22 January 2013

Analysis of Winter Long-eared Owl Pellets

The Long-eared owl (Asio otus) is a permanent resident of Nebraska, but is seldom seen due to its nocturnal habits. Days are spent roosting in dense wooded cover with the Owls venturing out at night to hunt. Typically, for roosting and nesting Long-eared owls select woodlands adjacent to open grassy fields in which they hunt (Getz 1961). Although dispersed during the breeding season, these Owls often form wintering groups which utilize communal roosts.

With a large number of birds using an area continuously for a short period, or with a small number of owls using an area for an extended period, regurgitated pellets containing prey remains are concentrated at one location. The remains in the pellets provide a representative sample of prey consumed by the owls. Pellets were collected at two different sites utilized by wintering Long-eared Owls near Lincoln, Lancaster County, Nebraska.

Site 1 was located on private land southwest of Lincoln. Sixteen Long-eared Owls had been observed at this site during the 1980 Lincoln Christmas Bird Count (Cindy Cochran pers. comm.). On 13 February 1981 site 1 was visited to assess the roost habitat and to collect pellets. Six to 8 Long-eared Owls were flushed when they were approached to within 3 meters (m). The Owls were hesitant to fly when disturbed. This delay in flight provided an opportunity to note that a Short-eared Owl (A. flammeus) was also in the roost area. The group of Owls flew only a short distance to the east and landed back in the shelterbelt.

The roost of these Owls was a shelterbelt planted to Eastern Red Cedar (Juniperus virginianus). The two hectare (ha) shelterbelt is oriented east to west and is 50 m wide and 400 m long. It is comprised of 12 rows of planted cedars with an average diameter at breast height of 25 to 30 centimeters. Vegetation noted around the shelterbelt is deciduous trees such as Cottonwood (Populus deltoides), and a pasture consisting primarily of a cool season grass, Smooth Brome (Bromus inermis), and scattered cedar trees.

Site 2 was located 18 kilometers northwest of Lincoln at Branched Oak Wildlife Management Area. A pair of Long-eared Owls had been recorded as year-round residents in the area (Richard Manning pers. comm.). The apparent nesting and roost site is located within a 1 ha planted cedar grove 10 to 15 rows wide and 20 to 25 trees deep. The branches of the trees within the grove are dead to height of about 1.7 to 2.5 m which makes access and movement within the grove difficult. Immediately surrounding the wooded area is a planting of Smooth Sumac (Rhus glabra), several Ponderosa Pine (Pinus ponderosa), poplars (Populus sp.), brushy plants including Wild Plum (Prunus sp.) and Rose (Rosa sp.), and large areas of brome. East of the grove is another brushy area, 75-100 m long, comprised of typical plant species invading a grassland. The grove is located within 150 m of Branched Oak Lake.

Pellet Analysis

Pellets were collected from site 1 on 13 February 1981 and site 2 on 14 March 1981. All pellet material observed, including complete pellets and weathered incomplete pellets and other debris, was collected. The pellets were then carefully picked apart and the contents identified. Mammal remains were identified with the aid of a cranium key (Glass 1951) and by comparison with representative skulls in the mammal collection at the University of Nebraska-Omaha (UNO). Bird remains were identified by skull remnants and by complete or partially complete bills. Each individual cranium/skull and pair of dentary bones were considered as one occurrence of the prey animal. The dentary bones were paired with a similar species skull and were not counted twice. Remaining skulls without corresponding dentary bones and paired dentary bones were each considered as an occurrence of the prey animal.

Prey item biomass was determined by utilizing an average body weight for the species recorded. Average body weights from mammals are from Schwartz (1981). Specific avian masses are from Amadon (1943) and other sources. Representative weight for a group of birds, i.e. Fringillidae, is from Marti (1974).

Results and Discussion

A total of 465 pellets and 236 incomplete pellets or debris pieces were collected from the two locations. Pellet contents included at least 10 mammal and 3 bird species (Table 1). Voles, genus Microtus, were the most common prey item that occurred and comprised almost half of the total biomass consumed (Table 2).

