Brief Observations on the Breeding Biology of the Flammulated Owl in Colorado

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, inconspicuousbird. Henry Henshaw suggestedthat the paucityof informationregarding this small owl of montane forestsmay be due to its secretivebehavior rather than actual rarity (Oberholser1899). In fact, trainedobservers have recently discoveredit to be locallycommon in some areas (Winter 1971). Duringa studyof the impactof pesticideson breedingbirds in 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, Col- orado. One nest produced two young birds which were orphaned, removed from the nest and raisedin captivity.Each of the other two nestsfledged three young. Becausethey were obtainedin research incidentalto other work, our data on nesting,growth and development of wild and captive nestlingsare not extensive.

STUDY AREA AND METHODS

All three nestswere found at 2650 to 2775 m altitudein the upper watershed of West Creek. This watershed of the South Platte River systemcontains a variety of upper montane stand-types,including Ponderosa Pine (Pinus ponderosa) on south-facingslopes and Douglas-fir(Pseudotsuga rnenziesii), covering the steeper north- facingslopes. Ponderosa Pine is dominant on the more open south- facingslopes; however, a dry grasslandcomplex occurs on 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 Distanceto Height (m) Exposure Trunk River nearestknown of cavity diameter at Nest tributary nest (km) Cavity Tree entrance cavity(cm) 1 Phantom 6.4 4.1 14.0 West 38 2 Manchester 1.6 2.4 12.0 East 45 3 Manchester 1.6 7.6 10.0 West 16

Table 2. Growth of nestlingand captive young FlammulatedOwls.

Nestlingweight (g) Date A B C Observation

Nest 1 10-17 Jun Adult nearor peeringfrom hole (layingor incubating) 30 Jun- 14 Jul Adult tight in nest, not flushed (incubating or broodingyoung) 16 Jul 44.0 39.3 Sawed inspectionhole; adult (66.8g) on nest 21 Jul 45.5 45.5 No adult seen; claw marks, and sectionknocked out 28 Jul 32.0 29.6 Dead adult below nest;took nestlingsinto captivity 1 Sep 61.2 64.6 Maximum captive weight 10 Sep 60.2 64.5 Young released into wild

Nest 2

30 Jun Adult incubatingor broodingyoung 7 Jul Adult perchednear nest;three nestlingsin nest 10 Jul Same as 7 July 15 Jul 46.5 41.6 47.4 Sawed inspectionhole; adult (58.4g) on nest 21-28 Jul Nestlingsfledged

Nest 3 2 Jul Adulttight on nest,not flushed (incubating or brooding young) 7 Jul Adult flushed off eggs; 2 pipping (1 advanced), 1 intact 10 Jul Two adultstaking food to cavity 13 Jul 29.5 44.4 37.1 Sawed inspectionhole; adult flushedfrom nest 20 Jul 36.9 44.0 31.9 Weighedat 1930 21 Jul 46.3 55.6 42.1 Weighed at 0700 28 Jul 50.6 fidgd 47.9 Weighedat 1530 29 Jul fidgd 58.1 Weighedat 0830

36 BREEDING BIOLOGY OF FLAMMULATED OWL

The nestswere found in live aspenby extensivesearching and in- spectingcavities for potentialnesting sites. Hole sizescorresponded to those of the Common Flicker (Colaptesauratus) which are apparently favored nestingsites for the species(Bull and Anderson 1978:26-27; Eckert and Karalus 1974:161). The nest cavities are describedin Table 1. None of the cavitiescontained obvious nesting materialsother than bits of wood which were presumablyleft by previousoccupants. We found two nests1.6 km apart along one stream;the third nestwas locatedin a paralleldrainage 6.4 km to the west. Cavitieswere initiallyinspected with 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 periodicallyphotographed and weighedthe nestlings.All weighed birds were banded with Fish and Wildlife Service bands. Turnbuckle style window-screenfasteners held the cutoutsection in its natural position.Cavity alterationswere made duringdaylight hours and, in two instances,when an adult was broodingin the cavity.During the cavityalteration the adult owlswere at firstsluggish and sleepywhen disturbedbut offeredlittle resistance, other than somebill snapping. In two nestsan adult remainedin the cavity,and in the third nestan adult flushedto a nearbyperch. All three cavitiesheld young when 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 nestlingsobtained during periodicvisits to the three nestsare presentedin Table 2.

