J. Ornithl. 140, 193-197 (1999) Deutsche Ornithlgen-Gesellschaft/Blackwell Wissenschafts-Verlag, Berlin ISSN 0021-8375 Delayed Maturatin f Plumage Clratin and Plumage Spttedness in the Barn Owl (Tyr alba) Alexandre Rulin Department f Zlgy, University f Bern, CH-3032 Hinterkappelen, Switzerland. E-mail: arulin@esh.unibe.ch Summary The Barn Owl (Tyt alba) varies in plumage frm dark reddish-brwn t white, and frm heavily marked with black spts t immaculate. Males are cmmnly lighter clured and less sptted than females. I assessed whether male and female Barn Owls delay the full expressin f plumage clratin and spttedness t the secnd year f life. In Switzerland, I quantified the tw traits f birds captured at the nestling stage, first, secnd and third year f life. Males and females became lighter clured nly frm the first t the secnd year. Males became less sptted nly frm the first t the secnd year, and females less sptted frm the nestling stage t the first year but mre sptted frm the first t the secnd year. Females were als similarly sptted at the secnd and third year f age. By cutting ff small pieces f feathers f females I culd recgnize which feathers had later been renewed. After a cmplete mult ld females did nt change in plumage characteristics. Key wrds: mult, plumage plymrphism, sexual dimrphism Zusammenfassung Verzfgerter Wechsel vm Juvenil- zum Adultgefieder bei der Schleiereule (Tyr alba) Das Gefieder der Schleiereule (Tyt alba) variiert yn rstbraun bis weiss und yn dicht gefleckt bis fleckenls. Die M~innchen sind eher hell und weniger gefleckt als die Weibchen. Es wurde untersucht, b m~nnliche und weibliche Schleiereulen die vlle Entfaltung ihrer Gefiederfarbe und -struktur auf das zweite Lebensjahr verlegen. In der Schweiz habe ich beide Gefiederplymrphismen bei Jungv6geln, ein-, zwei- und dreij~ihrigen V6geln quantifiziert. Bei M~innchen und Weibchen wurde das Auslichten der Gefiederfarbe nut zwischen dem 1. und 2. Lebensjahr bebachtet. M~nnchen wurden weniger dicht gefleckt nur zwischen dem 1. und 2. Lebensjahr, w~ihrend Weibchen zwischen dem Nestlingstadium und dem 1. Lebensjahr weniger gefleckt wurden und zwiscben dem 1. und 2. Lebensjahr wieder mehr Flecken hatten. Weibchen wurden als gleich gefleckt im 2. und 3. Lebensjahr. Um die erneuerten Federn zu erkennen, wurden bei Weibchen kleine Federstticke herausgeschnitten. Nach einer vllst~indigen Emeuemng der Bmstfedern hatte sich die Farbe und die Anzahl Punkte bei alten Weibchen nicht ge~indert. Intrductin In many bird species yearling males display fe- male r juvenile plumage attributes befre fully prducing a plumage sex-trait at tw years f age r even later. The plumage f U.S. Cpyright Clearance Center Cde Statement: 0021-8375/99/14002-0193 $ 11.00/0
194 Jurnal ftir Omithlgie 140,! 999 adults is cnsidered t be the standard and yearling males are said t delay its maturatin. Mst hyptheses put frward t explain the adaptative value f delayed plumage maturatin assume that the benefits accruing frm a fully expressed sex-trait utweigh the csts f wearing r prducing it nly in male adults (review in Muehter et al. 1997). This view hlds fr dichrmatic species where almst all adult males have a different plumage frm adult fe- males (Butcher & Rhwer 1988), but perhaps nt in plymrphic species where the tw sexes display a plumage trait in the same range f pssible values but are n average differently feathered. In that case, males may gain fitness benefits by delaying the prductin f the typical male-plumage pattern while females may d s by delaying the prductin f the typical female-plumage pattern. In plymrphic species it is thus f interest t test whether males and/r females change in plumage characteristics between years. The ventral bdy area f Barn Owls varies frm dark-reddish brwn t white, and frm heavily marked with black spts t immaculate. The expressin f bth traits has a genetic basis, but was nt fund t be influenced by the envirnment and bdy cnditin (Rulin et al. 1998). Althugh members f the tw sexes exhibit the tw traits in the same range f pssible values, males are cmmnly lighter clured and less sptted than females (Rulin 1996). Here, I examine whether sexual dimrphism in the tw plumage traits is already marked amng nestlings and whether within