Within-clutch repeatability of egg dimensions in the jackdaw Corvus monedula: a study based on a museum collection

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1 Biologia, Bratislava, 56/2: , 2001 Within-clutch repeatability of egg dimensions in the jackdaw Corvus monedula: a study based on a museum collection Piotr Tryjanowski 1, Lechos law Kuczyński 2, Marcin Antczak 1, Maciej Skoracki 2 & Martin Hromada 3 1 Department of Avian Biology and Ecology, Institute of Environmental Biology, Adam Mickiewicz University, Fredry 10, PL Poznań, Poland; ptasiek@amu.edu.pl 2 Department of Animal Morphology, Institute of Environmental Biology, Adam Mickiewicz University, Szamarzewskiego 91 A, PL Poznań, Poland; lechu@amu.edu.pl 3 Department of Natural History, Šarišské Museum, Radničné nám. 13, SK Bardejov, Slovakia; sarmus@nextra.sk Tryjanowski, P.,Kuczynski, L.,Antczak, M.,Skoracki, M.&Hromada, M., Within-clutch repeatability of egg dimensions in the jackdaw Corvus monedula: a study based on a museum collection. Biologia, Bratislava, 56: , 2001; ISSN Variation in size and shape of the jackdaw (Corvus monedula) eggs was studied on egg s museum collection data from Bardejov region, NE Slovakia, from 1961 to Mean clutch size was 4.14 ± 1.29 (n = 125). Coefficients of variation for clutch means ranged from 1.67 (breadth) to 4.85 (volume). Mean repeatability estimates were 0.54, 0.64, 0.60, 0.57 for length, breadth, volume and elongation index, respectively. The results suggest that in the studied population one should expect relatively low or intermediate heritability of egg dimensions. Key words: Corvus monedula, egg size, clutch size, repeatability, breeding biology. Introduction Many birds, including the Passerines, produce eggs on the basis of the daily gain of necessary nutrients (Perrins, 1996). As a consequence, the abundance and availability of nutrients is likely to set an upper constraint on egg size by influencing the process of egg formation (Murphy & Haukioja, 1986). However, only in very few species the effect of extra food provisioning on egg mass or egg size was shown (for review see Meier & Drent, 1999). This suggests that both clutch size and egg mass are more or less independent on female body reserves, and her actual condition. Nevertheless, the size of an egg from which a bird hatches may be important in determining fledgling s body size and condition, its probability of survival and ultimately its reproductive success (Schifferli, 1973; Williams, 1994). Consequently, it is of interest to determine, whether egg size traits in wild populations respond to natural selection. For selection to influence egg-size evolution, a portion of phenotypic variation must be heritable (Fisher, 1937). Obtaining the necessary measurements for determining heritability (h 2 ) values is difficult in case of many bird species in the wild (Boag & van Noordwijk, 1987; Banbura & Zielinski, 1990). 211

2 However, repeatability is relatively easy to estimate and provides an upper limit estimate for the degree of genetic determination and heritability (e.g. Boag & Van Noordwijk, 1987; Banbura & Zielinski, 1990; Falconer & Mackay, 1995). Large repeatability often indicates a significant heritable component to the phenotypic variance (Boag & van Noordwijk, 1987). Banbura & Zielinski (1998) showed that, despite the repeatability is certainly a worse measure of genetic variation than heritability, it seems to be as good or even better at measuring environmental components of variation. In this study we investigated the degree to which a phenotypic variation in traits associated with egg size (length, breadth, volume, and elongation) may be genetically determined in the population of the jackdaw (Corvus monedula Linnaeus, 1758). The jackdaw oology, despite the common occurrence of the species in Europe, was studied more extensively only in Spain. Soler (1988a, b) gave a description of egg variation in size and coloration, as well as discussed an adaptive significance of this variation. However, he was unable to demonstrate effects of experimental food provisioning on egg volume (Soler & Soler, 1996). The aim of this study is to show the effect of environment (in sense of Banbura & Zielinski, 1998) on egg measurements using repeatability estimates. We discuss possibilities of obtaining such estimates based on museum egg collections. Material and methods Between 1961 and 1964, 125 (3 in 1961, 8 in 1962, 108 in 1963 and 6 in 1964) jackdaw clutches were studied near Bardejov, NE Slovakia, (49 03 N N, E E). Each clutch was taken as full and after preparation located in museum collection. For all clutches information about date, place, and clutch size were recorded. Additional information, for instance, incubation stage or clutch size completeness were also gathered. All clutches were collected by Tibor Weisz, founder of Natural History Museum in Bardejov, Slovakia. The clutches were collected in jackdaw colonies situated in large buildings, church towers and lofts. Egg length and breadth were measured to the nearest of 0.1 mm with callipers. Coulson (1963) gave a formula for jackdaw s egg volume estimation: V = (π K B 2 L)/6, where B is breadth and L is length (in mm). K is a constant derived from a sample of eggs whose volume is known ( for the jackdaw; Soler, 1988a). This formula, after simplification, gives the following equation: V = B 2 L. An index of egg elongation was calculated by dividing egg length by egg breadth. To avoid problem of pseudoreplication, egg size was defined as the mean volume of all eggs laid in each nest (Soler & Soler, 1996). Within-clutch coefficient of variation (CV )ofegg measurements was calculated according to the formula: CV = s 100 Y where s is the standard deviation and Y is the mean of the egg measurement within the clutch. The coefficient of variation calculated with the above formula is known to be biased, especially for small sample sizes (Sokal & Rohlf, 1995). For this reason correction was applied: ( CV = 1+ 1 ) CV 4n where CV is the bias adjusted estimate of coefficient of variation and n is the clutch size. One-way analysis of variance (ANOVA) was carried out in order to obtain variance components. The repeatability were calculated as intra-class correlation coefficients (Sokal & Rohlf 1995), applying the formula: r =(MS A MS W )/[MS A +(n 0 1)MS W ] where MS A is the among-groups (i.e. between-clutch) mean square, MS W is the within-group mean square, and n 0 is a coefficient related to the sample size per group in ANOVA, given by: 1 n 0 = (a 1) [ n i ( n 2 i / n i )] where n i is the size in the i-th group and a is the number of groups (Lessells & Boag, 1987; Falconer & Mackay, 1995). Standard errors for repeatability values were calculated as described in Becker (1992). Mean values are given with standard deviations. Results Jackdaw clutches (n = 125) contained from 2 to 9 eggs. Mean clutch size was 4.14 ± 1.29 (mean ± S.D.), modal clutch size was 4 (Fig. 1). 1R RI FOXWFKHV &OXWFK VL]H Fig. 1. Frequency distribution of clutch size in jackdaw nests. 212

