j RaptorRes'.35(4):331-336
¸ 2001 The Raptor Research Foundation, Inc.
STATUS
AND
ATTEMPTED
REINTRODUCTION
OWLS IN MINNESOTA,
OF BURROWING
U.S.A.
MARK S. MARTELL 1
TheRaptorCenterat the University
of Minnesota,1920 FitchAvenue,St. Paul, MN 55108 U.&A.
JOHN SCHLADWEILER
Minnesota
Department
ofNaturalResources,
Box 756, New Ulm,MN56073 U.S.A.
FRANCESCA CUTHBERT
Department
ofFisheries
and Wildlife,University
of Minnesota,St. Paul, MN 55108 U.S.A.
ABsTRaCT.--BurrowingOwls (Athenecunicularia)have been recorded nesting in most of Minnesota's
westerncounties. Consideredcommon in the early 1920s,by the mid-1960sonly 9-10 breeding pairs
were known with estimatesof no more than 20 pairs in the west-centralpart of the state.Ten breeding
records exist for the period 1965-85. In 1984, Burrowing Owls were listed as Endangered by the State
of Minnesota. In 1986, we began surveysand site management for nesting Burrowing Owls and experimented with a reintroduction program. From 1986-90, 13 nestswere found at eight sites,with a mean
reproductive successof 3.5 fledglings/pain The maximum number of breeding pairs/yr wasfoun Nest
burrowswerefound in alfalfafields(37.5%), pastures(37.5%), roadsideditches(12.5%), andfencelines
betweenrow crop fields (12.5%). We released105 wild, preflightedjuveniles:nine in 1986, 18 in 1987,
21 in 1988, 27 in 1989, and 30 in 1990.Youngowlswere kept in hack penswith roofsand sidesmade
from cotton mesh fish netting. Burrowsinside each pen and in surroundingfieldswere availableto the
owls.Crippledadultswereplacedin eachpen with thejuvenilesbut werenot released.We documented
eight mortalities, all of which were fledglings recovered in the release area. No owlswere found, or
reported, after leavingtheir hack sites.No successful
nestingsoccurredfrom 1992-98.
KEYWOP,
DS: BurrowingOwl;Athene cunicularia;reintroduction;
status;endangered
species;
Minnesota.
Estadoy reintrocucci6nfallida de BfihosCavadoresen Minnesota,U.S.A.
RUSUMEN.---Los
Bfihos Cavadores(Athenecunicularia)han sido registradosanidando en la mayoria de
condadosdel occidentede Minnesota.Consideradocomfin a principiosde 1920, para la mitad de los 60's
finicamentese conocian9-10 parejasreproductorascon un estimativode no masde 20 parejasen la parte
oeste-centraldel estado.Diez registrosde reproducci6nexistianpara el periodo 1965-85. En 1984, los
BfihosCavadoresfueron puestosen la listade especiesen peligro para el estadode Minnesota.En 1986,
nosotrosiniciamosprospecciones,
el manejo de un sifiopara anidaci6nde BfihosCavadoresy experimentamoscon un programade reintroducci6n.De 1986-90, 13 nidosfueron encontradosen ochosiftos,con
una media en el 6xito reproductivode 3.5 volantonespor pareja. E1m•ximo numero de parejasreproductoras/afio fue cuatro.Las cuevasnido fueron encontradasen camposde alfalfa (37.5%), pastos(37.5%),
zanjasde carreteras(12.5%) y lineasde cercasentre lasfilasde loscamposde cultivo(12.5%). Nosotros
liberamos105juvenilespreviamenteadiestrados
para volar:nueveen 1986, 18 en 1987, 21 en 1988, 27
en 1989,y 30 en 1990. Losj6venesbfihospermanecieronen encierrosde caballoscon los techosy los
ladoscubiertoscon mallasde pescarhechasde algod6n. Estuvierondisponsiblespara los bfihos cuevas
dentro de cadacorralyen los camposcircundantes.
Los adultoslisiadosfueron colocadosen cadacorral
con los juveniles pero no se liberaron. Documentamosocho muertes, cada una de las cualesfueron
volantonesrecuperadosen el firea de liberaci6n. Ningfin bfiho fue encontrado,o reportado, despu6sde
abandonarsussitiosde encierro. No ocurri6 ninguna nidada exitosade 1992 a 1998.
