sign in sign up
<<
Home , Cyprinidae, Heteromyidae, Leporidae, Mountain cottontail, Perognathus

Western North American Naturalist

Volume 63 Number 2 Article 13

4-30-2003

Food habits of Great Horned in northeastern California with notes on seasonal diet shifts

Raymond J. Bogiatto California State University, Chico

Brian A. Sardella California State University, Chico

Jennifer J. Essex California State University, Chico

Follow this and additional works at: https://scholarsarchive.byu.edu/wnan

Recommended Citation Bogiatto, Raymond J.; Sardella, Brian A.; and Essex, Jennifer J. (2003) "Food habits of Great Horned Owls in northeastern California with notes on seasonal diet shifts," Western North American Naturalist: Vol. 63 : No. 2 , Article 13. Available at: https://scholarsarchive.byu.edu/wnan/vol63/iss2/13

This Note is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Western North American Naturalist 63(2), ©2003, pp. 258-263

FOOD HABITS OF GREAT HORNED OWLS IN NORTHEASTERN CALIFORNIA WITH NOTES ON SEASONAL DIET SHIFTS

Raymond J. Bogiatto!, Brian A. Sardella!, and Jennifer J. Essex!

Key tvOrd.s: Great Horned OwLs, Bubo virginianus,Jood habits, diet.

. The Great Horned (Bubo virginianus) into an open sagebrush-juniper woodland 0.2 is the most widely distributed owl in North km north of the collection sites, with a wet America, known to use a variety of wooded meadow and marsh located 0.8 km further to habitats ranging from to cities to the north. Collection sites were located 0.3 (Burton 1984, Johnsgard 1988). Although num­ and 0.8 km east ofthe lake. erous studies ofits foraging ecology have been We made 16 collections between 8 August completed in western (Fitch 1989 and 8 August 1996. Pine roost trees were 1947, Cunningham 1960, Seidensticker 1968, flagged and revisited throughout the study. All Marti 1974, Rudolph 1978, Weir and Hanson pellets were removed from beneath marked 1982, Knight and Jackman 1984, Hayward et trees dUring each visit, using only those pel­ al. 1993, Zimmerman et al. 1996, and others), lets (n = 106) for our seasonality data set. no published food habits data exist for this Materials from pellets collected during our from the Modoc Plateau region ofnorth­ initial visit and others collected from unmarked eastern California. Our data should provide areas at the ELBFS (n = 40) were included regional biologists, wildlife managers, and re­ within our annual diet data set (Table 1). search ecologists with a better understanding Seasons were partly defined by accessibil­ of food habits of this avian predator, both ity to our ELBFS study area during the initial annually and seasonally. field season. In addition, we attempted to stay We studied the diet of Great Horned Owls within temporal limits set by both the fall and at the Lake Biological Field Station spring equinoxes. Therefore, pellets deposited (ELBFS), located on the eastern shore of Eagle between 19 March and 22 September were Lake (40'N, 130'W), 25 km northwest ofSusan­ included in our spring-summer sample (n = ville, Lassen County, California. Eagle Lake, 56), and those deposited between 23 Septem­ Californias 2nd largest freshwater lake (surface ber and 18 March constituted our fall-winter area of 11,000 hal, lies on the Modoc Plateau, sample (n = 50). For standardization, collec­ east of the Cascades and north of the Sierra tion dates were included within the deposition Nevada. The ELBFS is owned by California period for those pellets collected, with subse­ State University, Chico (CSUe), and adminis­ quent deposition periods beginning the fol­ tered by CSUC and the University of Califor­ lowing day. nia Natural Reserve System. Upon collection, pellets were indiVidually We collected pellets from 2 known Great bagged and labeled prior to transport back to Horned Owl roost sites (separated by 0.5 km) our lab at CSue. We then dissected the pel­ within a of Jeffrey pine (Pinus jeffreyi) lets, removing all cranial and postcranial ele­ and western juniper (Juniperus occidentalis), ments as well as chitinous arthropod remains. with an open understory of widely scattered Osteological materials were identified using big sagebrush (), rubber comparative collections in the CSUC Verte­ rabbitbrush (Chrysothamnus nauseosus), and brate Museum (Department of Biological Sci­ gooseberry (Ribes spp.). This forest transitions ences) and Zooarchaeology Lab (Department

lDepartment ofBiological Scien~, D.Jifomia State Univenity, Chico, CA 95929-0515".