NEST 1 An adult was near or peeringout from its cavityon severalocca- sionsbetween 10 and 17 June, but no eggs were observedthen. From 30 June to 14 July, we were unableto see the nestcontents, becausethe adult sattightly within its cavity,refusing to flush. On 16 July we cut into the cavityto band and weighthe broodingadult and itstwo nestlings.On 21 July we found what appearedto be claw and tooth marksaround the hole. Althoughthe sawedsection had been knocked out, exposingthe two nestlings,both were safe insideand had gained 1 and 6 g during the previous5 days. On 28 July a portion of an unbandedadult FlammulatedOwl Car- cass(pelvic girdle with its legsstill attached) was found lyingat the 37 BREEDING BIOLOGY OF FLAMMULATED OWL baseof the cavitytree and the cut-outsection of tree was dislodged. Although it was not possibleto positivelyidentify the responsible animal, claw marks and other evidencesuggested a Bobcat (Lynx rufus). However, a Black Bear (Ursusamericanus) had ripped open a Common Flickercavity in an aspentree and depredatedthe nest- lingsless than 75 m away only 2 yearsprior (DeWeeseand Pillmore 1972). Our alterationsof the cavityhole may have givena predator the advantagenecessary to take the adult. We returned that evening to ascertainwhether or not the other adult (banded) was still tending its young. Since both sexes feed young, it was possiblethat the other adult could raisethem. Our con- cernwas well founded becausethe 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-usedperches until 2400 on 28 ,July without seeingan adult. At that ti'me we fed some moths to both nestlings.Apparently they were not satisfiedwith our limitedsupply and their beggingcalls became more intense.Their additionalweight lossovernight and 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 subse- quent visitsto the nest sites. The two youngwere about3 weeksold when captured,based on comparisonof their previousweights with 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 initiallyremoved the largejumping legs of 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 wild FlammulatedOwl nestlingsin the studyarea. However, the reporteddiet is almostexclusively made up of a varietyof insects and other arthropods(Jacot 1931, Marshall 1939, Ross 1969). Both birdswere firstmaintained in a large cardboardbox, and later moved into a screenedgarage where they spent much of their captivelife. Residencein the garagemay have helpedthem develophunting and flying skillsnecessary for survival;the young owls were observedat night capturinginsects which were attractedto a light in the garage. In captivitythe young FlammulatedOwls apparentlyderived ade- quate nutrition for growth from their predominatelygrasshopper diet. Both birdsgained weight and developedfull plumageand flight capabilitiesbefore release. Pellets of chitinousgrasshopper parts were noted in their cagesoon after they enteredcaptivity at 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 pro- bablytrampled beyond recognition any pelletsthat they regurgitated in their cavities. Our observationsagree with Eckert and Karalus (1974:160) who describedthe pelletsas "poorly formed and made up of bits of chitinousmaterial from insects." Near the end of captivity,at 50 to 60 daysof age, the owlswere occasionallyreleased into a large indoor room and fed grasshoppers whichwere placedon a smalllow perch. At thisage, the owlsflew well and easilycaptured fully active grasshoppers(Figure 1). On severalnights we saw the owlsfly up to catchsmall moths resting on the ceilingnear overheadlights. The owls capturedtheir prey with their feet while in an upside-downposition. They would return to their perch and swallowtheir catchesimmediately. The feet were alwaysused to initiallycapture, then hold the prey for pluckingwith the beak, if need be, beforeswallowing. Moths and similarsoftbodied insects were usuallyswallowed intact, but large grasshopperswere usuallyripped into pieces before swallowing. Grasshopperswere usually swallowedhead first, but sometimes were crushedand softenedwith the beak before being ingestedin either large pieces or sometimesintact. Drinkingwater was continuallyprovided the captiveowls, but not in largeenough quantities for bathing.Soon afterrelease, both owls plungedfull-body into a nearbystream and bathedextensively. Ap- parentlywe shouldhave given them opportunity 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 weightduring 44 daysin captivity.About 10 g of their overallgain, however, was apparentlyneeded to make up the weight losscaused by starvationand dehydrationduring their or- phanage in the wild.

NEST 2 This nest was first found on 30 June, but a tenaciousadult in the cavityprevented our seeingthe nest contents.On two occasions,7 and 10 July, we observedthree downy nestlings in the cavityand an adultperched nearby. Weights and plumage development were only obtainedon 15 July (Table2). This broodapparently left the nest from 21 to 28 July or about20 to 27 daysafter hatching, based on knownage and weightsof Nest 3 nestlings.

NEST 3 When we foundNest 3 on 2 July, a tenaciousadult obstructed our viewof the contents.On 7 Julywe flushedthe adultoff three eggs in 39 BREEDING BIOLOGY OF FLAMMULATED OWL variousstages of hatching.One egg waspipping; another was begin- ningto pip; and the thirdshowed no signof hatching.On 10 July we observedthe pair bringingfood to 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 indicated overnight feeding activity (Table 2). The weightsof the nestlingsfrom 1930 to 0700 the following morningincreased over 10g each. During a 28 July weighingat 1530, the two remainingnestlings (one had fledged)weighed more than their weighingat 1930 on 20 July by 14g and 16g. At 0830 the followingmorning, the heaviernestling at the 28 July weighinghad fledged, and the remainingnestling had gained again about 10g overnight.It is apparentthat weightsof nestlingsmay increaseby 20 to 25% overnightfrom feeding,but an actual20 to 25% weightin- creasemay requireabout 1 week. Only weightstaken at consistent timesbefore the usualfeeding periods may be usedfor a measureof actual weight gains. Assuming(1) that the heaviestnestling came from the firstegg to hatch, and (2) that the lightestnestling came from the last egg to hatch, then the three nestlingsin Nest 3 left the nestin <21 days, 21 daysand 20 to 27 daysafter hatching.This 21- to 27-day nestling period for 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 nestlings in Nest 3 (Table 2). Periodic weighingsof these nestlingsin- dicatedthat midwayinto nestlinglife (about2 weeksold), individual weightsvaried as much as 20 to 50%. At thistime they had attained about 70% of their eventualfledgling weight. At the time of fledging, their weights averaged about 52g. This weight is well within the range for 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,weight and developmentcomparisons with Nest 3 provided a rough estimateof their age. In addition, lack of parentalcare and food probablyretarded the 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 publishedphotographs of this speciesin juvenal plumage. BREEDING BIOLOGY OF FLAMMULATED OWL