[Traducci6n de Victor Vanegasy C•sar Mfirquez]
Minnesota's
western counties are at the eastern
breeding range in North America, excluding the
edge of the Burrowing Owl's (Athenecunicularia) disjnnct Florida population (A. c. fioridana, Ilaug
et al. 1993). The specieswasfirst recorded in MinE-mail address: [email protected]
nesotain July 1881, and there are historicalnesting
331
332
STATUSAND TRENDS
t
_
Vot.. 35, No. 4
[
F•gure1. Minnesota counties with historical 1881-1986,horizontalhatch)and modern(1987-91,verticalhatch)
BurrowingOwl nestingrecords.Counties:1 = Traverse,2 = Big Stone,3 = Lac Qui Parle,4 = YellowMedicine,5
= Lincoln, 6 = Pipestone,7 = Rock,8 = Grant, 9 = Stevens,10 = Swift,11 = Chippewa.
records
tbr
most
of the
state's
western
cormties
(Fig. 1). In the early 1920sBurrowing Owls were
thought to nest commonlythroughout Grant, Traverse,Pipestone,Lincoln, and Lac Qui Parle countms (counties8, 1, 6, 5, g; Fig. 1), aswell asfurther
norlh in the Red River Valley (Roberts 1932).
In tire mid-1960s, (;ran! (1965) reported 9-10
breeding pairs in Stevensand Traverse cormties,
and estimated no more than 20 pairs of owlsin the
five-county area that included Stevens,Traverse,
Grant, Big Stone, and Swift (counties9, 1, 8, 2, 10;
Fig. 1). Grant (1965) suggestedthe BurrowingOwl
was no longer common in that part of the state.
We fbund only 10 Minnesotabreeding recordsfor
the 20-yr period from 1965-85 (Martell 1990). The
specieswasstate-listedasEndangeredin 1984 (Coffin and Pfannmuller 1988).
To develop management strategies to recover
the speciesh-om its endangeredstatus,we began
to test reintroduction techniques (Martell 1990)
and to surveyfor nesting Burrowing Owls in Minnesota in 1986. This paper summarizesmethods
and results of the reintroduction,
as well as habitat
use, population status,and reproductiveperfor'mance of Burrowing Owls in Minnesota from
1986-98.
METHODS
Monitoring of Wild Population.To locatenestingBurrowing Owls,we solicitedinformationfYomthe pubhc,
conducted surveysin suitablehabitat, and searchedh•storical nest sitesduring the springand summerof 198690. Lessrigorousmonitoring and public contactcontinued from
1991-98.
In 1986, bird clubs and conservation organizations
were contacted, and television,radio, and newspapertoterviewswere used to increasepublic awarenessand encourage reporting of Burrowing Owls seen in the state
DECEMBER 2001
BURROWING OWLS IN MINNESOTA
333
Table 1. Location, habitat, and number of young fledged at nestsof wild Burrowing Owls in Minnesota, 1987-91.
NUMBER
COUNTY
OF YOUNG
FLEDGED
HABITAT
1987
1988
1989
1990
1991
Pipestone
Pipestone
Alfalfa
Roadside
->3
->3
->3
....
->3
--
--
->2
--
Rock
Pasture
--
-->2
Yellow Medicine
Fenceline
--
->2
--
5
--
--
Traverse
Pasture
--
-> 2
--
--
--
Rock
Alfalfa
--
--
8
Rock
Pasture
--
--
2
Rock
Alfalfa
--
--
--
7
--
5
->2
--
--
In 1988, a color poster featuring Burrowing Owlswasdis- pipe (10 cm diameter). This was positioned in the burtnbuted statewideto solicit nesting reports. During the row entrance with the door opening out, allowing owls
spring of 1989, 1000 black-and-whitepostersthat request- to leave but not reenter their burrow. The area above
ed reports of Burrowing Owls and Short-eared Owls (Asio and immediately around the burrow entrance was covfiammeus)were distributed to Minnesota Department of ered with a chickenwire cage,enabling us to capture the
Natural Resources(MNDNR) personnel and posted in
birds without their escaping.
