully is another important productivity parameter. Butts (1971) found that 80% of the nests he studied in Oklahoma produced at least one fledgling. Martin (1973b) found only one nonbreeding pair among 15 pairs he studied in New Mexico. 54

Thomsen (1971), however, determined in her two-year study that although

36 of 51 adults were breeding, only 13 of 24 pairs fledged young.

Renesting does occur occasionally in burrowing owls if. the eggs or nestlings are destroyed or removed {Dent, 1938; Butts, 1971; Rhoads, 1892;

Thomsen, 1971). Butts (1971) found that only l of 4 renesting attempts he observed was successful, however. Burrowing owls apparently never start a second clutch if the first one is successful.

There are, of course, numerous ecological and behavioral factors that could possibly affect the productivity of burrowing owls. Thomsen

(1971) has pointed out that both the amount of past experience in raising young and t 'he amount of "stability both within the pair and in relation to the activities of other pairs" are important to successful reproduction.

She also thought that a late, short growing season can influence ·produc­ tivity and that food can be a limiting factor to production. On the other hand, Coulombe (1971) found that food was not limiting but rather the availability of burrow sites was the most crucial factor limiting the population he studied. Martin (1973b) has suggested that the high productivity of a population of owls he studied in New Mexico compensates for a high annual death rate during migration. Productivity for a migratory population can be maximized because limiting resources should be more abundant in the breeding season than they are for a resident popu­ lation. Finally, Martin (1973b) also found ncfoorrelation between burrow position {direction) and the number of young fledp;ed.

Field Observations

Since my study was behaviorally oriented, I spent most of my time observing a f ew selected pa irs of owls, Consequently, I have very little 55 information on the productivity of the Columbia Basin population as a whole. I observed young at 13 of the 29 burrows I located but this figure is of little meaning since I visi~ed many burrows only once or twice. The minimum brood sizes for the four burrows I watched intensively were 3, 3, 6, and 8. I found it very difficult to determine the exact OT brood size as I was never sure that all~the juveniles were outside of the burrow when I was counting them; hence, the term minimum brood size. And of these four broods, I believe 2, 2, 6, and 6 of the juveniles respectively fledged successfully. 56 MORTALITY

~iterature Summary

There is very little information in the literature on the mortality rate for burrowing owls. Thomsen (1971} estimated for a California population of owls a nestling mortality rate of 18 to 23%, a first- year mortality rate of 70%, and an adult annual mortality rate of 19% for an overall total annual mortality rate of 35%. She also suggests that the mortality rate for adults is age constant as it is for most other raptors (course notes, Zool 501, Raptor Ecology, Dr. Don

Johnson). Nestling mortality probably occurs before the young begin to emerge from the burrow at 2 weeks of age (Martin, 1973b) although this is difficult to document. Juveniles are often found dead near the burrow in summer (Coulombe, 1971) with the cause of death unclear in many cases. Thomsen (1971) observed that 5 of 9 first-year survivors were the heaviest of their respective broods and two others were the next heaviest. Someti~es this apparent weight advantage to survival amounted to only a few tenths of grams, however. Incubation in burrowing owls begins with the first egg so the owlets often vary some­ what in size (Butts, 1971).

Of course, there are many possible causes of mortality in F<>r uaP'pfe., · burrowing owls. /\Natural mortality may result from predation (see section on Predators), disease and parasitism, starvation (rarely), stress during migration; and the occasional collapse of a burrow

(Thomsen • 1971; Martin, 1973b).

Burrowing owls can probably contract several avian diseases 57 although I found no information in the literature on this subject.

However, they do act as hosts for at least ecto- and endoparasites

(Table 5), The extent of parasitism apparently depends on the location

with some populations exhibiting a high infestation and others a very

low infestation (Bent, 1938; Rhoads, 1892; Thomsen, 1971). The stick­

tight f l ea (Echidnophaga gallinacea) is of particular interest to

humans as it has been shown by Wheeler et al. (1941) to be a vector

for the Plague (Pasteurella pestis).

Starvation resulting directly from a small prey base is probably

not a significant cause of mortality for these owls , even in winter

(Thomsen, 1971). Martin (1973b) has argued that migratory populations v~(lT•'(ol,lf\d of burrowing owls probably suffer a higher death rate than~resident populations.

Burrows undoubtedly collapse at least occasionally and perhaps frequently. Domestic stock passine within 3 to 4 ft of a burrow elicit calline and feather ruffling behavior (see section on Behavioral

Responses to Predators) from adult owls (Grant, 1965). Thomsen (1971) regularly observed damaged burrows in her study of owls inhabiting the

Oakland Municipal Airport. She estimated that dogs caused 20% and humans caused 65% of the damaged burrows. Other owl populations not located in urban areas probably do not suffer quite as high a rate of bu:rrow collapse, however.

