Spring 1995 (v7n2)

Summary of California studies analyzing the diet of barn owls.

Chuck Ingels

Article written for Sustainable Agriculture/Technical Reviews. 1995

Farmers and ranchers are looking closely at the benefits barn owls offer as an alternative method of controlling vertebrate pests (see Sustainable Agriculture Vol. 5. No. 1). The diet of the barn owl (Tyto alba) is relatively easy to ascertain, and several dozen studies have been conducted throughout the U.S. to determine the prey species consumed (Clark and Bunck, 1991). Barn owls swallow their prey whole and later regurgitate one to two inch pellets containing undigested bones, teeth, and fur. The owls usually produce one to two pellets per day, often dropping one at their nesting site and one at a distant roosting site (Evans and Emlen, 1947). Skulls found in these pellets can be keyed out to determine the identity of the prey species.

Over 95 percent of the diet usually consists of small mammals (mostly rodents), however in some studies substantial bird remains have been found. According to Colvin (1986), each adult barn owl may consume about one or two rodents per night; a nesting pair and their young can eat over 1,000 rodents per year. Dietary studies from California and other states show that a barn owl consumes an average 50 to 60 grams of prey per day (0.11-0.13 pounds per day, 40-48 pounds per year). The actual species consumed depends on the species abundance and availability in the area.

Overview

Table 1 shows the results of several barn owl prey studies conducted in California. In many studies, meadow voles and/or pocket gophers were consumed most often, while pocket, white-footed, and house mice were also important. One notable species missing in nearly all these studies is the California ground squirrel (Spermophilus beecheyi). This species ventures above ground only during the day, while the barn owl hunts almost strictly at night.

Study Findings

Berkeley 1926-27. Because each of these studies took place in Berkeley and because the results of each were very similar, the percentages of each species were averaged and combined into one column. In one study (Foster, 1926), pellets were collected on one sampling date from under a nest in Wildcat Canyon, just northeast of Berkeley. In another study (Foster, 1927), pellets were collected over a period of one and a half years from a nest located in a cave in Wildcat Canyon. Prey counts were separated by the dry season vs. the wet season. More shrews, Jerusalem crickets, and white-footed and pocket mice were taken during the dry season than the wet season, while the opposite was true for pocket gophers. In a third study (Hall, 1927), accumulated pellets were collected on one sampling date from a location in Berkeley.


Table 1.  Food Items in Barn Owl Pellets:  Summary of California References                      

                     ---------------------------------  Study  -----------------------                                                                  

Common    Scientific Berkeley  SF     Central Davis  Madera    LA    Placer   Siskiyou No     % of   Ave.    
Name       Name      1926-27   Bay            1947   Foothills Co.   Co.       Co.     prey.  Total  Weight  
                               Area   Calif.          Co.      1960  1974     1978     of     prey   (g)     
                               1937   1945           1947                                     found          

Calif.    Microtus   60        50     6       15     2         10    42       61       2,398  31     54      
meadow    californic                                                                                         
vole      us                                                                                                 

Pocket    Thomomys   8         21     24      26     37        2     28       0        1.053  18     156     
gopher    bottae                                                                                             

White-foo Peromyscus 14        14     6       12     7         0     25       37       878    14     25      
ted mice   sp.                                                                                               

Pocket    Perognathu 1         0      36      0      43        0     0        0        609    10     15      
mice      s sp.                                                                                              

Wood rat  Neotoma    0         0      3       0      0         65    0        0        90     8      271     
          fuscipes                                                                                           

House     Mus        3         4      2       38     0         0     0        0        348    6      18      
mouse     musculus                                                                                           
                                                                                                             
                                                                                                             

Harvest   Reithrodon 8         4      4       3      0         2     0        0        280    3      12      
kangaroo  -tomys                                                                                             
          megalotis                                                                                          

Kangaroo  Dipodomys  0         0      6       0      7         0     0        1        112    2      65      
rat       heermanni                                                                                          

Roof rat  Rattus     0         0      0       1      0         12    1        0        32     2      183     
          rattus                                                                                             

Other                6         7      13      5      4         9     4        1        388    6              
species                                                                                                      

No. of               2480      338    958     749    513       92    660      398      6,188                 
individua                                                                                                    
l prey                                                                                                       
found                                                                                                        

Number               796       87     NA      280    240       NA    538      143                            
of                                                                                                           
pellets                                                                                                      
found                                                                                                        



San Francisco Bay region (Smith and Hopkins, 1937). In this study, 12 boxes were installed in trees and barns in these counties: Marin (4), Contra Costa (2), Alameda (3), and San Mateo (3). A total of 141 pellets were collected over three years. California meadow voles were found most frequently except in Marin County, where pocket gophers predominated.

Central California (Hawbecker, 1945). Pellets were collected over a wide area during the nesting seasons of several years. Three types of habitat were included in this study, ranging from a well-forested, humid region to one that is treeless and shrubless. The specific regions and important prey findings are as follows:

1. Santa Cruz and western Monterey counties (coastal Transition Zone): pocket gophers-33%, meadow voles-17%, and birds-16%.