Release sites were located within
historical
Minnesota
public locations throughout western Minnesota. In both
1989 and 1990, 10 000 copiesof this poster were mailed nesting range. The siteswere availablefor future Burrowto farmers in the southwestregion of the state. An ad- ing Owl management needs, allowed us to control un&tional 10 000 copies were mailed in the northwest re- wanted human intrusion during the release, and could
glon in 1990.
be managed and modified to suit the needs of the proBetween 17 May-7 June 1988, a 150-km route wassur- ject. Owls were kept in hack pens made from cottonveyed through Lac Qui Parle, Chippewa, Big Stone, and mesh fisheriesnetting (1.5 cm diameter) strung along
Stevenscounties (counties 3, 11, 2, 9; Fig. 1). This route metal fence poles. Pens were approximately7.6 m long
encompassedthe reintroduction area (Martell 1990) and
X 5.5 m wide X 1.7 m high. Wooden artificial burrows
the area where historical concentrations of Burrowing (40 cm X 40 cm, Henderson 1984) were placed 0.6 m
Owls were recorded (Grant 1965). The route was driven
underground and connected to the surfaceby a wooden
between0600-1000 H, three times/wk. Using binoculars tunnel.
and a 15-60X spotting scope, we searched for owls in
While in the hack pens,owlswere fbd dead laboratory
fields and along roadsides.In 1989 we surveyed1000 km
mice and weanling rats daily. Daily feeding of mice, weanof roads in nine southwestern
Minnesota
counties. All
ling lab rats, European Starlings (Sturnusvulgaris),and
occupied nest sites were visited in years subsequentto
House Sparrows(Passerdomesticus)
then continued for 33
their use, and all public reports of owlswere checked in
d post-release.To protect releasedjuveniles from predaall years.
tion by Great Horned Owls (Bubovirginianus),we used
Nest siteswere mapped and entered into the State of
Minnesota's Natural Heritage database. Land use and adult Burrowing Owls as "parental models," increased
ownership were recorded for each nest. We calculated the number of burrows around the site, and removed
the number of fledglings as the maximum number of local Great Horned Owls under federal and state perprefiedgedjuveniles seen at a burrow, minus known mor- mits.
Banding and Marking. Wild and released juveniles
tality prior to fiedging. Reproductive successwas meawere banded with a standard U.S.G.S. band and one red,
sured as the number of young fledged/pair.
Land management focused on protection and en- yellow, or green leg marker (Martell 1990).
hancement of nesting sites.We encouraged landowners
to maintain fields used by nesting owls in their current RESULTS
rotation (e.g., alfalfh), or enroll those fields in federal
agricultural set-asideprograms. In fall 1989, 24 artificial
Monitoring the Wild Population.Between1987burrows (Henderson 1984) were placed near natural bur- 91, 14 successfulnestingswere recorded at eight
rows to provide alternate nest sitesfor returning pairs of
sites in four counties (Rock, Pipestone, Traverse,
owlsor their oft•pring in future years.
Reintroducfion. Young owlswere obtained for reintro- and YellowMedicine, counties7, 6, 1, 4; Fig. 1) in
duction by trapping on black-tailedprairie dog (C]yn0mys western Minnesota (Table 1). Four of the eight
ludovicianus) colonies on the Fort Pierre National Grass-
lands, located approximately8 km south of Pierrc, Sonth
Dakota. Juvenile owlswere trapped using "Haug traps"
(Haug 1985), consistingof a piece of clear Plexiglasattached by a hinge to a 30-cm section of black drainage
siteswere used only once. The maximum number
,,•
•c nests found .,.;t•.:_
......... any year wasfour (1988 and
1989). A minimum of 49 young was produced for
a minimum reproductive rate of 3.5 young/pair.
334
STATUS AND TRENDS
Table 2. Number of Burrowing Owls released,mortality,
and number of daysseen after release.
MINIMUM
No.
YEAR
1986
OF OWLS
RELEASED
NO.
OF
MORTALITIES
9
POST-RELEASE
3
1.5
1987
18
1
37
1988
21
0
21
1989
27
2
3O
1990
30
2
15.5
105
8
Tot•
NO.
OF DAYS SEEN
No nestingBurrowing Owlswere recorded in Minnesota from 1992 through 1998.
VOL. 35, NO. 4
that the population is extremelysmall.Therefore,
Endangered statusisjustified in Minnesota.