41 BREEDING BIOLOGY OF FLAMMULATED OWL

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At about 1 week old, the nestlingswere stillnearly covered with densedown, all white exceptfor a few darkeningprimary feathers emergingas bloodquills (Figure 2). Tail featherswere not at all evi- dent at this age. At 3 weeks,the owls had noticeablychanged to a generalgray appearance(Figure 3); the body appearedto be com- pletelycovered with juvenal feathers. Also, the breastand bellywere predominatelybarred with alternatingtransverse markings of dark gray and pale gray. Wing and tail feathersshowed considerable developmentand typical adult barring,but the tail was still quite short.By 6 weeksof age, wing and tail featherswere considerably longerand bothcaptive owls could fly up to 30 m. After 7.5 weeks, bothowls were capable of sustainedflight of about50 m, and had attained their maximum weight in captivity(Table 2). At the time of release(about 9 weeksold) the owlshad developeda few adultlike, barredfeathers on breast,throat, wingsand tail; however,the overall plumagewas clearlymore immaturethan adult.

DISCUSSION

Physicalabilities displayed by the capturedowls may indicatethe capabilitiesof wild FlammulatedOwls at fledging.At about 25 days, the captiveowls, weighingabout 40 and 45 g, would fly 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 observedfly- ing upward to a perch or chasingcrippled insectsuntil they were about 34 days old. The timing of this behavior, however, was con- ceivablya littledelayed due to theirweight loss and possibleretarded development of physicalstrength prior to captivity. Immediately beforefledging, the nestlingsfrom Nest 3 weighedabout the sameas the 25-day old captives.This fact suggeststhat the fledglingFlam- mulated Owls from Nest 3 could branch-climb into a bush or low tree and could move around in the forestin short flightsfrom higherto lowerperches, although they were certainlyincapable of full flight.A 20 m flight made by one captiveowl when it was 5 weeks old suggeststhat the youngowls could easily move around in 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 likelycavities in the adjacentarea were observedagain during the 1972 breedingseason, but no FlammulatedOwls were found during extensivesearching. Whether their apparentabsence I year lateris due to unpredictability of the speciesor resultedfrom our disturbancesis unknown. Nest 1 was severelydisturbed by 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 thesespecific nesting areas. Future studies should not involveeither te/mperingwith or destroyingthe nestingcavity. Sincethis studywas completed,other observershave attempted to determinethe species'status in Colorado.Winn (1979) foundon- ly five activenests and observed11 owlsduring intensive searching in the Pike National Forest. She suggestedthat suitablebreeding habitat and nestingsites were probablyimportant in limitingthe distribution and abundance of Flammulated Owls in Colorado. Her conclusionwas that it is a rare speciesin Colorado.

SUMMARY

Locationsof two Flammulated Owl nestsless than 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 probablyattributable more to behaviorthan scarci- ty. Three nestswere found in cavitiesin aspensin the upper montane vegetativetype found in moistbottomlands between 2650 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 encouraged by cutting an enlarged opening into the nestingcavity. Cutting open the cavitieswas effectivefor handling adultsand nestlingsbut is discouragedas a routinemethod for study- ing these birds. Weight of three nestlingsin one nest increasedby about 10g after an overnightfeeding, but an actual averageweight gainfor these6- to 14-day old nestlingswas about 11 g eachduring 8 days.These wild nestlingowls weighed about 50 g beforeleaving the nest,and two captivenestlings weighed about 60 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 Americanbirds of 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. Birdnests in an aspentree robbedby 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.Doubleday and Co., Garden City, NY. Jacot, E.C. 1931. Notes on the Spottedand Flammulatedowls in 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 northeasternScreech Owl. Biol. Leafl. 50. Cornell Univ., NY. Marr, J.W. 1961. Ecosystemsof the eastslope of the Front Rangein Colorado. Inst. of Arctic and Alpine Res., Contrib. No. 4. Univ. Colorado Press,Boulder, CO. Marshall, J.T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl. Con- dor 41:71-78. Oberholser,H.M. 1899. The FlammulatedScreech Owls, Megascopsflammeolus (Kaup) and Megascopsflammeolus idahoensis Merriam. Ornis 10:23-38. Reilly, E.M., Jr. 1968. Audubon illustratedhandbook of American birds. McGraw- Hill, NY. Ross,A. 1969. Ecologicalaspects of the food habitsof insectivorousScreech Owls. 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 criticalnotes on 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