Table 1. Vertebrate remains in Long-eared Owl pellets from two sites near Lincoln, Lancaster County, Nebraska
  Prey Items Total Biomass
  Number Percent Grams Percent
Species Site 1 Site 2 Site 1 Site 2 Site 1 Site 2 Site 1 Site 2
Least Shrew (Cryptotis parva) 6 10 2.2 2.5 21 35 T T
Short-tailed Shrew (Blarina brevicauda) - 17 - 4.3 - 357 - 3
White-footed/Deer Mouse (Peromyscus spp.) (a) 83 76 29.9 19.2 1,577 1,444 21 12
Meadow Vole (Microtus pennsylvanicus) 65 167 23.4 42.2 2,730 7,014 36 56
Prairie Vole (Microtus ochrogaster) 43 26 15.5 6.6 1,655 1,001 22 8
Vole spp. (Meadow or Prairie, #2 upper cheek tooth missing, so not determinate) - 11 - 2.8 - 456 - 4
Harvest Mouse (Rethrodontomys spp.) (b) 75 67 27.0 16.9 1,425 737 19 6
House Mouse (Mus musculus) 4 - 1.4 - 84 - 1 -
Eastern Cottontail (Sylvilagus floridanus) - 1 - 0.2 - 750* - 1
House Sparrow (Passer domesticus) - 4 - 1.0 - 100 - T
Red-winged Blackbird (Agelaius phoeniceus) - 5 - 1.3 - 275 - 2
Cardinal (Cardinalis cardinalis) 1 - 0.3 - 44 - 1 -
Unidentified Passeriformes (c) 1 4 0.3 1.0 30 120 T 1
Unidentified Fringillidae (c) - 8 - 2.0 - 240 - 2
  278 396 100.0 100.0 7,566 12,529 100 100

(a) Either White-footed (P. leucopus) or Deer (P. maniculatus) Mouse, extreme fragmentation prohibits specific identification.

(b) Probably Western (R. megalotus) as Plains (R. montanus) Harvest Mouse is locally uncommon. Skulls greatly fragmented.

(c) Identified by bill remains only.

* Average mass x .75 = consumed biomass (Grear and Gilstrap 1970).

T Trace amounts of less than 1%.

Pellets collected from site 2, Branched Oak Lake, contained 4 species not recorded at site 1. These were the Short-tailed Shrew (Blarina brevicauda), Eastern Cottontail (Sylvilagus floridanus), House Sparrow (Passer domesticus), and Red-winged Blackbird (Agelaius phoeniceus). Pellets from the cedar shelterbelt/pasture area of site 1 contained the House Mouse (Mus musculus) and Northern Cardinal (Cardinalis cardinalis) which were not recorded from the lake site. The average mass of a prey item was 29.8 grams. This weight most closely approximates the mass of the small rodents which were the most frequently consumed prey items found in the pellets. (Tables 1 through 3 provide a complete analysis of prey composition and frequency).

Table 2. Combined contents of Long-eared Owl pellets from two sites near Lincoln.
Prey Group Prey Items Biomass
  Number Percent Grams Percent
Least Shrew (Cryptotis parva) 16 2.4 56 0.3
Short-tailed Shrew (Blarina brevicauda) 17 2.5 357 1.8
White-footed/Deer Mouse (Peromyscus spp.) 159 23.6 3,021 15.0
Voles (Microtus spp.) 312 46.3 12,856 64.0
Harvest Mouse (Reithrodontomys spp.) 142 21.1 2,162 10.8
House Mouse (Mus musculus) 4 0.6 84 0.4
Eastern Cottontail (Sylvilagus floridanus) 1 0.1 750 3.7
Birds 23 3.4 809 4.0
  674 100.0 20,095 100.0

Table 3. Number of prey items (i.e., individual animals) recorded per pellet analyzed, from Long-eared Owls from two sites near Lincoln.
Items per Pellet Number of Pellets Percent
  Site 1 Site 2 Total  
0 0 43 43 9.3
1 156 186 342 73.5
2 37 28 65 14.0
3 5 3 8 1.7
4 2 4 6 1.3
5 0 1 1 0.2
  200 265 465 100.0