Reproductivesuccess
recordedduring this study
(3.5 fledglings/pair) was similar to the historical
estimate of 3.8 fledglings/pair for Minnesota
(Grant 1965). Our resultswere also similar to other
productivityestimatesof 2.2 fledglings/pair in California (Thomsen 1971), 4.0 in North Dakota
(Konrad and Gilmer 1984), 4.4 in Saskatchewan
(Wedgwood1976), and 4.9 in New Mexico (Martin
1973). In our opinion, theseestimatessuggestthat
Burrowing Owl population sizein Minnesota is not
limited by reproduction. Other factors,historical
and current, probablyhave causedthe population
decline.
Reasonsfor Population Decline. Burrowing Owl
populations have declined in other parts of theJr
ported by local citizensor MNDNR personnelre- breeding range, where habitat loss,predation, and
spondingto postersor personalcontacts.No nest- pesticideshave been identified as important probing Burrowing Owls were located during road lems (Haug 1985,Jamesand Espie1997). In Minsurveys. Fledging occurred during the last two nesota,the population decline hasbeen attributed
weeks of July. Two adults and one immature bird to three factors:intensivecultivationof agricultural
died during our study:the immature and one adult lands, plowing of native prairie and pastureland,
were killed by collisions with vehicles, and the and the decimation of burrowing mammalsin the
cause of death for the other adult was unknown.
western part of the state (Grant 1965). However,
Land usesat the eight nest burrowswere alfalfh Coffin and Pfhnnmuller (1988) noted that suitable
fields (37.5%), pastures(37.5%), roadsideditches unoccupied habitat still seemed to exist in the
(12.5%), and fencelines between row crop fields state, a situation also noted for Endangered pop(12.5%; Table 1). Sevenof 14 nestings(50%) were ulations in Canada (De Smet 1997, Schmutz 1997,
•n alfalfa fields and produced 32 young (63% of Wellicome 1997). The use of alfalfa fields by nesttotal). All but one of the nests were located on ing Burrowing Owls in our study indicates that
privately-ownedland. One pair of owlsfledgedsev- thesebirds may have somecapacityto adapt to agen young from an artificial burrow the year after ricultural habitatsprovidedthat burrowsare availtheir natural burrow collapsed.The artificial struc- able.
Lack of suitable nest burrows may also contribture was located in the same field, approximately
ute to the population decline in Minnesota. Bur40 m from the original burrow.
Reintroduction. From 1990-96, we released 105
row availabilityhas been suggestedas a factor limjuvenile Burrowing Owls (Table 2). We document- iting Burrowing Owl populationsin other parts of
ed eight mortalities at or near release sites.With the United States (Coulombe 1971, Zarn 1974). In
the exception of 1996, almost all birds were seen Minnesota, Burrowing Owlshave been reported to
nest in burrows abandoned by I)adgers (Taxidea
well past fiedging (Table 2). No birds were fbu•d
taxus) and Richardsoh'sground squirrels ($•ermoor reported after they left their hack sites.
philus,Jchardsonii)
(Roberts 1932, Grant 1965). We
I)ls½;I ]SSI()N
recorded no use of Richardsoh'sground squirrel
Current Status and Reproductive Success.The burrowsduring our study,despitethe presenceof
Burrowing Owl is currently listed as Endangered a large colony near the Rock County nest sites(B.
by the state of Minnesota. The number of nesting Lane pets. comm.). Roberts (1932) stated that
owlsfound from 1987-91 wasthe highestrecorded holes made by Miunesota'sground squirrelswere
in Minnesota since the mid-1960s (Grant 1965),
too small to be used by Burrowing Owls until badbut thiswaslikely a result of our intensivesearches. gers enlarged them. Badgers may be a critical
Lack of nesting from 1992-98, despite continued sourceof nestingburrowsin Minnesota,a situation
interest and monitoring of sites,leaveslittle doubt similar to that reported in Canada (Wellicome
All new nest records between
1987-98
were re-
DECEMBER 2001
BURROWING OWI,S IN MINNESOTA
1997) arid in the Columbia Basin of Oregon
335
to expect some resightingsin subsequentyears.A
(Green 1983).
