BRIEF OBSERVATIONS
ON
THE BREEDING
BIOLOGY
OF THE
FLAMMULATED
OWL IN COLORADO
MERLE L. RICHMOND, U.S. Fishand WildlifeService,Lloyd 500 Bldg,Suite 1692,
500 NE Multnomah St., Portland, Oregon 97232
LAWRENCE R. DeWEESE, U.S. Fish and Wildlife Service, 220 West ProspectRd.
Fort Collins, Colorado 80526
RICHARD E. PILLMORE, U.S. Fish and Wildlife Service, 1300 Blue Spruce Dr.,
Fort Collins, Colorado 80524
Although the Flammulated Owl (Otus fiamrneolus)is found in
western North America from Guatemala to Canada (AOU 1957),
Reilly (1968) and Winter (1974) concurthat littleis known concern-
ing the biologyof this small, inconspicuous
bird. Henry Henshaw
suggestedthat the paucityof informationregardingthissmallowl of
montane forestsmay be due to its secretivebehavior rather than actualrarity (Oberholser1899). In fact, trainedobservershave recently
discoveredit to be locallycommon in some areas (Winter 1971).
Duringa studyof the impactof pesticides
on breedingbirdsin the
Pike National Forest, Colorado, we locatedthree FlammulatedOwl
nests.The nestswere all found in 1971 within 6.4 km of each other,
about 13 km north and northwestof Divide, Teller County, Colorado. One nest produced two young birdswhich were orphaned,
removed from the nestand raisedin captivity.Each of the other two
nestsfledgedthree young. Becausethey were obtainedin research
incidentalto other work, our data on nesting,growthand development of wild and captive nestlingsare not extensive.
STUDY
AREA
AND METHODS
All threenestswere foundat 2650 to 2775 m altitudein the upper
watershed of West Creek. This watershed of the South Platte River
systemcontainsa variety of upper montane stand-types,including
Ponderosa Pine (Pinus ponderosa) on south-facingslopes and
Douglas-fir(Pseudotsugarnenziesii),coveringthe steeper northfacingslopes.PonderosaPine is dominanton the more open southfacingslopes;however, a dry grasslandcomplexoccurson the lower
reaches of these slopes (Marr 1961). Moist bottomlandswhere the
nestswere found have complexesof willow (Salix spp.), Shrubby
Cinquefoil (Potentilia fruticosa), open meadows and scattered
clumps of Quaking Aspen (Populustrernuloides),spruce (Picea
spp.) and fir (Abiessp.).
Western Birds 11: 35-46, 1980
35
BREEDING BIOLOGY OF FLAMMULATED
OWL
Table 1. Characteristicsof Flammulated Owl nest trees (Quaking Aspen) in the Pike
National Forest, Colorado.
S. Platte
River
Nest
tributary
Distanceto
Height (m)
Exposure
nearestknown
of cavity
nest (km) Cavity
Tree entrance
Trunk
diameterat
cavity(cm)
1
Phantom
6.4
4.1
14.0
West
38
2
3
Manchester
Manchester
1.6
1.6
2.4
7.6
12.0
10.0
East
West
45
16
Table 2. Growth of nestlingand captiveyoung FlammulatedOwls.
Nestlingweight (g)
Date
A
Nest
B
1
10-17 Jun
Adultnearor peeringfrom hole (layingor incubating)
30 Jun14 Jul
16 Jul
44.0
39.3
21 Jul
28 Jul
45.5
32.0
45.5
29.6
1 Sep
10 Sep
61.2
60.2
64.6
64.5
Nest
Adult tight in nest, not flushed (incubating or
broodingyoung)
Sawed inspectionhole; adult (66.8g) on nest
No adult seen; claw marks, and sectionknocked out
Dead adult below nest;took nestlingsinto captivity
Maximum captive weight
Young releasedinto wild
2
30 Jun
7 Jul
10 Jul
15 Jul
46.5
41.6
47.4
21-28 Jul
Nest
Observation
C
Adult incubatingor broodingyoung
Adult perchednear nest;three nestlings
in nest
Same as 7 July
Sawed inspectionhole; adult (58.4g) on nest
Nestlingsfledged
3
2 Jul
Adulttighton nest,notflushed(incubating
or brooding
young)
Adult flushed off eggs; 2 pipping (1 advanced), 1
7 Jul
intact
10 Jul
13 Jul
29.5
44.4
20 Jul
36.9
44.0
31.9
21 Jul
46.3
55.6
42.1
28 Jul
29 Jul
50.6
fidgd
fidgd
47.9
58.1
36
37.1
Two adultstakingfood to cavity
Sawed inspectionhole; adult flushedfrom nest
Weighedat 1930
Weighed at 0700
Weighedat 1530
Weighedat 0830
BREEDING BIOLOGY OF FLAMMULATED
OWL
The nestswere found in live aspenby extensivesearchingand inspectingcavitiesfor potentialnestingsites.Hole sizescorresponded
to those of the Common Flicker (Colaptesauratus)which are apparently favored nestingsitesfor the species(Bull and Anderson
1978:26-27; Eckert and Karalus 1974:161). The nest cavities are
describedin Table 1. None of the cavitiescontainedobviousnesting
materialsother than bits of wood which were presumablyleft by
previousoccupants.We found two nests1.6 km apart along one
stream;the third nestwaslocatedin a paralleldrainage6.4 km to the
west.
