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Molt of the Spotted Owl

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Molt of the Spotted Owl MOLT OF THE SPOTTED OWL ERIC D. FORSMAN • CooperativeWildlife ResearchUnit, OregonState University,Corvallis, Oregon 97331 USA ABSTRACT.--The molt of both wild and captive SpottedOwls was studied in Oregon between 1970and 1980. SpottedOwls developedthree plumages in their first year. The white natal down wasreplaced by the fluffy juvenalplumage before owlets left the nest.Replacement of the juvenal feathersby the basicI plumagebegan when owletswere 47-56 daysold and wascomplete by late Septemberor October. The only juvenal feathersnot replacedduring the latter molt were the remiges, rectrices, and greater primary coverts. Adults underwenta prolongedprebasic molt each year betweenApril and mid-October.The molt of the body featherswas completeannually, but 2 yr (rarely 3) were requiredto molt all of the remiges,rectrices, alulars, and greaterwing coverts.The mostcommon pattern of tail molt was a rapid or "simultaneous"molt, but irregularand partial molts alsooccurred. Received 12 March 1980, accepted 5 May 1981. LITTLE is known about the molt of the Spotted Owl (Strix occidentalis). Bent (1938) and Miller (1974) presentedincomplete descriptions of plumagedevelopment in young SpottedOwls but did not discusslater molts. Oberholser(1974) statedthat adult Spotted Owls underwent a completeannual molt but he did not indicate how he determined that the molt was complete. Indeed, although some authors (e.g. Bent 1938, Dement'ev et al. 1951, Oberholser 1974) have assumedthat the annual molt in Strix was complete, I have been unable to find any studies in which a completemolt was documentedin this genusexcept for the Tawny Owl (Strix aluco) (Piechocki 1968a). The pattern of tail molt in the SpottedOwl is also unclear. Mayr and Mayr (1954) suggestedthat the tail molt occurredgradually in all owls in the genusStrix, but Piechocki(1968a) found that Tawny Owls molted the rectricesrapidly or "simul- taneously."Two Spotted Owls collectedby Huey (1913) and one collectedby John- son and Russell (1962) had also molted their rectrices rapidly. Because so many statements in the literature concerning the molt in Strix are either unverified or incorrect, carefully documentedstudies on the molt in this group of owls are needed. Between 1970 and 1980, I conducted a series of studies on the distribution, re- productivebiology, and ecologyof the SpottedOwl in Oregon(Forsman 1976, 1980). During these studiesI observedmany Spotted Owls in the field and raised two SpottedOwls in captivity. Recordswere kept of the molt of all owlsobserved. This reportdescribes the molt of the SpottedOwl basedon my observationsof both wild and captive birds. METHODS Owls observedin the field included 339 adults and 95 fledglings.Most observationswere conducted betweenMarch and Septembereach year, when I made regular visits to SpottedOwl nestsand roosts. Duringthese visits, any recently molted feathers were collected from belowroost trees, and the owlswere examined at closerange with binocularsfor evidenceof molt. Sixteen adults were examinedwhen I trappedthem to installradiotransmitters and whenI retrappedthem to removethe transmitters.Almost daily observationswere madeof eight of the radio-taggedowls between May 1975and June 1976and Presentaddress: 4655 N. E. Elliott Circle, Corvallis,Oregon 97330 USA. 735 The Auk 98: 735-742. October 1981 736 ER•c D. FORSMAN [Auk, Vol. 98 A B C Fig.1. Sequenceoffeather replacement during molt from juvenal plumage into basic I plumage.A. 42-46days old, 10 days after leaving nest. B. 90-100days old. C. 140-150days old; note white tips of juvenal rectrices. of sixother radio-tagged individuals between April and September1980. Six nestlings between the age of 5 and 25 dayswere examined to determinethe patternof molt in nestlings. Thetwo captive owls (both females) were confined in separateoutdoor cages with wire mesh sides and corrugatedmetal roofs. Cage dimensions were 2.5 x 3.6 x 2.5 m and2.5 x 3.0 x 2.1 m, respectively. Onecaptive (IA) wasobtained as a 3-week-oldnestling in 1970.A recordof itsmolt was kept every. _vear until 1980,except for 1971,when only a partialrecord was kept. The otherowl (2A) wascaptured as a 4-month-oldfledgling in 1976;its moltwas subsequently recorded every year through 1980. During the molt, the plumageof bothcaptive owls was examined every 7-10 days,and all moltedfeathers were collectedand cataloguedas they were shed. RESULTS Molt sequenceduring the first year of life.