return rate of 14% for fledglingswasreported •n
Burrow arid burrowing mammal (e.g., Richardson'sground squirrels,badgers)managementmay
benefit Burrowing Owls in Minnesota. Artificial
burrows are readily accepted by nesting pairs in
other parts of their range (Collins and Landry
1977, Wellicome et al. 1997). Promotion of artificial burrow construction through a "Burrowing
Owl Trails" program similar to that done for East-
British Columbia (Haug et al. 1993), although De
Smet (1997) reported a return rate of only 3.5%
from 538 wild-banded fledglings in Manitoba. Becausewe could not document any positive restilts
ern Bluebirds (Sialia sialis) and Wood Ducks (Aix
Nongame Wildlife Program. Given the lack of recent breeding recordsand the uncertain future for
this speciesin Minncsota, no management or research is planned beyond protection under current state and federal legislation (e.g., Migratory
Bird Treaty Act, Minnesota Endangered Species
sponsa)may benefit this speciesin Minnesota and
m other parts of its range. However,with the current low population levels in Minnesota, location
of burrows would be critical to success.Low priority should be given to this effort in Minnesota.
Causesof decline may also operate away from
breeding areas. Burrowing Owls are migratory in
the northern portion of their range (Bent 1938,
Haug et al. 1993). No specific information exists
on the migration routes or wintering areasof Minnesota Burrowing Owls. Based on nine band recoveries,Brenckle (1936) describedthe wintering
range of the Northern Plains population as "central Texasand adjoining Oklahoma." Lossof grassland habitat in the wintering range has been suggested as the cause of decline in the midwestern
population of Loggerhead Shrikes (Lanius ludoviczanus)(Brooksand Temple 1990). This possibility
also needs to be consideredfbr Burrowing Owls.
Continued monitoring for Burrowing Owls in
Minnesotais probably best accomplishedthrough
public contact. Landowner reluctance to report
owlsin someparts of their range (De Smet 1997)
may argue againstexclusivereliance on this means
of locatingbreeding pairs. Publicitythrough posters, mailings,and media producedall nestreports
during this study and seemsto be an effective and
efficient method for locatingnestingpairs in Minnesota.Field surveysin areas traditionally used by
Burrowing Owls were important to establishpresence or absenceof nesting owls.However, surveys
proved ineffective in locating new sites.
Feasibilityof Reintroduction.We suggestthat reintroducing Burrowing Owls into western Minnesota is not a wise management strategy.The techniques used were successfulin getting juvenile
b•rds through the fledging stage, and we documented foraging, burrow use, and successful
predato• avoidance (Martell
1990), but no released
owls returned to breed. Although the numbers of
b•rds releasedwas not large, enough were released
from these translocations, we discontinued them.
Conclusion.
Future
conservation
efforts
for Bur-
rowing Owls in Minnesotawill depend on the status of the speciesand the priorities of Minnesota's
Act). Should this situationchange, habitat protection, management, and public education and cooperation will become important. Selectiveuse of
reintroduction may also be useful in enhancing
these effbrts (Martell 1990). Specific research
needs include information on population demographics,migration, and winter ecology.
AGIOqOWLEDGMENTS
Financial support for this work was provided by the
Minnesota Department of Natural ResourcesNongame
Wildlife Program, the Minnesota Chapter of The Nature
Conservancy,the Minnesota Agricultural Experiment Station, The Raptor Center at the Universityof Minnesota,
and the St. Paul Audubon Society.We were assistedm
the field by G. Buhl, C. Curran, E. Lawlet', M. Linder, J
Nibe, and L. Pohglase.Special thanks are due to D. and
D. Soehren. We are especially grateful to E. Haug, L.
Pfhnnmuller,H. Totdoff; D. Smith, and P. Redig for advice during various stagesof this project. T. Wellicome,
K. Hasselblad, and D. Low provided valuable comments
and
review.
LITERATURE
CITED
BENT, A.C. 1938. Life histories of North American
b•rds
of prey. U.S.Natl. Mus. Bull. 170.
BRENGKLE,
J.F. 1936. The migration of the western Burrowing Owl. Bird-banding
7:166-168.
BROOKS,
B.L. ANDS.A. TEMPLE.1990. Dynamicsof a Loggethead Shrike population in Minnesota. WilsonBull
102:441-450.
COFFIN, B. AND L. PFANNMUIJ,ER. 1988. Minnesota's
en-
dangered flora and fauna. Univ. Minnesota Press,
Minneapolis, MN U.S.A.
COLLINS, C.T ^ND R.E. LANDRY.1977. Artificial nest burrows for Burrowing Owls. N. Am. Bird Bander2:151154.
