Short-eared Owl Habitat Model
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Short-eared Owl, Asio flammeus
Use of Study Area Resources:
Reproduction (potential) and wintering: Holt and Leasure (1993) designated areas in the northern half of Maine as potential breeding habitat, and the remainder of the study area as potential wintering habitat. Melvin et al. (1989) portrayed coastal Maine and New Hampshire, and most of Massachusetts as wintering habitat, but Tate (1992) excluded Maine and New Hampshire from his map of the short-eared owl's range. These owls are known to breed just south of the study area on Nantucket and Tuckernuck Islands, south of Cape Cod, Massachusetts (Combs-Beattie 1993). They may also winter in suitable breeding habitat, and vice versa, depending on prey availability (Craighead and Craighead 1956, Clark 1975, Tate 1992). Base on the above, we regarded the near-coastal portions of the study area as having the greatest potential for use to the species, for reproduction, migration, and/or wintering.
Short-eared owls require broad expanses of open land with low vegetation, such as grasslands or low-structured open shrublands, for hunting and for nesting (Holt and Leasure 1993).
Breeding habitat includes areas used for foraging, roosting or nesting. In the northeast U.S., optimal breeding cover consists of grasslands, heath, saltmarsh, freshwater marsh, dunes or old pasture, so long as there is an abundant prey base and dry upland for the nest sites (Tate 1992). Combined statistics for 9 Manitoba nest sites (Clark 1975) and 28 Montana nest sites (Holt and Leasure 1993) showed 81% in grasses, 13% in herbs/herb-grass combination, and 6% in stubble. No nesting occurs in actively grazed areas (Kantrud and Higgins 1992 in Dechant et al. 1999). Nests consist of shallow scrapes on the ground, minimally lined with grasses or other vegetation, from which the young disperse within a few days of hatching (Clark 1975). Short-eared owls usually hunt on the wing, quartering fields or hovering, but trees, fence posts or other structural components may rarely be used as perches for hunting (Clark 1975)
Short-eared owls avoid developed areas; average distance of nests from the nearest buildings, on Nantucket Island (Massachusetts), was over 789 m (Combs-Beattie 1993). Minimum distance was 250 m. On the other hand, Bosakowski (1986) encountered owls roosting within 15 m of an unused parking lot at the edge of an extensive salt marsh.
Short-eared owls are area sensitive. Grassland areas of 50 ha or larger offer greatest potential as breeding or wintering habitat (Tate 1992, Dechant et al. 1999). Patches as small as 8 ha may be used in winter, as compared to the 28 ha minimum nesting habitat area observed in Illinois (Herkert et al. in Dechant et al. 1999).
Wintering habitat for short-eared owls is similar to breeding habitat. Short-eared owls typically roost on the ground, moving to adjacent woodlands with the arrival of significant snowfall (Craighead and Craighead 1956, Clark 1975, Holt and Leasure 1993, Bosakowski 1986). Bosakowski (1986) noted that as little as 5 cm of snow caused owls to move roosts to nearby conifers. Heavy snowfall may cause short-eared owls to abandon winter roost sites (Smith 1989 in Tate 1992).
Suitable wintering/foraging and potential reproductive habitats were identified by selecting areas within the known range of the owl and having appropriate cover types, distance from development, and patch size. Habitat was mapped only within the winter range as depicted in Melvin et al. (1989) and as described in Tate (1992). This range was replicated by selecting the winter snowfall depth boundary most closely matching the owl's winter range. Based on the average February snow depth for 1980 through 2000, a depth of 8" or less was found to correspond closely with the published accounts. This approach seems justified since excessive snowdepth is likely to impede foraging by owls.
Cover suitability: Foraging/wintering/reproduction cover first was scored as shown in the following table. Some types (e.g., cultivated land) are relatively dynamic, and their scoring represents an "averaged" value (conditions ranging from planting, growth, harvest, and residue of crops). The upland scrub/shrub category of our landcover is actually dominated by clearcut and early regenerating forest, and so was not considered to be suitable as foraging or nesting habitat. However, shrub/scrub and other suitable cover types within 30 m of the wintering/foraging/reproductive sites were mapped as supplementary winter roosting and perching areas. These supplementary cover types were given the same habitat value as the foraging habitat they abut. We were not able to obtain data confirming breeding occurrences within the study area. Therefore, the maximum habitat score within the study area was 0.7 on a 0 to 1.0 scale.