258 2003] NOTES 259 of Anthropology). Arthropod remains were (Dipodomys californicus), Great Basin pocket identified using the ELBFS comparative col­ (Perognatlw.s parous), and voles (Micro­ lection, as well as teaching collections of the tus spp.) suggests that Great Horned Owls, in primary author and of Dr. D.H. Kistner. We addition to their foraging activities in woodland attempted to identify all remains to the lowest environments, also hunt in more open (sage­ taxonomic level possible. Materials from each brush-juniper and meadow) environments. pellet were quantified as the minimum num­ Eared Grehes, hatch-year Aechmophorus ber of individuals (MNI) from each taxou uec­ grebes, which include hoth the Western (A. oed­ essary to account for the assemblage of bone dentalis) and Clark's Grehe (A. clarkii), as well and exoskeletal remains (White 1953). The as American Coots were the most common occurrence of large prey such as and avian prey throughout the year, occurring in in the diet of the Great Homed Owl has 17.8%, 9.6%, and 6.2% of the pellets, and been well documented (Fitch 1947, Marti accounting for 8.4%, 4.5%, and 2.9% of the 1974, Mclnvaille and Keith 1974, Zimmerman total number ofprey items, respectively (Table et al. 1996, and others). As a result, it has been 1). As grebes conduct their activities on the shown that one or more owls may forage on a water surface, under the water, or in over-water single carcass, often returning to an unfinished nests, it is clear that Great Horned Owls cap­ carcass during subsequent foraging efforts tured water birds from the surface or shore­ (Marti 1974). Ifthis is in fact the case at Eagle line of Eagle Lake. All unidentified large and Lake, then osteological remains ofany individ­ medium-sized bird remains were likely from ual prey item could occur in multiple pellets, Eared Grebes, coots, and ducks, although thus inflating the relative importance oflarger these remains lacked taxon-specific diagnostic prey taxa. As a result, we acknowledge that our features. numbers of individuals may be biased high for Large nocturnal arthropods snch as scorpi­ larger taxa such as rabbits and ducks. As most ons, Jerusalem crickets (Stenopelmatus sp.), unidentified materials in our sample were avian, and heetles (Coleoptera) were also common we used the following arbitrary size categories: prey items, collectively accounting for 26.6% , "large" refers to birds weighing >601 g; "med­ of the total annual prey item sample. Never­ iuni' refers to species ranging from 151 g to 600 theless, although we did not consider biomass g; and "smalf' refers to birds weighing <150 g. contribution in our study, the contribution of and birds were the most abun­ , relative to bird and dant and commonly occurring prey items in prey, would be quite low. Other less common the diet of Great Horned Owls throughout the prey in the annual diet included , amphi­ year at the ELBFS (Table 1). These findings bians, and reptiles, which occurred in 2.1%, are consistent with diet literature for this 4.8%, and 1.4% ofall pellets, comprising 0.9%, species (Fitch 1947, Korschgen and Stuart 2.6%, and 0.6% of all prey items, respectively. 1972, Marti 1974, Weir and Hanson 1989, As all identified amphibian and reptile taxa Zimmerman et al. 1996, Trejo and Grigera could have been taken from either a terrestrial 1998, and others). Woodland taxa such as or lakeshore context, they are not as useful as woodrats (Neotoma spp.), indicators of specific foraging hahitat type, rabbits (Sylvilagus nuttallii), and white-footed whereas the presence offish remains is unam­ mice (Peromyscus spp.) were the most com­ hignous. Bone and scale materials in our sample mon taxa in the diet, occurring in 30.1%,15.8%, were from adult-sized tui chuhs ( bieolor) and 13.0% ofall pellets (n = 146), and account­ and Tahoe suckers (Catostomus tahoensis). ing for 17.7%, 7.4%, and 10.9% of the total The most likely scenario is that carcasses number of prey items (n = 311), respectively. were scavenged from the shoreline or surface The most common local woodrat species, the of the lake. As prevailing winds at Eagle Lake dusky-footed woodrat (N. juscipes), likely typically blow from the southwest during the accounts for most if not all woodrat remains, spring and summer, chub, sucker, and water although we were unable to rule out the bushy­ bird carcasses are generally quite abundant tailed woorat (N. cinerea) with any confidence, along the shore near the ELBFS. However, due to similarities in their osteology. This was we cannot discount the possibility that live also the case with the white-footed mouse. fishes swimming near the surface are occa­ The occnrrence of the California sionally taken hy Great Horned Owls. 260 WESTERN NORTH AMERICAN NATURALIST [Volume 63