Cavitieswere initiallyinspectedwith a viewingtube designedfor
suchuse (DeWeeseet al. 1975). When the nestlingswere about 1 to
2 weeks old, we sawed a section about fist-size at the hole, and
periodicallyphotographedand weighedthe nestlings.All weighed
birds were banded with Fish and Wildlife Service bands. Turnbuckle
style window-screenfastenersheld the cutoutsectionin its natural
position.Cavity alterationswere made duringdaylighthoursand, in
two instances,when an adultwasbroodingin the cavity.Duringthe
cavityalterationthe adult owlswere at firstsluggishand sleepywhen
disturbedbut offeredlittleresistance,otherthan somebill snapping.
In two nestsan adultremainedin the cavity,and in the third nestan
adultflushedto a nearbyperch. All three cavitiesheld youngwhen
altered, and the adults did not abandon their nestsas a direct result
of the cavityalterations.However, we made no attemptsto observe
discrete behavioral effects.
ACCOUNTS
OF NESTS
Weightsof nestlingsobtainedduringperiodicvisitsto the three
nestsare presentedin Table 2.
NEST
1
An adult was near or peeringout from its cavityon severaloccasionsbetween 10 and 17 June, but no eggswere observedthen.
From 30 June to 14 July, we were unableto see the nestcontents,
becausethe adult sattightlywithinitscavity,refusingto flush.On 16
July we cut into the cavityto bandand weighthe broodingadultand
itstwo nestlings.On 21 July we found what appearedto be clawand
tooth marksaround the hole. Althoughthe sawedsectionhad been
knocked out, exposingthe two nestlings,both were safeinsideand
had gained 1 and 6 g during the previous5 days.
On 28 July a portionof an unbandedadult FlammulatedOwl Carcass(pelvicgirdlewith its legsstillattached)was found lyingat the
37
BREEDING
BIOLOGY
OF FLAMMULATED
OWL
baseof the cavitytree and the cut-outsectionof tree was dislodged.
Although it was not possibleto positivelyidentify the responsible
animal, claw marks and other evidencesuggesteda Bobcat (Lynx
rufus). However, a Black Bear (Ursusamericanus)had ripped open
a Common Flickercavityin an aspentree and depredatedthe nestlingslessthan 75 m away only 2 yearsprior (DeWeeseand Pillmore
1972). Our alterationsof the cavityhole may have givena predator
the advantagenecessaryto take the adult.
We returned that evening to ascertainwhether or not the other
adult (banded) was still tending its young. Since both sexesfeed
young, it was possiblethat the other adult couldraisethem. Our concernwaswellfoundedbecausethe nestlingshad lost 13.5 and 15.9g
(30 to 35%) of their body weightfrom 21 to 28 ,July.We observed
the nest site and frequently-usedperchesuntil 2400 on 28 ,July
without seeingan adult. At that ti'mewe fed some moths to both
nestlings.Apparentlythey were not satisfiedwith our limitedsupply
and their beggingcallsbecamemore intense.Their additionalweight
lossovernightand the absenceof an attendingadult convincedus
that they would soon die from starvation;thus, we took the two
nestlingsinto captivity.The banded adult was not seen on subsequent visitsto the nest sites.
The two youngwere about3 weeksold when captured,basedon
comparisonof their previousweightswith the weightsof known age
nestlingsin Nest 3. We fed them mostly live adult grasshoppers
(Acrididae),some live moths (Lepidoptera)and an occasionalhouse
cricket(Gryllidae).We initiallyremovedthe largejumpinglegsof the
grasshoppers,but after a few days the nestlingsdid so themselves
before ingestingthem. Food was usuallyoffered in late afternoon,
once per day ad libitum. No attempt was made to determine the
foodsof wildFlammulatedOwl nestlings
in the studyarea. However,
the reporteddiet is almostexclusivelymade up of a varietyof insects
and other arthropods(Jacot 1931, Marshall1939, Ross1969). Both
birdswere firstmaintainedin a largecardboardbox, and later moved
into a screenedgaragewhere they spentmuch of their captivelife.