Man-caused mortr11ity is probul>ly quite hir,h for burrowine owls. llecause of their tendency to nest along hi?;hway road banks, they are often hit by passing automobiles (Scott, 1940; Zarn, 1974), Burrows located along irrigation canals are occasionally flooded out (Grant,

1965) and burrows located in agricultural fields are often destroyed TABLE S. &ome ecto- and eodoparasites of the burrowing owl (Speotyto cunicularia).

CLASS FAMILY SCIENTIFIC NAME REFERENCES

Cestoda Dilepididae Choanotaenia speotytonis Buscher and Tyler (1975)

Trematoda Echinostomidae Echinoparyphium sp. Buscher and Tyler (1975 )

Strigeidae Strigea sp. Buscher and Tyler (1975)

Nematoda Filariidae Hamatospiculum cylindricum Buscher and Tyler (1975)

' Spiruridae Cyrnea sp. Buscher and Tyler (1975) Arachnida Argasidae Argas dalei Clifford --et al. (1976) Insecta Menoponidae Colpocephalum pectinatum Tyler and Buscher (1975)

Philopteridae Philopterus syrnii Stoner (1932b); Tyler and Buscher (1975) Tungidae Echidnophaga gallinacea Wheeler --et al. .( 1941)

u, 00 59

accidentally by agricultural equipment (Grant, 1965; Zarn, 1974).

Some owls probably suffer secondary poisoning from rodenticides picked

up from carrion ( Zarn, 1974). Finally, some owls are shot either by

misinformed individuals or by hunters mistaking them for upland game

(Zarn, 1974) .

There is some controversy in the literature over whether burrowing

owls occasionally exhibit cann.ibalism. Many investieators (Coulombe, .

1971; Grant, 1965; ; Robinson, 1954; Thomsen, 1971) have found

dead owls (usually juveniles); sometimes partially devoure

the burrow site and Uarti (1974) even found the remains of several owls

in some pellets. Predation is a possible explanation in some cases although

it seems unlikely that a predator would devour a fresh kill right at the

burrow site. The more likely explanation may simply be that the .owls die

from any number of causes and the survivors merely _rcgard the dead owls

as carrion to be eaten. The actual killing and subsequent cannibalism of

one owl by another has never been seen in the wild as far as I can determine

although it has occurred in captivity (Coulombe, 1971; Rhoads, 1892).

Field Observations

Virtually all of my owls supported a considerable population of

what I believe to be sticktight fleas. It was common for me to find up

to 10 fleas on my person after walkine about a burrow site for several minutes. It is not known whether the birds were affect adversely by the

high flea infestation.

I observed the collapse of only one burrow among those I studied

but I am not sure of the fate of the birds involved. I occasionally 60

observed collapsed burrows from previous years.

One burrow along an irrigation ditch was flooded out but the

birds using it were not killed. One pair with 6 young inexplicably

disappeared without a trace. Two piles of feathers were all that

remained of still another pair but the cause of death is unknown.

Highway mortality is probably considerable for the Columbia

Basin burrowing owl population. Many pairs locate their burrows in

road banks and frequently fly low across the road to favorite• perchesf

(such as road signs) and hunting areas. Doth juveniles and adults often stand out in the middle of the road in the morning or evening and at night, possibly because the road is warm. · Although I came across only 3 road-kills, I believe that hiehway mortality is probably a major cause of death for the burrowing owls I studied.

Twice I found the wing of a dead owl at the burrow site. Whether the two owls were killed and cannibalized I of course do not know but

I am inclined to believe that the owls died from any one of a number of causes and then were fed upon by the surviving owls. 61 MIGRATION AND DISPERSAL

Literature Summary

MIGRATION

Bent {1938) has described burrowing owls as being migratory in their northern range and more or less year-round reside~ts elsewhere.

They winter north to California, southern Arizona, southern New Mexico,

Texas, and southern Florida. In parts of Central America, burrowing owls may only be winter residents. The Florida subspecies is essentially eoxly non-migratory. For Washington, Bent (1938) lists the followingAsprine arrival dates: Walla Walla - March 1, Yakima - Narch 7, and Prescott -

March 19. lie also lists two fall late departure dates, Yakima - October l and Grays Harbor - October 19. Also according to Bent (1938), burrowing owls rarely winter in Washinr,ton;

Brenckle (1936) in the Dakotas, Butts (1976) in Oklahoma, Coulombe

(1971) in California, Grant (1965) in Minnesota an

(1973b) in New Mexico, and Scott (1940) in Iowa have all reported migrations to occur in the owl populations they studied. Coulombe (1971), however, found that 20 to 25% of the popul~tion in the Imperial Valley of

California overwinters with some immigration from the north and some emigration to the south occurring also in this period. Butts (1976) reported that less than 0.5% of the summer population he studied in Okla­ homa wintered in the area as well. Martin (1973b) observed owls sporad­ ically during the winter in New Mexico. The population that Thomsen (1971) studied at the Oakland Municipal Airport was essentially non-migratory.

