SEASONAL VARIATION AND DIET SELECTION FROM PELLET REMAINS OF SHORT-EARED OWLS (ASIO FLAMMEUS) IN WYOMING PDF
Preview SEASONAL VARIATION AND DIET SELECTION FROM PELLET REMAINS OF SHORT-EARED OWLS (ASIO FLAMMEUS) IN WYOMING
GreatBasinNaturalist54(2), ©1994,pp. 191-192 SEASONAL VARIATION AND DIET SELECTION FROM PELLET REMAINS OF SHORT-EARED OWLS {ASIO FLAMMEUS) IN WYOMING Eric StonedJocelyn Smith^-, and PollyThornton^ Keywords:Short-earedOtvh, Asioflammeus,dietselection,predators, Wyoming. Short-eared Owls {Asio flammeus) are was located within 20 m of the ditch and medium-sized predators ofopen country, sage another within 2 km. We observed as many as flats, grasslands, and roadsides. Often active four owls roosting along the ditch, either on well after sunrise, they are more diurnal than the ground, in the shade oftrees, or perched other owls in northwest Wyoming (Karulus on the lowerbranches. and Eckert 1974, Clark 1985). Their foraging At the end ofeach month (March-October) areas significantly overlap those ofboth small- all pellets were collected at the study site. er and larger owls, namely Great Horned Thus, each group ofpellets collected and their Owls {Bubo virginianus) and Burrowing Owls contents could be assumed to have originated {Speotyto cunicularia) (E. Stone unpublished during that month. Short-eared Owls were no data, Karulus and Eckert 1974). Prey sources, longer seen in the study area in late October including small mammals, birds, and insects, and were presumed to have migrated to areas are diverse and overlap those used by Great with ample winter prey, shallower snowjjack, Horned Owls. or both. Owls were first seen using the roost This study examines shifts in prey sources site in early March. To assure large enough through the breeding season by identifying sample sizes, we combined sample months prey remains in Short-eared Owl pellets from into the following seasonal groups: spring wild birds. Shifts in prey sources may be the (March, April, and May), summer (June and result ofchanges in prey abundance or avail- July), and fall (August, September, and Octo- ability, or competition with other owl species ber). for the same resources. Additionally, shifts Prey items were identified using skaill and may result from changes in dietary require- teeth parts found in individual pellets. Pellet ments ofadults or their developing dependent remains ofMicrotus montanus and M. longi- offspring. In this study we sought to describe caudus were not distinguishable by skull or whether shifts in diet occurred and, ifso, what teeth parts and were combined into a prey types. This descriptive study may serve as a category hereafterreferred to asM. mont-long. useful baseline of data upon which future studies canbebased. Results Methods and Study Area Short-eared Owl pellets contained 11 dif- ferent prey items. Of these, the 6 most com- The Short-eared Owl study areawas an old monprey types constituted 94.42% ofthe diet. irrigation ditch located approximately 2.2 km A significant decline in sage voles {Lagurus southwest of the Teton Science School in curtatus) in the fall diet of Short-eared Owls Grand Teton National Park, Wyoming. Short- was augmented by an increase in the propor- eared Owl pellets were located by searching tions of northern pocket gophers {Thijmomys on the ground and at the base ofwillows {Salix talpoides) and southern red-backed voles sp.), mountain alder {Alnus tenuifolio), aspen {Clethhonomijs gapped) (Table 1). The com- {Populus tremuloides), and narrowleaf cotton- plete disappearance ofL. curtatus in fall, with wood {Populus angustifolia). One active nest increases in both the number and proportion 'TetonScienceSchool,Box68,Kelly,Wyoming83011. ^Presentaddress:454Route32North,NewPaltz,NewYork12561. 191 192 Great Basin Naturalist [Volume 54 Table 1.Seasonalpercentagesofpreyitemsfoinidin Short-earedOwlpellets Preytype
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