Residencein the garagemay have helpedthem develophuntingand
flyingskillsnecessaryfor survival;the young owls were observedat
night capturinginsectswhich were attractedto a lightin the garage.
In captivitythe young FlammulatedOwls apparentlyderivedadequate nutrition for growth from their predominatelygrasshopper
diet. Both birdsgainedweightand developedfull plumageand flight
capabilitiesbefore release. Pellets of chitinousgrasshopperparts
were noted in their cagesoonafterthey enteredcaptivityat about25
daysof age. We did not find any pelletsin the naturalcavities.At this
age young in the other two nestswere leavingtheir cavitiesand may
38
BREEDING BIOLOGY
OF FLAMMULATED
OWL
have been fed more completely digestiblefoods. Wild young probablytrampledbeyondrecognitionany pelletsthat they regurgitated
in their cavities. Our observationsagree with Eckert and Karalus
(1974:160) who describedthe pelletsas "poorlyformed and made
up of bits of chitinousmaterial from insects."
Near the end of captivity,at 50 to 60 daysof age, the owlswere
occasionallyreleasedinto a largeindoorroom and fed grasshoppers
whichwere placedon a smalllow perch. At thisage, the owlsflew
well and easilycapturedfully active grasshoppers
(Figure1). On
severalnightswe sawthe owlsfly up to catchsmallmothsrestingon
the ceilingnear overheadlights.The owlscapturedtheir prey with
their feet while in an upside-downposition.They would return to
their perch and swallowtheir catchesimmediately.
The feet were alwaysusedto initiallycapture,then hold the prey
for pluckingwith the beak, if need be, beforeswallowing.Mothsand
similarsoftbodiedinsectswere usuallyswallowedintact, but large
grasshoppers
were usuallyripped into piecesbefore swallowing.
Grasshopperswere usually swallowedhead first, but sometimes
were crushedand softenedwith the beak before beingingestedin
either large pieces or sometimesintact.
Drinkingwater was continuallyprovidedthe captiveowls,but not
in largeenoughquantitiesfor bathing.Soon afterrelease,bothowls
plungedfull-bodyinto a nearbystreamand bathedextensively.Ap-
parentlywe shouldhavegiventhemopportunity
to bathein captivity.
When we releasedthe young owls near their natal area on 10
September,they were about 63 days old and had nearly doubled
their body weightduring44 daysin captivity.About 10 g of their
overallgain, however, was apparentlyneeded to make up the
weightlosscausedby starvationand dehydrationduringtheir orphanage in the wild.
NEST 2
This nest was firstfound on 30 June, but a tenaciousadult in the
cavitypreventedour seeingthe nest contents.On two occasions,7
and 10 July,we observedthreedownynestlings
in the cavityand an
adultperchednearby.Weightsandplumagedevelopment
wereonly
obtainedon 15 July (Table2). This broodapparentlyleft the nest
from21 to 28 Julyor about20 to 27 daysafterhatching,basedon
knownage and weightsof Nest3 nestlings.
NEST 3
When we foundNest3 on 2 July, a tenaciousadultobstructed
our
viewof the contents.On 7 Julywe flushedtheadultoffthreeeggsin
39
BREEDING
BIOLOGY
OF FLAMMULATED
OWL
variousstagesof hatching.One eggwaspipping;anotherwasbeginningto pip; and the thirdshowedno signof hatching.On 10 July we
observedthe pairbringingfoodto the cavity,and on 13 July we sawed
out a sectionof the nest tree to band and weigh the three nestlings.
Over a 12-hour period during the night of 20 July, the weight
changesof thesenestlings
indicatedovernightfeedingactivity(Table
2). The weightsof the nestlingsfrom 1930 to 0700 the following
morningincreasedover 10g each. During a 28 July weighingat
1530, the two remainingnestlings(one had fledged)weighedmore
than theirweighingat 1930 on 20 Julyby 14g and 16g. At 0830 the
followingmorning,the heaviernestlingat the 28 July weighinghad
fledged, and the remainingnestlinghad gained again about 10g
overnight.It is apparentthat weightsof nestlings
may increaseby 20
to 25% overnightfrom feeding,but an actual20 to 25% weightincreasemay requireabout 1 week. Only weightstaken at consistent
timesbeforethe usualfeedingperiodsmay be usedfor a measureof
actual weight gains.