individuals the tw traits change frm the nestling stage t the third year. Fr this purpse birds frm a free-living Swiss ppulatin were captured and their plumage characteristics quantified. General methd Methds The study was carried ut frm 1994 t 1998 in western Switzerland. The study area f 190 km 2 was prvided with I l0 nestbxes fastened n barns, allwing the capture f breeding Barn Owls and their ffspring. Adult females were distinguished frm adult males by the presence f a brd patch at the time f incubatin. T test whether nestlings are already sexually dimrphic with respect t plumage clratin and spttedness, the sex f 75 nestlings frm 39 randmly chsen families was determined in 1996 using the laparscpy methd ~Richner 19891. Thirty-six nestlings belnging t 27 families were males, and 39 thers belnging t 27 families were females. T check whether within individuals the expressin f plumage clratin and plumage spttedness changes between years. I cnsidered birds captured in at least tw successive years when they were nestlings, 1-. 2- r 3-year-ld breeders. Age was determined with precisin by cnsidering nly birds ringed as nestlings (35 different males and 21 different females ~. r birds that multed the 6th primary (8 different males and 5 different females/, a criteria used t determine tw-year-ld individuals (Taylr 1993. Balker 1993, pers. bs.). The assessment f plumage clratin and plumage spttedness T recrd plumage clratin I cmpared the breast, belly, flank and underside f the wing clratin with eight numbered clur chips ~ 1 fr dark reddish-brwn t 8 fr white The mean value frm the fur bdy parts prvided an index f verall plumage clratin. T recrd plumage spttedness I cunted the number f spts within a 60 x 40 mm frame n the same bdy parts. The aumbers fund n the tw flanks, r n the tw wings resp.) were averaged, and the mean number f spts n the fur bdy regins gave an index f verall plumage spttedness. Crrelatins between tw measurements f the same individuals taken within the same seasn demnstrate that the methds f assessing plumage clratin ISperman rank crrelatin, nestlings: rs = 0.92. n = 68. p < 0.001. breeding males: r~ = 0.92. n = 31. p < 0.001: breeding females: rs = 0.95. n = 125, p < 0.001) and plumage spttedness are repeatable cpearsn crrelatin, nestlings: r = 0.90. n = 68. p < 0.001: breeding males: r = 0.96. n = 31. p < 0.001 : breeding females: r = 0.88, n = 125. p < 0.001). Mult and change in plumage characteristics T recgnize feathers f the ventral bdy area still present frm the year befre, I cut ut a small piece f the feathers f 33 females breeding in 1997. This
A. Rulin. Plumage Maturatin in the Barn Owl 195 peratin was carried ut when their ffspring were n average 6 days f age. The same was dne n 58 nestling females at the age f 55 days (fr the methd used t sex nestlings see Rulin et al. 1998). In 1998 1 clsely checked the state f the feathers f the (re)nesting females n tile 20th days f incubatin. I assumed that a bird did nt mult between 1997 and 1998 when all feathers present in 1998 had been cut in 1997. A bird was cnsidered t have partially multed when a few feathers present in 1998 had been cut in 1997 and t have multed cmpletely when all feathers had been renewed. The mutilatin f the feathers had n detectable harmful effect at least in breeding females. Mre treated females (94%) prduced fledglings in 1997 than thse that were nt marked frm 1990 t 1996 (306 ut f 346 (88%) breeding attempts resulted in the prductin f fledglings). The prprtin f females that renested the year after a capture was the same fr manipulated females (52%) as fr unmanipulated nes (data frm 1994 t 1996:93 ut f 174 captured females renested the year after, 53%). _ & = E. 60,50 40 3O Male Female Statistics AI1 tests have been cmputed with the Systat statistical package (Wilkinsn 1989). When tw r mre same-sex siblings appeared in the same statistical analysis I calculated their mean trait value t avid pseudreplicatin, i.e. t ensure that all gentypes have the same weight in the analyses. T test whether males and females changed in plumage clratin (r else in plumage spttedness) between tw successive years I cmputed a repeated measure ANOVA with the plumage characteristics fund in the tw years as repeated measures and with sex