Here I report on repeatability of clutch size. This study was carried out during the breeding season (first and second clutches) of 2003 in the village Mokrice (46°00’N, 15°55’E; NW Croatia). Repeatability of clutch size in first and second clutches (within-year) of female starlings was low and not significant.U ovome su radu prikazani rezultati istraživanja ponovljivosti veličine pologa unutar jedne godine (prvi i drugi polog) u ženke čvorka. Uzorkovanje je obavljeno u razdoblju gniježđenja 2003. godine na području sela Mokrice (46°00’N, 15°55’E; sjeverozapadna Hrvatska). Ponovljivost veličina pologa je niska i nije statistički značajna

Z. Dolenec

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short communication / kratko priop}enjeREPEATABILITY OF CLUTCH SIZE IN FEMALESTARLINGS (STURNUS VULGARIS)ZDRAVKO DOLENECDepartment of Zoology, Faculty of Science,University of Zagreb, Rooseveltov trg 6, HR–10000 Zagreb, Croatia(e-mail: dolenec@zg.biol.pmf.hr)Dolenec, Z.: Repeatability of clutch size in female starlings (Sturnus vulgaris). Nat. Croat.,Vol. 14, No. 4., 357–361, 2005, Zagreb.Here I report on repeatability of clutch size. This study was carried out during the breeding sea-son (first and second clutches) of 2003 in the village Mokrice (46°00’N, 15°55’E; NW Croatia). Re-peatability of clutch size in first and second clutches (within-year) of female starlings was low andnot significant.Key words: starling, Sturnus vulgaris, clutch size, repeatabilityDolenec, Z.: Ponovljivost veli~ine pologa u `enki ~voraka (Sturnus vulgaris). Nat. Croat., Vol.14, No. 4., 357–361, 2005, Zagreb.U ovome su radu prikazani rezultati istra`ivanja ponovljivosti veli~ine pologa unutar jednegodine (prvi i drugi polog) u `enke ~vorka. Uzorkovanje je obavljeno u razdoblju gnije`|enja 2003.godine na podru~ju sela Mokrice (46°00’N, 15°55’E; sjeverozapadna Hrvatska). Ponovljivost ve-li~ina pologa je niska i nije statisti~ki zna~ajna.Klju~ne rije~i: ~vorak, Sturnus vulgaris, veli~ina pologa, ponovljivostINTRODUCTIONTheories of clutch size determination have focused on first broods or on seasonaltrends in clutch size. Clutch size is controlled by combination of genetic, ontologi-cal and environmental factors (KENNEDY & WHITE, 1991). In recent years, an in-creasing number of studies have demonstrated the existence of significant repeat-ability in ecologically important traits in natural bird populations (e.g. GOODBURN,1991; JERZAK et al., 2000; AMAT et al., 2001; YOSEF & ZDUNIAK, 2004). Repeatabilityexpresses the proportion of the variation between measurements that is due to con-sistent differences between the objects measured (HÕRAK et al., 1995). According tovan NOORDWIJK et al. (1980), no single optimum value exists for clutch size. In someyears small clutches are optimal, in other years large clutches contribute more toNAT. CROAT. VOL. 14 No 4 357¿361 ZAGREB December 31, 2005Croatian Natural History Museum, Demetrova 1, Zagreb, Croatiathe next generation. The variation probably mirrors the range of values that are atsome moment or at some place more productive than others.The aim of this study is to show the effect of environmental factors on egg num-ber using repeatability estimates.STUDY AREA AND METHODSThis study took place in the village Mokrice (46°00’ W, 15°55’ E), in NW Croatia.My studies comprise the period from March to July 2003. The birds bred in nestboxes placed at a height of about 2.5–5 m. The dimensions of the nest-box were ap-proximately 16x16x25 cm, the diameter of the entrance hole being 4.5–5.0 cm. Thestudy included first and second clutches. Nest-boxes were checked regularly to de-termine clutch size. Analyses are conducted only on females; the same female onthe same territory. I ringed adult females with numbered metal rings (Fig. 1). Clutch358 Dolenec, Z.: Repeatability of clutch size in female starlingsFig. 1. Ringed starling female (Photo Z. Dolenec)size was assigned when the same number of eggs was recorded on two consecutivevisits to the nest. In this study population, starlings usually produce two regularbroods per year (DOLENEC, 1997). Repeatability is a measure used in quantitativegenetics to describe the proportion of variance in a character that occurs amongrather than within individuals (LESSELLS & BOAG, 1987). Repeatability, r, is given by:r = (VG + VEg)/ VP,where VG is the genotypic variance, VEg the general environmental variance, andVP the phenotypic variance (FALCONER, 1981). Repeatability (or intra-class correla-tion coefficients) can also be calculated using the among-group variance component(S²A) and the within-group variance (S²), derived from one-way analysis of variance(ANOVA, see SOKAL & ROHLF, 1981; LESSELLS & BOAG, 1987):r = S²A/ (S² + S²A)These variance components are calculated from the mean squares in the analysisof variance as:S² = MSWandS²A = (MSA – MSW) / n0,where n0 is a coefficient related to the sample size per group in the analysis of vari-ance. The value of n0 is calculated as:n0 = 1 / (a–1) ni – (ni² / ni),where a is the number of groups and ni is the sample size in the ith group.Standard errors of the repeatability estimates were calculated following BECKER(1984). Statistical analyses were performed using the SPSS 12.0 statistical package.P-values higher than 0.05 were considered non-significant.RESULTS AND DISCUSSIONClutch size characteristics of starlings in the present study site are presented inTab 1. The repeatability of clutch size in first and second clutches of female star-Nat. Croat. Vol. 14(4), 2005 359Tab. 1. Average clutch size in a single breeding season in starlings (same females andsame territory) in 2003 (SD = standard deviation)First clutch Second clutchParameter Mean SD N Mean SD NClutch size 5.83 0.72 12 5.00 0.74 12Tab. 2. Within-year repeatability of clutch size in individual female starlings in 2003 (r =repeatability; SE = standard error; s² = within-female variance; s²A = among-females variance)Parameter S² S²A r (SE) n0 F df pClutch size 0.583 0.11 0.16 (0.11) 2.00 1.377 11;12 0.295lings in 2003 was low (r = 0.16) and not significant (ANOVA, F11,12 = 1.377, P > 0.05;Tab. 2). This suggests that most of the variation in clutch size was due to intra-indi-vidual variation in response to environmental conditions. Females in the housewren (Troglodytes aedon) showed relatively low but significant consistency in theirclutch sizes across broods (KENNEDY & WHITE, 1991). Similarly, relatively low re-peatability of clutch size has been reported by other authors (e.g. MURPHY, 1978). Insome bird species we can also see different results. In the magpie (Pica pica), repeat-ability was highly significant for clutch size of a subsequent breeding attempt with-in a single season when the first breeding attempt had failed (GOODBURN, 1991).Unfortunately, the genetic component of variation is inseparable from that re-sulting from general environmental effects; because of this the value of repeatabilitymay be treated only as a first indication of genetic variation, or an upper limit ofcorresponding (unknown) heritability (BAÑBURA & ZIELIÑSKI, 1990).Received October 28, 2004REFERENCESAMAT, J. R., FRAGA, R. M. & ARROGO, G. M., 2001: Variations in body condition and egg char-acteristics of female Kentish Plovers Charadrius alexandrinus. Ardea 89, 293–299.BAÑBURA, J. & ZIELIÑSKI P., 1990: Within-clutch repeatability of egg dimensions in the Black--headed Gull Larus ridibundus. J. Orn. 131, 305–310.BECKER, W. A., 1984: Manual of guantitative genetics. Academic Enterprises, Pullmain, Wash-ington.DOLENEC, Z., 1997: Supplement to nesting habits of Starlings (Sturnus vulgaris L.) on the terri-tory of North-West Croatia. Larus 46, 120–125.DOLENEC, Z., 2005: Within-year repeatability of egg parameters in female starlings Sturnusvulgaris. Isr. J. Zool. 51(1), 53–55.FALCONER, D. S., 1981: Introduction to quantitative genetics. Longman, London.GOODBURN, S. F., 1991: Territory quality or bird quality? Factors determining breeding successin the Magpie Pica pica. Ibis 133, 85–90.HÕRAK, P., MÄND, R., OTS, J. & LEIVITS, A., 1995: Egg size in the Great Tit Parus major individ-ual, habitat and geographic differencces. Ornis Fennica 72, 97–114.JERZAK, L., BOCHEÑSKI, M., KUCZYÑSKI, L. & TRYJANOWSKI, P., 2000: Repeatability of size andshape of eggs in the urban Magpie Pica pica (Passeriformes: Corvidae) population. Actazool. Crac. 43, 165–169.KENNEDY, E. D. & WHITE, D. W., 1991: Repeatability of cluch size in House Wrens. WilsonBull. 103, 552–558.LESSELLS, C. M. & BOAG, P. T., 1987: Unrepeatable repeatabilities: A common mistake. Auk104, 116–121.MURPHY, E. C., 1978: Seasonal variation in reproductive output of House Sparrow: the deter-mination of clutch size. Ecology 59, 1189–1199.VAN NOORDWIJK, A. J., VAN BALEN, J. H. & SCHARLOO, W., 1980: Heritability of ecologicallyimportant traits in the Great Tit. Ardea 68, 193–203.SOKAL, R. R. & ROHLF, F. J., 1981: Biometry. Freeman and Co., San Francisco.YOSEF, R. & ZDUNIAK, P., 2004: Within-clutch variation in egg dimensions of LoggerheadShrike (Lanius ludovicianus) in south-central Florida. Biological Lett. 41(2), 155–162.360 Dolenec, Z.: Repeatability of clutch size in female starlingsSA@ E TAKPonovljivost veli~ine pologa u `enki ~voraka (Sturnus vulgaris)Z. DolenecU kvantitativnoj genetici ponovljivost daje gornju granicu stupnja geneti~ke od-re|enosti pojedinog obilje`ja. Iz rezultata istra`ivanja proizlazi niska ponovljivostveli~ine drugog pologa u odnosu na prvi (r = 0.16; iste `enke na istom teritoriju) istatisti~ki nije zna~ajna (ANOVA, p > 0.05). Niska ponovljivost veli~ine pologa sugerirave}i utjecaj ~imbenika stani{ta (»territory quality«) u odnosu na utjecaj `enke (»femalequality«). Istra`ivanja ponovljivosti dimenzija jaja istih `enki dala su suprotne re-zultate – zna~ajniji utjecaj `enki (DOLENEC, 2005).Nat. Croat. Vol. 14(4), 2005 361

Ponovljivost veličine pologa u ženki čvoraka (Sturnus vulgaris)

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