A comparison of the pellet contents of Long-eared Owls from several states is given in Table 4. Data from Michigan (Geis 1952), Minnesota (Christenson and Fuller 1975), Indiana (Kirkpatrick and Conway 1947), Illinois (Cahn and Kemp 1930, Birkenholtz 1958) and Ohio (Randle and Austing 1952) are given for comparative purposes. Although results obviously vary from location to location, an interesting difference of the Nebraska data presented is the relatively high , combined percentage of White-footed/Deer Mouse spp. and Harvest Mouse spp. Only Illinois had a higher percent composition of White-footed/Deer Mouse spp. Harvest Mice occurrence in Nebraska was almost 20 percentage points higher, in relation to total prey consumed, than values for the two other sites where they were recorded. The percentage of total avian prey items is notably consistent around 2-3% for all of the studies cited.

Table 4. A comparison of contents of Long-eared Owl pellets from six states, in percentage of prey items.
Prey Group Michigan Minnesota Indiana Illinois* A Illinois* B Ohio* Nebraska*
Least Shrew (Cryptotis parva) - - 7.6 - 20.8 10.4 2.4
Short-tailed Shrew (Blarina brevicauda) 1.4 4.4 1.0 3.5 - 10.2 2.5
White-footed/Deer Mouse (Peromyscus spp.) 10.0 7.7 10.3 40.7 13.1 5.3 23.5
Voles (Microtus spp.) 84.3 81.3 75.9 29.2 47.8 62.9 46.3
Harvest Mouse (Reithrodontomys spp.) - 2.2 - - - 0.5 21.2
House Mouse (Mus musculus) - - - 23.0 11.1 3.2 .6
Birds 2.6 3.3 2.8 2.7 1.5 3.3 3.4
Other 1.7 1.1 2.4 0.9 5.7 4.2 0.1
  100.0 100.0 100.0 100.0 100.0 100.0 100.0

* Pellets were collected from wintering birds

A Cahn and Kemp 1930; B Birkenholtz 1958.

Other variations in pellet contents represent many possible environmental differences. This could include differences in individual owl prey selection, prey type occurrence and density, and the physical characteristics of the hunting territory. Additional factors could include the presence of competing predatory birds and mammals which might affect the Owls behavior or possibly affect prey composition in the diet.

The results of this study indicate that Long-eared Owls appear to be indiscriminate feeders that prey on a diverse group of suitably sized animals that are available in the area of the roost.

Literature Cited:

Amadon, D. 1943. Bird weights and egg weights. Auk 60: 221-234.

Birkenholtz , D. 1958. Notes on a wintering flock of long-eared owls. Illinois Academy of Science Transactions 51: 83-86.

Cahn, A.R. and J.R. Kemp, 1930. On the food of certain owls in east-central Illinois. Auk 47: 323-328.

Christenson, G. and M.R. Fuller. 1975. Food habits of two long-eared owl families in east-central Minnesota. Loon 47: 58-61.

Geis, A.D. 1952. Winter food habits of a pair of long-eared owls. Jack-Pine Warbler 30: 93.

Getz, L.L. 1961.Hunting areas of the long-eared owl. Wilson Bulletin 73: 79-82.

Glass, B.P. 1951. A key to the skulls of North American mammals. Oklahoma University. Stillwater, Oklahoma. 53 pp.

Greer, J.K. and R.L. Gilstrap. 1970. Vertebrate remains in barn owl pellets. Bulletin of the Oklahoma Ornithological Society 3: 25-29.

Kirkpatrick, C.M. and C.H. Conaway. 1947. The winter food of some Indiana owls. American Midland Naturalist 38: 755-766.

Marti, C.D. 1974. Feeding ecology of four sympatric owls. Condor 76: 45-61.

Randle, W. and R. Austing. 1952. Ecological notes on long-eared and saw-whet owls in southwestern Ohio. Ecology 33: 422-426.

Schwartz, C.W. and E.R. Schwartz. 1981. The wild mammals of Missouri. University of Missouri Press, Columbia. 356 pp.

Jim Ducey and John Kirby. September 1983. An analysis of winter long-eared owl pellets from Lancaster County, Nebraska. Nebraska Bird Review 51(3): 79-82.