as a factr. A significant interactin between the repeated measures and the sex factr indicated that members f ne sex changed mre strngly in plumage characteristics than thse f the ther sex. Statistics are tw-tailed and the significance level is 0.05. Results Sexual dimrphism at the nestling stage Nestling males were n average lighter clured than nestling females (Student t-test: t = 3.48, df= 52, p = 0.001; Fig. la), and less sptted (t = -5.31, df= 52, p < 0.001; Fig. lb). Male Female Fig. 1. Plumage clratin (a) and plumage spttedness (b) f nestling males and females. Bars shw mean _+ ne standard errr. In Figure a the clur scre 1 is fr dark reddish-brwn and the clur scre 8 fr white. In figure b the numbers indicate the mean number f spts n the breast, belly, flank and underside f the wings. Abb. 1. Gefiederfarbe (a) und Gefiederfleckung (b) yn msnnlichen und weiblichen Nestlingen. Die Abbildungen zeigen Mittelwerte _+ Standardfehler. In Abbildung a reicht die Skala yn 1 (rstbraun) bis 8 (wei6) und in Abbildung b yn wenig bis dicht gefleckt. Thus, as nestlings Barn Owls are already sexually dimrphic with respect t plumage clratin and plumage spttedness. Age- and sex-related changes in plumage characteristics Frm the nestling t the first-year stage males and females did nt change in plumage clratin (repeated measures: F1,21 = 0.59, p = 0.45; interactin: F1,21 -- 0.56, p = 0.47). Frm the
196 Jurnal ffir Ornithlgie ~ 40, 1999 0.9 0.7 _~ 05 8 g 0.3 E ~- 0.1 g = -0.1-0.3-0.5 10-5- g 0- E -~ -5- g = -10J 14 27 15 19 14 NS NS first- t the secnd-year stage bth males and females became significantly lighter clured (repeated measures: F1,40 = 8.32, p = 0.006; interactin: FI,40 = 0.87, p = 0.36). Frm the secnd t the third year f life n change in plumage clratin was bserved (repeated measures: F1,31 = 0.07, p = 0,79; interactin: F1,31 = 0!55, p = 0.46). The figure 2a illustrates these results. Frm the nestling t the first-year stage females tended t becme less sptted than males (repeated measures: F1,21 = 1.02, p = 0.32; interactin: F1,21 = 3.92, p = 0.06). Frm the firstt the secnd-year stage nly females became significantly mre sptted (repeated measures: F1,40 = 15.97, p < 0.001; interactin: F1,40 = 20.21, p < 0.001), and frm the secnd t the third year birds did nt change significantly in plumage spttedness (repeated measures: F1,31 = 0.74, p = 0.40; interactin: F1,3~ = 0.45, p = 0.51). The figure 2b illustrates these results. -15 ~ 0-1 1-2 2-3 Fig. 2. Change in plumage clratin (a) and plumage spttedness (b) between the nestling stage and the first-year stage (0-1), the first- and secnd-year stage (1-2) and the secnd and third year f life (2-3). Bars shw mean ± ne standard errr. The hrizntal line indicates n change in plumage characteristics, and psitive values birds that became either lighter clured r mre sptted. Numbers at the tp f figure a indicate sample size. A paired t-test tested whether individuals f ne sex changed in plumage characteristics between tw successive years (NS: p > 0.05, *: p < 0.05, **: p < 0.01, ***: p < 0.001). Abb, 2. )knderungen der Gefiederfarbe (a) und Gefiederfleckung (b) zwischen dem Nestlingsstadium und dem 1. Lebensjahr (0-t), zwischen dem 1. und 2. Lebensjahr (1-2) und zwischen dem 2. und 3. Lebensjahr (2-3). Die Abbildungen zeigen Mittelwerte + Standardfehler. Die hrizntale Linie zeigt keine Anderungen beim Gefieder, und psitive Werte stellen Gefieder dar, das heller der dichter gefleckt wurde. Die Zahl fiber den Sfiulen geben die Stichprbengr/Sge an. Mit einem gepaarter t-test wurde geprfift, b sich die Gefiedereigenschaften vn Individuen eines Geschlechts zwischen zwei aufeinander flgenden Jahren ~nderten (NS: p > 0.05, *: p < 0.05, **: p < 0.01, ***: p < 0.001). Mult and change in plumage characteristics Tw ut f 58 nestling females whse feathers [ ntched in 1997 bred in 1998. Between these tw years they did nt mult. In 1997 and 1998 their mean plumage clratin was 3,81 and 3.75 respectively, and their mean plumage spttedness 63 and 55.5 respectively. Seventeen ut f 33 adult females with feathers ntched in 1997 renested in 1998. One f them was tw-year-ld in 1997 and the thers lder. All birds either partially r ttally multed. The breast was the bdy part where feathers had been mst ften all renewed. T test whether adult females changed in plumage traits after a cmplete mult I therefre cmpared the 1997 and 1998 breast plumage characteristics f the birds that renewed all feathers f this bdy regin: plumage clratin t median in bth years = 6.0: Wilcxn matched-pairs signed-rank test: z = 1.0, n = 12. p = 0.32) and plumage spttedness did nt change significantly between 1997 and 1998 ~mean ± ne standard deviatin in 1997 and 1998:36 ± 23