|Cover Types||Cover Suitability
(0 - 1 scale)
|Upland deciduous forest||
|Upland coniferous forest||*|
|Upland mixed forest||*|
|Upland shrub, regenerating forest||*|
|Cultivated (assumed to be fallow with stubble part of the time)||0.4**|
|PEM, L2EM||Lake/pond, emergent vegetation||0.3**|
|PFOcon||Palustrine forest, conifer||*|
|PFOdec||Palustrine forest, deciduous||*|
|PSSdec||Palustrine scrub shrub, deciduous||*|
|PSScon||Palustrine scrub shrub, conifer||*|
|PAB, L2AB||Lake/pond, aquatic vegetation|
|L1UB, PUB||Lake/pond, unconsolidated bottom|
|L2US||Lake, unconsolidated shore|
|L2RS||Lake, rocky shore|
|R1UB||Riverine subtidal unconsolidated|
|E1AB||Estuarine subtidal vegetated|
|E1UB||Estuarine subtidal unconsolidated bottom|
|E2AB||Estuarine intertidal algae|
|E2EM||Estuarine intertidal emergent (high salt marsh only)||0.4**|
|E2RS, R1RS||Estuarine, tidal river rocky shore|
|E2SS||Estuarine intertidal shrub||*|
|E2US||Estuarine intertidal unconsolidated shore|
|M1AB||Marine subtidal vegetated|
|M1UB||Marine subtidal unconsolidated bottom|
|M2AB||Marine intertidal algae|
|M2RS||Marine intertidal rocky shore|
|M2US||Marine intertidal unconsolidated shore|
|NOTES||* supplementary roost cover: scored the same as the final habitat suitability
for the foraging cover to which it is adjacent.
** foraging/wintering/reproduction cover
Disturbance suitability: Habitat was regarded suitable (1.0) only if it was more than 90 m from development, otherwise it was scored 0.
Area suitability: The minimum suitable patch size for wintering/foraging was 8 ha; that for reproduction was 28 ha. Based on Tate (1992) we scored sites over 50 ha as 1.0; sites over 28 ha were scored 0.5, and sites smaller than this, but over 8 ha were scored 0.3. Smaller sites were scored 0.
Habitat Suitability = (cover suitability x disturbance suitability) x area suitability
Bosakowski, T. 1986. Short-eared owl winter roosting strategies. American Birds 40(2):237-240.
Clark, R.J. 1975. A field study of the short-eared owl, Asio flammeus (Pontoppidan), in North America. Wildl. Monogr. 47:1-67.
Combs-Beattie, K. 1993. M.S. Thesis: Ecology, Habitat Use Patterns and Management Needs of Short-eared Owls and Northern Harriers on Nantucket Island, Massachusetts. University of Massachusetts - Amherst, Dept of Forestry and Wildlife Management.
Craighead, J.J. and F.C. Craighead, Jr. 1956. Hawks, Owls and Wildlife. Dover Publications, Inc., NY. pp 93-98.
Holt, D.W. and S.M. Leasure. 1993. Short-eared Owl. In The Birds of North America, No. 62 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, and The American Ornithologists' Union, Washington, DC.
Dechant, J.A., M.L. Sondreal, D.H. Johnson, L.D. Igl, C.M. Goldade, M.P. Nenneman and B.R.Euliss. 1999. Effects of management practices on grassland birds: short-eared owl. Northern Prairie Wildlife Research Center, Jamestown, ND. Northern Prairie Wildlife Research Center Home Page: http://www.npwrc.usgs.gov/resource/literatr/grasbird/shortear/shortear.htm (Version 19 Feb 1999).
Melvin, S.M., D.G. Smith, D.W. Holt, and G.R. Tate. 1989. Small owls, pp. 88-96. In B.G. Pendleton, M.N. LeFranc Jr., M.B. Moss, C.E. Ruibal, M.A. Knighton and D.L. Krahe [eds.] . Proceedings of the northeast raptor management symposium and workshop. National Wildlife Federation, Washington, DC. May 16-16, 1988, Syracuse, NY. (Scientific and Technical Series; no.13)
Tate, G.R. 1992. Short-eared Owl, Asio flammeus, in Schneider and Pence (eds) Migratory Nongame Birds of Management Concern in the Northeast. USFWS, R5, Hadley, MA, p. 171-189.