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Montana Field Guide

Montana Field Guides

Milbert's Tortoiseshell - Aglais milberti

Native Species

Global Rank: G5
State Rank: S5

Agency Status


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General Description
[From Ferris and Brown 1981; Scott 1986; Opler and Wright 1999; Glassberg 2001; Pyle 2002] Forewing 2.0-2.8 cm. Tip of forewing squared-off; upperside dark with a wide orange submarginal band grading inward to yellow, forewing cells with two orange to red-orange bars; a narrow black marginal border on both wings upper surface, the hindwing border may contain blue spots; undersurface dark striated brown with paler submarginal band.

One flight in the far north and higher elevations, mostly late July overwintering to June; two flights at lower elevation and in the east, late June to early August and late August overwintering to May (Scott 1986). Mid January to early October in Oregon and Washington with peaks in April to June and July to August (Pyle 2002);

Diagnostic Characteristics
Best determined by a combination of the outer margins irregular with a short tail-like hindwing projection, uppersurface outer third orange grading inward to yellow-orange, a narrow black marginal border on hindwing sometimes including blue spots; undersurface of wings with a pale submarginal band.

Species Range
Montana Range Range Descriptions


Range Comments
Boreal North America, from southern Alaska east to Newfoundland and West Virginia, south in the west to California, Nevada, Arizona, and New Mexico; migrants in the east infrequently appear south to Arkansas and Georgia (Scott 1986; Opler and Wright 1999; Glassberg 2001; Pyle 2002); to at least 3990 m elevation in California (Garth and Tilden 1963), 4206 m elevation in Colorado (Scott and Scott 1978). In Montana, reported throughout the state (Kohler 1980; Stanford and Opler 1993). Mainly rare to uncommon, but common in the Pacific Northwest (Glassberg 2001).

Observations in Montana Natural Heritage Program Database
Number of Observations: 250

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Semi-migratory; appears to make seasonal elevational movements in the western US mountains, flying upslope in summer, downslope in autumn (Shapiro 1974, 1979; Scott 1986, 1992; James and Nunnallee 2011). In areas of alpine karst, this species may overwinter in caves (Taylor et al. 2009).

Montane wet meadows, moist streamsides, riparian areas, open woodlands, parks, gardens, above treeline in alpine habitats (Ferris and Brown 1981; Opler and Wright 1999; Glassberg 2001; Pyle 2002). In Glacier National Park, Montana reported from mesic montane meadows and above treeline in alpine habitat (Debinski 1993).

Ecological Systems Associated with this Species
  • Details on Creation and Suggested Uses and Limitations
    How Associations Were Made
    We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
    1. Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
    2. Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
    3. Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
    4. Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
    Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.  In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.  However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system, even if point observations were associated with that system.  Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.  The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.  If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.

    Suggested Uses and Limitations
    Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.  These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at: or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.  Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.  Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.  Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).  Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.

    Literature Cited
    • Adams, R.A.  2003.  Bats of the Rocky Mountain West; natural history, ecology, and conservation.  Boulder, CO: University Press of Colorado.  289 p.
    • Dobkin, D. S.  1992.  Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34.  Missoula, MT.
    • Foresman, K.R.  2012.  Mammals of Montana.  Second edition.  Mountain Press Publishing, Missoula, Montana.  429 pp.
    • Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998.  Montana atlas of terrestrial vertebrates.  Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT.  1302 p.
    • Hutto, R.L. and J.S. Young.  1999.  Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32.  72 p.
    • Maxell, B.A.  2000.  Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species.  Report to U.S. Forest Service Region 1.  Missoula, MT: Wildlife Biology Program, University of Montana.  161 p.
    • Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath.  2004.  Amphibians and reptiles of Montana.  Missoula, MT: Mountain Press Publishing Company. 262 p.

Food Habits
Larval food plants include Urtica in particular (several species), rarely Helianthus, Laportea, Ribes, Salix (Ferris and Brown 1981; Scott 1986, 1992, 2006; Pyle 2002). Adults feed on flower nectar (including Achillea, Allium, Anaphalis, Apocynum, Arctostaphylos, Arnica, Barbarea, Ceanothus, Chrysothamnus, Cirsium, Crypthantha, Erigeron, Erioganum, Erysimum, Helianthella, Heterotheca, Jamesia, Medicago, Monarda, Monardella, Phlox, Prunus, Salix, Sedum, Senecio, Solidago, Taraxacum), sap, fermetted fruit, and mud (Ferris and Brown 1981; Scott 1986, 2014; Pyle 2002; James and Nunnallee 2011).

Reproductive Characteristics
Females lay eggs in clusters (as many as 713) on the undersides of host plant leaves. Eggs hatch in about 5-7 days (depending on temperature), with pupation occurring 21 days after egg-hatch; 4, 3, 4, 3, and 7 days spent in L1-L5 instars, respectively (depending on temperature). L5 instars pupate away from host plant, adults eclose (emerge) from pupae in 7 days. Larvae gregarious when young, live in a silk nest atop host plant. Older larvae (L4-L5) usually solitary, may live in rolled-up leaves tied with silk; L5 may leave nests and feed openly. Overwinter (hibernate) as pupae or adults (Ferris and Brown 1981; Scott 1986, 1992, 2006; Guppy and Shepard 2001; James and Nunnallee 2011). Spring males perch behind shrubs and on logs mostly in the afternoon and usually on rocky places below hilltops, sometimes on banks where no hilltops are present, to await passing females; hill-topping behavior not noted in autumn (overwintering) males (Scott 1975b, 1982, 1986).

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Citation for data on this website:
Milbert's Tortoiseshell — Aglais milberti.  Montana Field Guide.  .  Retrieved on , from