2. Eastern Monterey and western San Benito counties (Upper Sonoran zone): pocket gophers-52%, pocket mice-17%.

3. Western Merced and Fresno counties (Lower Sonoran Zone): pocket mice-66%.

Based on rodent trapping in several of the study areas, the barn owl was found to serve as a good sampler of the small mammals of a given area. However, the author noted that the selection of species appeared to be based partially upon numbers and ease of capture.

Davis (Evans and Emlen, 1947). An average of one pellet per day was found beneath a palm tree over a one year period. The palm tree served as a daytime roost to one barn owl. Based on nighttime observations, the owl was determined to have a hunting range of about 165 acres. About 140 acres were in open fields planted largely to grain and alfalfa and 25 were in wooded areas along Putah Creek. Animals typically associated with wooded or brushy cover, including house mice, deer mice (Peromyscus), harvest mice, and roof rats, comprised 57 percent of the total food items. Open field habitats, more than six times as extensive on the owl's range, contributed the remaining 43 percent of the items, which included pocket gophers and meadow voles. During the fall, the numbers of house and deer mice taken declined, while pocket gopher numbers steadily increased from winter through fall.

Madera County foothills (Fitch, 1947). This study was conducted in the blue oak-Digger pine belt of the Upper Sonoran Zone of Madera County. The region is comprised of rolling foothills broken by numerous ravines, and includes substantial grassland. Barn owl pellets were collected over four years at four sites. Computed on a prey weight basis, the pocket gopher accounted for 71 percent of the diet of the barn owls. Pellets were also collected from day roosts of great horned owls, which were far more numerous than barn owls in this area. The diet of the great horned owls consisted largely of Jerusalem crickets, woodrats, cottontails, kangaroo rats, and pocket gophers. On a weight basis, 56 percent of the diet was cottontails. For comparison, the diet of red-tailed hawks was also presented from a related study. On a weight basis, 50 percent of the diet of the hawks consisted of ground squirrels.

Coastal Los Angeles County (Cunningham, 1960). Pellets were collected once from the base of a date palm tree. Because of the abundance of wood rats and the low percentage of pocket gophers and meadow mice, the author concluded that the barn owls foraged largely in the chaparral-covered Santa Monica Mountains about two miles north of the collection site. Two samples of great homed owl pellets were also taken; their diet consisted mostly of pocket gophers, house mice, meadow mice, and wood rats.

Placer County (Clark and Wise, 1974). Pellets were collected at eight sites along the eastern edge of the Sacramento Valley, mostly from barns just northwest of Lincoln. On a weight basis, over half of the diet of the barn owl consisted of pocket gophers, while white-footed mice accounted for only about seven percent.

Siskiyou County (Rudolph, 1978). This study examined the coexistence and diets of barn owls and great horned owls at Tule Lake National Wildlife Refuge. The owls roosted on rock cliffs with a hunting range that included natural vegetation and agricultural fields. Pellets were collected at weekly intervals from the roosting sites. The diet of the great horned owls was very similar to that of the barn owls. Barn owls were found to hunt primarily on the wing, while great horned owls hunted primarily from telephone poles.

References

Clark, D.R. and C.M. Bunck. 1991. Trends in North American small mammals found in common barn owl dietary studies. Canadian Journal of Zoology 69:3093-3102.

Clark, J.P. and W.A. Wise. 1974. Analysis of barn owl pellets from Placer County, CA. The Murrelet 55(l):S-7.

Colvin, B.A. 1986. Barn owls: Their secrets and habits. Illinois Audubon, No. 216, Spring 1986.

Cunningham, J.D. 1960. Food habits of the horned and barn owls. The Condor 62:222.

Evans, F.C. and Emlen, J.T., Jr. 1947. Ecological notes on the prey selected by a barn owl. The Condor 49:3-9.

Fitch, H.S. 194 7. Predation by owls in the Sierran foothills of California. The Condor 49:137-151.

Foster, G.L. 1927. A note on the dietary habits of the barn owl. The Condor 29:246.

Hall, E.R. 1927. The barn owl in its relation to the rodent population at Berkeley, CA. The Condor 29:274-275.

Hawbecker, A.C. 1945. Food habits of the barn owl. The Condor 47:161-166.

Jameson, E.W., Jr. and H.J. Peeters. 1988. California Mammals. University of California Press, Berkeley, CA.

Rudolph, S.G. 1978. Predation ecology of coexisting great horned and barn owls. The Wilson Bulletin 90(l):134-37.

Smith, C.E and C.L. Hopkins. 1937. Notes on the barn owls of the San Francisco Bay region. The Condor 39:189-191.

For more information write to: Chuck Ingels, UC SAREP, University of California, Davis, CA 95616.

(CI-PEST.137) Contributed by Chuck lngels