Assuming(1) that the heaviestnestlingcamefrom the firsteggto
hatch, and (2) that the lightestnestlingcame from the last egg to
hatch,then the three nestlingsin Nest3 left the nestin <21 days,21
daysand 20 to 27 daysafter hatching.This 21- to 27-day nestling
periodfor FlammulatedOwls would be shorterthan the 30 to 33
days (Sherman1911), 31 to 35 days (Kelso1950) and 26 days
(Bent 1938) reportedfor ScreechOwls (Otu$ a$io).
GROWTH
OF NESTLINGS
The only weightsof known age nestlingswere obtainedfrom nestlingsin Nest 3 (Table 2). Periodicweighingsof these nestlingsindicatedthat midwayinto nestlinglife (about2 weeksold), individual
weightsvariedas muchas 20 to 50%. At thistime they had attained
about70% of their eventualfledglingweight.At the time of fledging,
their weights averaged about 52g. This weight is well within the
rangefor adult FlammulatedOwls reportedby Johnsonand Russell
(1962), Johnson (1965) and Earhart and Johnson (1970).
Exact age of the captive owls from Nest 1 was undetermined;
however,weightand developmentcomparisons
with Nest3 provided
a rough estimateof their age. In addition, lack of parentalcare and
food probablyretardedthe growthand developmentof both captive
owls. Nonetheless,we photographedthe captive owls when they
were approximately1 and 3 weeksold (Figures2 and 3). We are not
aware of publishedphotographsof this speciesin juvenal plumage.
BREEDING
BIOLOGY
OF FLAMMULATED
OWL
41
BREEDING
BIOLOGY
OF FLAMMULATED
OWL
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BREEDING
BIOLOGY
OF FLAMMULATED
OWL
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BREEDING BIOLOGY
OF FLAMMULATED
OWL
At about 1 week old, the nestlingswere stillnearlycoveredwith
densedown, all white exceptfor a few darkeningprimaryfeathers
emergingasbloodquills(Figure2). Tail featherswerenot at all evident at thisage. At 3 weeks,the owlshad noticeablychangedto a
generalgray appearance(Figure3); the body appearedto be completelycoveredwithjuvenalfeathers.Also,the breastandbellywere
predominately
barredwith alternatingtransverse
markingsof dark
gray and pale gray. Wing and tail feathersshowedconsiderable
developmentand typicaladult barring,but the tail was still quite
short.By 6 weeksof age, wing and tail featherswere considerably
longerand bothcaptiveowlscouldfly up to 30 m. After 7.5 weeks,
bothowlswerecapableof sustained
flightof about50 m, and had attained their maximumweight in captivity(Table 2). At the time of
release(about9 weeksold) the owlshad developeda few adultlike,
barredfeatherson breast,throat, wingsand tail; however,the overall
plumagewas clearlymore immaturethan adult.
DISCUSSION
Physicalabilitiesdisplayedby the capturedowls may indicatethe
capabilitiesof wild FlammulatedOwls at fledging.At about 25 days,
the captiveowls, weighingabout40 and 45 g, wouldfly 0.5 m down
from their perchto their cagefloor for food. Prior to that age, rather
than actuallyfly, they would either continueto beg for food from
their perchor clumsilyjump down to it. They were not observedflying upward to a perch or chasingcrippledinsectsuntil they were
about 34 days old. The timing of this behavior, however, was conceivablya littledelayeddue to theirweightlossand possibleretarded
developmentof physicalstrengthprior to captivity. Immediately
beforefledging,the nestlingsfrom Nest3 weighedaboutthe sameas
the 25-day old captives.This fact suggeststhat the fledglingFlammulated Owls from Nest 3 could branch-climb into a bush or low tree
and could move around in the forestin shortflightsfrom higherto
lowerperches,althoughthey were certainlyincapableof fullflight.A
20 m flight made by one captiveowl when it was 5 weeks old suggeststhatthe youngowlscouldeasilymovearoundin the forestfrom
tree to tree about 2 weeks after fiedging. Well-controlled and
directedflight was not observeduntil they were about 53 days old.
Cavitiesthat were occupiedin 1971 and other likelycavitiesin the
adjacentarea were observedagainduringthe 1972 breedingseason,
but no FlammulatedOwls were found during extensivesearching.
Whethertheir apparentabsenceI year lateris due to unpredictability
of the speciesor resultedfrom our disturbances
is unknown. Nest 1
was severelydisturbedby a predator,and the tree containingNest 2
BREEDING
BIOLOGY
OF FLAMMULATED
OWL
was cut down duringthe winter of 1971. Our activities,combined
with otherfactors,may have preventedthe owlsfrom returningto
thesespecificnestingareas.Futurestudiesshouldnot involveeither
te/mperingwith or destroyingthe nestingcavity.