TABLE 1. Prey items identified from pellets collected at Eagle Lake, Lassen County, California, 1989-1996. Percent occurrence in Percent oftotal owl pellets (n ~ 146) prey items (n = 311) Prey taxa Number Percent Number Percent

INSECTS Stenopelmatidae Stenopelmatus sp. 21 14.4 65 20.9 Hydrophilidae Hydrophduo triangularis I 0.7 I 0.3 Ceramhycidae Prionus californica 2 1.4 2 0.6 Scarabaeidae Phyllophaga 'p. I 0.7 5 1.6 Unidentified Tenebrionidae 1 0.7 2 0.6 Unidentified Coleoptera I 0.7 3 1.0 Total insectsa 24 16.4 78 25.0 ARACHNIDS Scorpionida 4 2.7 5 1.6 Total arachnids 4 2.7 5 1.6 FISHES Cyprinidae (= Gila) bwolor I 0.7 1 0.3 Catostomidae Catostomus tahoensis 2 1.4 2 0.6 Total fishes 3 2.1 3 0.9 AMPHIBIANS Bufonidae Bufo boreas 6 4.1 7 2.3 Pelobatidae Scaphiopus intermontanus 1 0.7 I 0.3 Thtal amphibians 7 4.8 8 2.6 REPTILES Unidentified ColubridaelBoidae 2 1.4 2 0.6 Total reptiles 2 1.4 2 0.6 BIRDS Podicipedidae Podiceps nigricolUs 26 17.8 26 8.4 Aechmophorus spp,b 14 9.6 14 4.5 Unidentified Podicipedidae 3 2.1 3 1.0

Whereas the activity patterns of most prey Seasonal diet data are presented in Table 2. species identified are at least somewhat, ifnot Results show that fewer prey taxa were taken fully, nocturnal, the presence of diurnal taxa during fall and winter months at Eagle Lake, such as the California ground (Sper­ with a diet comprising primarily medium to mophUus beecheyi) and golden-mantled ground large waterbirds and woodrats (Table 2). Notice­ squirrel (S. lateralis; lugles 1965) suggests at ably absent during fall and winter are ecto­ least some crepuscular foraging by Great thermic and invertebrates. At an Horned Owls at Eagle Lake. This is consistent elevation of 1524 m, late fall and winter tem­ with the findings of others including Marti peratures often fall below -lOoC during the (1974), who observed that Great Horned Owls night at Eagle Lake. As a result, ectotherms in north central Colorado begin foraging "long are essentially unavailable as prey items at Eagle before full darkness," with average departure Lake from mid-November through March. times of 20 minutes after sunset during the Those arthropod prey items in our fall-winter spring. sample were found in pellets deposited 2003J NOTES 261

TABLE 1. Continued. Percent occurrence in Percent oftotal owl pellets (n = 146) prey items (n = 311) Prey taxa Nwnber Percent Number Percent

BIRDS (continued) Anatidae Aythya affini8 3 2.1 3 1.0 Unidentified Anatinae 4 2.7 4 1.3 Rallidae FuUca americana 9 6.2 9 2.9 Landae Lams sp. 1 0.7 1 0.3 Picidae Colaptes auratus 2 1.4 2 0.6 Unidentified Aves Oarge)c 1 0.7 1 0.3 Unidentified Aves (medium) 10 6.8 10 3.2 Unidentified Aves (small) 9 6.2 10 3.2 Total birds 75 51.4 83 26.7 MAMMALIA Thlpidae Scapanus latitnllmlS 1 0.7 1 0.3 Unidentified Chiroptera 1 0.7 1 0.3 Sylvilagw nuttalli 23 15.8 23 7.4 Sciuridae Spermophilus law..aJis 1 0.7 1 0.3 Spermophilt