A. Rulin. Plumage Maturatin in the Barn Owl 197 and 40 _+ 28, respectively; paired t-test: t = 0.87, df = 11, p = 0.40). Discussin Sexual dimrphism in plumage clratin and spttedness was already marked amng nestlings. Frm the nestling stage t the first-year f life females were nt seen t mult partially r cmpletely. Thus, the bservatin shwing that yearling birds retained the plumage clratin they had at the nestling stage demnstrates that in ne year feather abrasin is nt sufficiently far advanced t cause a change in this plumage trait. With respect t plumage spttedness nly females had less spts at the first-year stage than at the nestling stage because they had lst sme sptted feathers (Tay- 1r 1993) maybe fr instance t frm the brd patch. Frm the first- t the secnd-year stage males and females became lighter clured t a cmparable extent (see Taylr 1993, Rulin 1996 fr similar results), and nly females became mre sptted. The change in plumage clratin was prbably due t a renewal f dark feathers by lighter clured nes as suggested by the bservatin f a yearling female that was multing the whle bdy underside in late summer 1998. The mechanism with which females became mre sptted in the secnd year is unknwn. Maybe females had mre feathers than the year befre. Alternatively, a larger prprtin f the feathers may have a spt, r each feather may have mre spts at the secnd- than the first-year stage. Frm the secnd t the third year birds did nt change in plumage characteristics. At subsequent ages females at least did changed neither in plumage clratin nr in plumage spttedness in spite f a cmplete mult. Thus, at this time the tw traits appeared t be stable and their prductin is prbably nt cnditin-dependent as already fund in nestlings (Rulin et al. 1998). In ther wrds, the amng-individuals variance in plumage clratin and spttedness has a very strng genetic basis. This further suggests that these traits are genetic plymrphisms. In cnclusin, the mst intense changes in the tw sex-limited traits arse between the first- and secnd-year f life after a mult as in mst passerines (Butcher & Rhwer 1988). Males and females became lighter clured and nly females mre sptted. Acknwledgements I thank Anne-Lyse Ducrest, Martin Epars and Henri Etter fr their help in the field. Heinz Richner determined the sex f the 75 nestlings with the laparscpy methd under authrisatin n 956 delivered by the "Services vdt6rinaires des Cantns de Vaud et Friburg". The mdificatin f the feathers was made by the permissin f the "Service vdt&inaire du cantn de Vaud, n 1146". Philipp Heeb and an annymus referee prvided helpful suggestins n a first draft f this paper. Stephanie Fauser helped me t translate the summary and figure captins int German. References Baker, K. (1993): Identificatin Guide t Eurpean Nn-Passerines. BTO Guide 24. Tring, Lndn. Butcher, G. S., & Rhwer, S. (1988): The evlutin f cnspicuus and distinctive clratin fr cmmunicatin in birds. Curr. Ornithl. 6: 51-108. Muehter, V. R., Greene, E. & Ratcliffe, L. (1997): Delayed plumage maturatin in Lazuli buntings: tests f the female mimicry and status signalling hyptheses. Behav. Ecl. Scibil. 41 : 281-290. Ricbner, H. (1989): Avian laparscpy as a field technique fr sexing birds and an assessement f its effects n wild birds. J. Field Ornithl. 60: 137-142. Rulin, A. (1996): Dimrphisme sexuel dans la clratin du plumage chez la Chuette effraie (Tyt alba). Ns Oiseaux 43: 517-526. Rulin, A., Richner, H. & Ducrest, A.-L. (1998): Genetic, envirnmental and cnditin-dependent effects n male and female plumage rnamentatin in the barn wl Tyt alba. Evlutin 52: 1451-1460. Taylr, I. R. (1993): Age and sex determinatin f Barn Owls Tyr alba alba. Ring. Migr. 14: 94-102. Wilkinsn, L. 1989. SYSTAT: the system fr statistics. SYSTAT, Inc., Evanstn, IL. Accepted: 15 Nvember 1998