De?er.dine on the area, fall migratio~ may begin as early as July 62

(late) or as late as October (Bent, 1938; Brenckle, 1936; Coulombe, 1971;

Grant, 1935; Martin, 1973b; Scott, 1940). The birds return to most of the breeding areas in March and April (Bent, 1938; Brenckle, 1936; Butts ,

1976; Martin, 1973b).

It is not well-known where many of the migratory owls winter.

Brenckle (1936) was able to document from band recoveries and returns that SOM~ owls from South Dakota were wintering in central Texas and in Oklahoma.

Butts (1976) reported that at least one of the birds he studied in Okla- homa mip,rated to west-central Mexico.

According to Butts (1976), overwintering owl.sin Oklahoma do not establish extensive food caches nor do they exhibit hibernation or torpor; however, they may be able to fast during bad weather. Communal burrow utilization during the winter has been reported by G. S. Agersborg in

Bent (1938) although it apparently only occurs very rarely.

Because burrowing owls seldom fly more than a few hundred meters at a time when on the breeding grounds , they do not appear to be strong fliers ideally suited for long migrations. This is not the case, however.

In fact, they have been reported several times at sea as far as 125 miles off the coastline (Patterson, 19~6; Sykes, 1974).

DISPERSAL

Soon after fledging in June, July, or August, the family structure slowly begins to break down and the birds begin to lose their site attach­ ment to the burrow (Coulombe, 1971; Grant , 1965; Martin, 1973b; Thomsen,

1971). Thomsen (1971) noted that the birds may change burrows up to 3 times and the young become increasingly independent. Owls in northern areas .f.jMJly begin their migration in September and October. For a non-migratory 63

population in California, however, Thomsen (1971) found a dispersal wave A"cl in September instead. ftln January, she observed a second wave of dispersal

that was apparently related to the approaching breeding season; Females

appeared to disperse farther than males in her study.

Field Observations

MI GRAT I OH

I believe that the owl population I studied is migratory. Based on information in the literature, talks I had with local farmers, and my own observations, it appears that some of the Grant County owl popu­ lation begins to migrate in August. By October, most of the population has left the area. I am not sure of the extent of overwintering since I did not visit the study area during the winter. However, there are reports in the literature o(overwinterinr.; owls in Washington. Woodby

(1976) sighted a single owl on 30 January 1976 right in my study area,

DISPERSAL

After fledr,inr, occurred in June and July, the owls began to wander more, sometimes shifting to nearby accessory burrows. It also became increasingly difficult to find all of the members of a family group together- or even to locate a single bird. 64

LITERATURE CITED

Bendire, C. E. 1892, Life histories of North American Birds, Vols. I & II_. U.S. National Museum Special Bulletin No. l. Washington, D.C., Government Printing Office. Bent, A, c. 1938, Life histories of North American birds of prey, Part 2, U.S. Nat. Mus. Bull. No. 170. Washington, O.C., Government Print­ Office, 466pp.

Berger, R. J, and J.M. Walker. 1972. Sleep in the burrowing owl (Speotyto cunicularia hypugaea). Behav. Biol. 7:183-194,

Bond, R. M. 1942. Food of the burrowing owl in western Nevada. Condor 44:183.

Brenckle, J, F. 1936. The migration of the western burrowing owl. Bird Banding 7:166-168.

Brown, L. H. and A. Watson. 1964. The golden eagle in relation to its food supply. Ibis 106:78-100. Buscher, H. N. and J. D. Tyler. 1975. Parasites of vertebrates inhabiting prairie dog towns in Oklahoma: II. Helminths. Proc. Okla. Acad. Sci. 55:108-111. Butts, K. o. 1971. Observations on the ecology of burrowing owls in western Oklahoma. A preliminary report. Proc. Okla. Acad. Sci. 51:66-74.

1976. Burrowing owls wintering in the Oklahoma Panhandle. Auk 93:510-516.

Carson, J. A. 1951. Burrowing owl with Jerusalem cricket. Condor 53:46,

Clifford, C. M., J.E. Keirans, H, Hoogstraal and D. Corwin. 1976. Observations on the subgenus Argas (Ixodoidea: Argasidae: Argas). A. (A.) dalei, new species, parasitizing the burrowing owl in Peru. Ann.-Entomol. Soc, AM. 69:917-925,

Coues, E. 1903, Key to North American birds, Vol. II. Boston, D. Estes.