COULOMBE,
H.N. 1971. Behaviorand populationecolog
7
of the Burrowing OM, $•eotytocunicularia,in the Imperial Valleyof California.Condor73:162-176.
336
STATUSAND TRENDS
VOL. 35, NO. 4
DE SMET,K.D. 1997. BurrowingOwl (•5)Oeotyto
cunicularia) MARTELI,,M.S. 1990. Minnesota Burrowing Owl reintroduction: a feasibility study.M.S. thesis,Univ. Minnemonitoring and managementactivitiesin Manitoba,
sota, St. Paul, MN U.S.A.
1987-1996. Pages123-130 inJ.R. Duncan, D.H.Johnson, and T.H. Nicholls lEDs.I, Biology and conserva- MARTIN,DJ. 1973. Selected aspectsof Burrowing Owl
ecologyand behavior.Condor75:446-456.
tion of owls of the northern hemisphere: 2nd international symposium.USDA Gem Tech. Rep. NC-190. ROBERTS,T.S. 1932. The birds of Minnesota. Univ. MinnesotaPress,Minneapolis, MN U.S.A.
St. Paul, MN U.S.A.
J.K. 1997. Selectedmicrohabitvariablesnear
GRANT,R.A. 1965. The Burrowing Owl in Minnesota. SCHMUTZ,
nests of Burrowing Owls compared to unoccupied
Loon 37:2-17.
sites in Alberta. Pages 80-83 in J.L. Lincer and K
Green, G.A. 1983. Ecologyof breeding Burrowing Owls
Steenhof lEDs.I, The Burrowing Owl, its biology and
m the Columbia Basin, Oregon. M.S. thesis,Oregon
management including the proceedingsof the first
State Univ., Corvalis, OR U.S.A.
international Burrowing Owl symposium.J. Raptor
HAUG,E.A. 1985. Observationson the breeding ecology
Res. Report 9.
of BurrowingOwlsin Saskatchewan.
M.S. thesis,Univ.
THOMSEN,
L. 1971. Behavior and ecologyof Burrowing
Saskatchewan, Saskatoon, SK Canada.
Owls on the Oakland municipal airport. Condor73
--,
B.A. MILLSAP, AND M.S. MARTELL. 1993. Burrowmg Owl (5)Oeotyto
cunicularia).In A. Poole and F. Gill
lEDs.I, The birds of North America, No. 61. The
Academy of Natural Sciences,Philadelphia, PA and
American Ornithologists' Union, Washington, DC
U.S.A.
HENDERSON,
C.L. 1984. Woodworkingfor wildlife: homes
for birds and mammals. Minn. Dept. Nat. Res., St.
Paul, MN U.S.A.
JAMES,
P.C. ANDR.H.M. EsPIE.1997. Current statusof the
Burrowing Owl in North America: an agency survey.
Pages3-5 inJ.L. Lincer and K. Steenhof [EI)s.], The
Burrowing Owl, its biology and management including the proceedingsof the first international Burrowmg Owl symposium.
J. Raptor Res. Report 9.
KONRAD, P.M. AND D.S. GILMER. 1984. Observations on
the nesting ecology of Burrowing Owls in central
North
Dakota.
Prairie Nat. 16:129-30.
177-192.
WEDGWOOD,
J.A. 1976. Burrowing Owlsin south-central
Saskatchewan.
BlueJay34:27-45.
WF.I.I.ICOMF,
T.I. 1997. Statusof the BurrowingOwl (Speotyrocuniculariahypugaea)in Alberta. Alberta Environmental Protection, Wildlifb Management Division,
Wildl. StatusRep. 11, Edmonton, AB Canada.
, G.L. HOLROYD, K. SCALISE, AND E.R. WILTSE.
1997. The effectsof predator exclusionand food supplementation on Burrowing Owl ($peotyto
cunicularia)
populationchangein Saskatchewan.
Pages487-497 2n
J.R. Duncan, D.H. Johnson and T.H. Nicholls [EDs.],
Biology and conservation of owls of the northern
hemisphere: 2nd international symposium. USDA
Gem Tech. Rep. NC-190. St. Paul, MN U.S.A.
Z•P,N,M. 1974. Habitat managementseriesfor uniqueor
endangeredspecies.USDI Bureau of Land Management, Tech. Note T/N-250 (No. 11), Denver, CO