Sincethisstudywas completed,other observershave attempted
to determinethe species'statusin Colorado.Winn (1979) foundonly five activenestsand observed11 owlsduringintensivesearching
in the Pike National Forest. She suggestedthat suitablebreeding
habitatand nestingsiteswere probablyimportantin limitingthe
distribution and abundance of Flammulated Owls in Colorado. Her
conclusionwas that it is a rare speciesin Colorado.
SUMMARY
Locationsof two FlammulatedOwl nestslessthan 1.6 km apart
and a third nest about 6.4 km away suggestthat the Flammulated
Owl is more numerousin partsof Coloradothan previouslyknown.
Their obscurityis probablyattributablemore to behaviorthan scarcity. Three nestswere found in cavitiesin aspensin the upper montane vegetativetype found in moistbottomlandsbetween2650 and
2775m
elevation about 50 km southwest of Denver. Predation on
an adult by a large mammal occurredat one of three nestsand may
have been encouragedby cutting an enlarged opening into the
nestingcavity. Cutting open the cavitieswas effectivefor handling
adultsand nestlings
butisdiscouraged
asa routinemethodfor studying these birds. Weight of three nestlingsin one nest increasedby
about 10g after an overnightfeeding, but an actualaverageweight
gainfor these6- to 14-day old nestlingswasabout 11 g eachduring8
days.Thesewild nestlingowlsweighedabout50 g beforeleavingthe
nest,and two captivenestlingsweighedabout60 g when old enough
for full flight; two adultsweighed 67g and 58g. Two captive owls
could not fly well for short distancesuntil they were about 7 weeks
old. The time from hatchingto fledgingis estimatedto be about21
days and perhapsup to 27 days.
LITERATURE
CITED
AmericanOrnithologists'
Union. 1957. Check-listof North Americanbirds,5th ed.
Am. Ornithol. Union, Baltimore, MD.
Bent, A.C. 1938. Life historiesof North Americanbirdsof prey. U.S. Natl. Mus. Bull.
170.
Bull, E.L. and R.G. Anderson. 1978. Noteson the FlammulatedOwl in northeastern
Oregon. Murrelet 59:26-28.
45
BREEDING
BIOLOGY
OF FLAMMULATED
OWL
DeWeese,L.R. and R.E. Pillmore.1972. Birdnestsin an aspentreerobbedby Black
Bear. Condor 74:488.
DeWeese, L.R., R.E. Pillmoreand M.L. Richmond.1975. A devicefor inspecting
nest cavities.Bird Banding46:162-165.
Earhart,C.M. and N.K. Johnson. 1970. Size dimorphismand food habitsof North
American owls. Condor 72:251-264.
Eckert,A.W. and K.E. Karalus.1974. The owlsof North America.Doubledayand
Co., Garden City, NY.
Jacot, E.C. 1931. Notes on the Spottedand Flammulatedowlsin Arizona. Condor
33:8-11.
Johnson, N.K. 1965. Montane avifauna of southern Nevada. Condor 67:93-124.
Johnson, N.K. and W.C. Russell. 1962. Distributional data on certain owls in the
western Great Basin. Condor 64:513-514.
Kelso, L.H. 1950. The postjuvenal molt of the northeasternScreechOwl. Biol. Leafl.
50. Cornell Univ., NY.
Marr, J.W. 1961. Ecosystems
of the eastslopeof the Front Rangein Colorado.Inst.
of Arcticand Alpine Res., Contrib. No. 4. Univ. Colorado Press,Boulder, CO.
Marshall, J.T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl. Condor 41:71-78.
Oberholser,H.M. 1899. The FlammulatedScreechOwls, Megascopsflammeolus
(Kaup) and MegascopsflammeolusidahoensisMerriam. Ornis 10:23-38.
Reilly, E.M., Jr. 1968. Audubon illustratedhandbookof Americanbirds. McGrawHill, NY.
Ross,A. 1969. Ecologicalaspectsof the food habitsof insectivorous
ScreechOwls.
West. Found. Vert. Zool. Monogr. 1:301-344.
Sherman, A.R. 1911. Nest life of the Screech Owl. Auk 28:155-168.
Winn, R. 1979. Status of breeding Flammulated Owls in Colorado. Audubon
Warbler, December: 2-4. Denver, CO.
Winter, J. 1971. Some criticalnoteson finding and seeingthe FlammulatedOwl.
Birding 3:204-209.
Winter, J. 1974. The distributionof the Flammulated Owl in California. West. Birds
5:25-44.
Accepted $ December 1979
46