Coulombe, H. N. 1971 . Behavior and population ecology of the burrowing owl, Speotyto cunicularia, in the Imperial Valley of California. Condor 73:162-176.

Earhart, C. M. and N. K. Johnson. 1970, Size dimorphism and food habits of North American owls. Condor 72:251-262.

Errington, P. L. and L. J. Bennett. 1935. Food habits of burrowing owls in northwestern Iowa. Wilson Bull. 47:125-128. 65

Glading, B., D. r. Tillotson and D. M. Selleck. 1943. Raptor pellets as indicators of food habits. Calif. Fish and Game 29:92-117. Glover, r. A, 1953. Summer foods of the burrowing owl. Condor 55:275. Grant, R. A. 1965, The burrowing owl in Minnesota. Loon 37:2-17.

Hamilton, W, J,, Jr. 1941. A note on the food of the western burrowing owl. Condor 43:74.

Howell, T. R. 1964. Notes on incubation and nestling temperatures and behavior of captive owls. Wilson Bull. 76:28-36. James, T. R. and R. W, Seabloom. 1968. Notes on the burrow ecology and food habits of the burrowing owl in southwestern North Dakota. Blue Jay 26:83-84. Kennard, r. H. 1915, The Okaloacoochee Slough. Auk 32:154-166. Longhurst, w, M. 1942. The summer food of burrowing owls in Costilla County, Colorado. Condor 44:281-282,

Marti, C. D. 1969. Some comparisons of the feeding ecology of four owls in northcentral Colorado. Southwestern Nat. 14:163-170.

1973. Food consumption and pellet formation rates in four owl species. Wilson Bull. 85:178-181.

---. · 1974. Feeding ecology of four sympatric owls. Condor 76:45-61.

Martin, D. J. 1971. Unique burrowing owl pellets. Bird Banding 42: 298-299.

1973a. A spectrographic analysis of burrowing owl vocalizations. Auk 90:564-578.

1973b. Selected aspects of burrowing owl ecology and behavior. Condor 75:446-456.

Maser, c., E.W. Hammer and S. H. Anderson. 1971. Food habits of the burrowing owl in central Oregon. Northwest Sci. 45:19-26.

Murray, Gale A. 1976. Geographic variation in the clutch sizes of seven owl species. Auk 93:602-613.

Neff, J. A. 1941. A note on the food of burrowing owls. Condor 43: 197-198.

Patterson, R. A. 1946. Burrowing owl at sea. Wilson Bull. 58:53.

Peeters, H.J. 1963, Two observations of avian predators. Wilson Bull. 75:274. 66 Platt, J.B. 1971. A survey of nesting hawks, eagles, falcons, and owls in Curlew Valley, Utah. Great Basin Nat. 31:51-65.

Rhoads, s. N. 1892. The breeding habits of the Florida burrowing owl (Speotyto cunicularia floridana). Auk 9:1-8.

Robertson, J. McB. 1929. Some observations on the feeding habits of the burrowing owl. Condor 31:38-39,

Robinson, T. S. 1954. Cannibalism by a burrowing owl. Wilson Bull. 66: 72.

Ross, P. V. and D. J. Smith. 1970. Notes on the ecology of the burrowing owl, Speotyto cunicularia, in the Texas High Plains. Tex. J. Sci. 21:479-480.

Scott, T. G. 1940. The western burrowing owl in Clay County, Iowa in 1938. Amer. Midl. Nat. 24:585-593.

Sperry, C. C. 1941. Burrowing owls eat spadefoot toads. Wilson Bull. 53:45.

Stoner, E. A. 1932a. Notes on the western burrowing owl. Condor 34: 228-229.

1932b. The approximate louse population of a burrowing owl. Ccndor 34:230-231.

Sykes, P. w., Jr. 1974. Florida burrowing owl col lected in North Carolina. Auk 91:636-637.

Thomsen, L. 1971. Behavior and ecology of burrowing owls on the Oakland Municipal Airport. Condor 73:177-192.

Tyler, J. D. and H. N. Buscher. 1975. Parasites of vertebrates inhabiting prairie dog towns in Oklahoma: I. Ectoparasites. Proc. Okla. Acad. Sci. 55:166-168.

Webster, H. 1944. A survey of the prairie falcon in Colorado. Auk 61: 609-616.

Wheeler, c. M., J. R. Douglas and F. C. Evans. 1941. The role of the burrowing owl and the stickt-ight flea in the spread of plague. Science 94:560-561.

Woodby, D. A. 1976. Winter owl records for eastern Washington. Murrelet 57:16-17.

Zarn, M. 1974, Habitat management series for unique or endangered species. Report No. 11, burrowing owl, Speotyto cunicularia hypugaea. Bureau of Land Management, U.S. Dept. of Interior. Denver, Colo. 32pp.