--When they hatched,owlets were sparselycovered by whitenatal down on mostof the tractsthat wereoccupied by contourfeathers in adults. When owletswere 7-9 days old, their eyesopened, and, whenthey were about 10 days old, the juvenalplumage began to replacethe natal downon the wings,back, and top of the head.Juvenal contour feathers were soft, fluffy, and palebrown with darkerbrown transverse barring (Fig. 1A). Natal down featherswere attachedto the tips of many of the developingjuvenal feathers.Re- placementof thenatal down by thejuvenal contour feathers was essentially complete when owlets left the nest at 34-36 days of age (Fig. IA). The juvenalremiges became visible when nestlings were about 10 days old and reachedfull developmentat about65 days.The rectricesbecame visible slightly laterthan the remigesand reached full developmentat about75 days.When owlets left the nest,their remigeswere about two-thirds developed and their rectrices were aboutone-fourth developed (Fig. 1A). As a result,owlets flew weaklyfor about2 weeksafter they left thenest and frequently resorted to climbingfrom one perch to another rather than flying. October1981] SpottedOwl Molt 737 A B Fig.2. Contrastbetween juvenal rectrices (A) and rectrices acquired in subsequentmolts (B). Note white,slightly pointed tips of juvenalrectrices; rectrices acquired in subsequentprebasic molts had rounded tips that were not clear white. Replacementof the juvenal contour feathers by thebasic I plumage(prebasic molt)began at 47-56days (between 1 June and $ July)and was- complete by late Septemberor October.The onlyfeathers not replaced during this molt were the juvenalremiges and rectrices and the greaterprimary coverts. The rateof molt wasslow at first,reached a peakin Julyand August, then declined in Sep- tember.With some minor differences, the prebasic molt followed the same pat- ternas describedfor youngScreech Owls (Otus asio) by Kelso(1950). The molt beganwith the lesser, middle, and marginal wing coverts and progressed to the scapularsand greater wing coverts by thetime owlets were 56 daysold. At about the sametime that the scapularsbegan to molt,feathers on the dorsaltract also beganto molt. Between 57 and 67 days, molt of the wing coverts, back, and scap- ularscontinued, and molt of thefacial area and neck began. On the neck, the molt begannear the posterior end of the interscapular tract and proceeded anteriorly. At 68-82days, the scapulars and feathers on the back emerged rapidly, and the feathers on top of the head beganto molt, beginningnear the anteriorend of the coronal tractand proceeding posteriorly and laterally. At 83-93days, basic I feathersbegan to appearin twoparallel rows down the sides of the breast,and the last greater wingcoverts were molted. Between 94 and 120 days, the rows of feathers along the sidesof thebreast gradually merged to coverthe entire ventral area, and the molt of thewings, back, and scapulars was completed (Fig. lB). At 120days, most feathershad been replaced on theneck and head except for a bandof juvenal feathersacross the back and sides of thehead. By 135to 155days (10 September- 15 October),the basicI plumagewas complete (Fig. 1C). Individualsin the basic I plumage were indistinguishable fromadults, except that thetips of theirrectrices were clear white and pointed. By comparison,rectrices acquiredin subsequentmolts had rounded tips that were white with brown blotches onthe white background (Figs. 1C, 2). Thewhite-tipped rectrices acquired in the firstsummer were not molted until owls were 26 monthsold and thus served as an age-specificmarker for thefirst 2 yr of life (seebelow). Moltsequence after the first year of life.--After their first year, both captive owls 738 Emc D. FORSMAN [Auk, Vol. 98 TABLE 1. Order of primar.v molt in two captive Spotted Owls (1A and 2A). a Year 1A 2A 1971 No data b 1972 R 1,4,5,6,7,8,9,10 L 1,4,5,2,3,7,8,9,10 1973 None molted 1974 R3,2,1,4,5,6,7,8,9,10 L 3,1,2,5,6,7,8,9,10 1975 R 3 L 3,4 1976 R5,4,6,10 L 3,1,612,7,8,9 1977 R3,1,21718,9 R6,7,8,9,10 L 311,6,2,7,8,9 L 6,9,10 1978e R2,4,5,6,10 R 17,2,3,4,5 L 2,4,5,10 L 17,2,3,4,5,7,8 1979 R4,7,8,9 R 1,6,7,8,9,10 L 2,6,7,8,9 L 1,6,9,10 1980 R2,3,1,5,6,10 R4,5,2 L 3,1,4,5,10 L 3,4,5,7,8 • Owl 1A was 1 yr old in 1971; 2A was 1 yr old in 1977. b Owl 1A moltedseveral primaries in 1971,but no recordwas kept of which onesthey were. They were probablythe onesnot molted in 1972. ½A questionmark after a primary number indicatesthat I was uncertainwhether a primary was molted or not. underwent an incompleteprebasic molt annually, beginningbetween 2 April and early June and ending in late Septemberor October. The averageduration of this molt (from first to last feathers shed)was 141 days (range = 112-184 days). During eachprebasic molt, all of the bodyfeathers were replacedin approximatelythe same order as in the first prebasic molt. Molt of the primaries, secondaries, and alulars occurred gradually and was incomplete; the typical pattern was the replacement of some of the feathers one year and the remaining feathers the next year (Table 1). This pattern was disrupted occasionallywhen one or more feathers were molted 2 yr in a row or were retained for 2 yr in a row. The pattern of molt of the remiges becameincreasingly complex as the owls grew older (Table 1).
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