3 Table 1. Repeatability (R) of the jackdaw egg length, breadth, volume and elongation index. Trait F-ratio d.f. R 95% CI Length 5.77* 124, Breadth 8.02* 124, Volume 6.98* 124, Elongation 6.21* 124, Key: * all F-ratios (one-way ANOVA) are significant at P< (JJ YROXPH &9 >@ &OXWFK VL]H 0HDQ HJJ EUHDGWK 0HDQ HJJ OHQJWK Fig. 2. Correlation between within-clutches mean values of jackdaw egg length and egg breadth (egg dimensions in mm). Mean egg length was ± 1.78 mm (range mm), mean egg breadth ± 0.80 mm (range mm), mean egg volume ± 0.99 cm 3 (range cm 3 ) and mean elongation index ± (range ) (n = 518). The greatest within-clutch variation was recorded for the egg volume (CV* = 5.16 ± 2.93), then for the elongation index (CV* = 3.40 ± 1.83), egg length (CV* = 3.23 ± 1.67) and egg breadth (CV* = 1.78 ± 1.06). Mean repeatability estimates were 0.54, 0.64, 0.60, 0.57 for length, breadth, volume and elongation index, respectively (Tab. 1). The coefficient of within-clutch variation differed significantly for egg dimensions (Friedman ANOVA; χ 2 = 163.5, d.f. = 3, P < ). Correlation coefficient between mean egg length and mean egg breadth was significantly different from zero (r = 0.22, n = 125, P = ) (Fig. 2). To test if the trade-off between egg size and clutch size exist, one-way ANOVA was performed. No significant differences in egg dimensions were found between clutch size classes (F 6,118 = 1.38, P = ; F 6,118 = 0.40, P = ; F 6,118 = Fig. 3. Relationship between jackdaw clutch size and within-clutch coefficient of variation in egg volume. Points represent means, bars standard errors. 0.26, P = ; F 6,118 = 2.00, P = for egg length, breadth, volume and elongation index, respectively). Significant differences between withinclutch coefficient of variation for different clutch size classes were found for egg volume (Kruskal- Wallis ANOVA, χ 2 = 16.4, P = ). The larger the clutch size, the greater the variation in egg volume (r s = 0.33, P = , n = 125) (Fig. 3). Discussion The mean clutch size recorded in this paper is slightly smaller than expected by the model of Soler & Soler (1992) (4.2 vs. 4.7 for latitude 49 ) and smaller than the mean clutch size recorded in Europe (Chesney, 1986; Dwenger, 1989; Heeb, 1994; Strebel, 1991; Soler & Soler, 1992; and papers cited there). It is possible, that beside the local environmental conditions, the main reason of above differences could be the way of data acquisition. In this paper we used the museum collection of eggs without any knowledge about its reliability. Mean egg length, breadth, volume and elongation index recorded in the studied population are similar to that reported by other authors in different parts of the jackdaw breeding range (Soler & Soler, 1992; Dwenger, 1989). Mean egg volume was slightly (10.1 vs cm 3 ) smaller than estimated by the model of Soler & Soler (1992). The difference seems to be not biologically significant and may arise from local and temporal changes of weather or food resources. These factors probably influence egg volume in greater extend than clutch size and thus estimation of egg 213

4 measurements from any model may be seriously biased. Egg measurements differ in their withinclutch variation. The greatest variation was recorded for egg volume, the least for egg breadth. This observation seems to be a more general rule and was reported for many bird species (review in Ojanen, 1983; Banbura, 1996). The low variation in egg breadth may arise from the fact that this parameter is in greatest extend constrained by the female reproductive organs structure (oviduct diameter) (van Noordwijk et al., 1981). Thus, maximisation of egg volume may be easier achieved by maximisation of egg length, which is less forced by physiological or mechanical conditions. The evolution of egg dimensions is complicated by the fact that both egg length and egg breadth may be under strong selection acting for each trait independently and possibly in opposite directions (Chylarecki, 1993; Banbura, 1996; Chylarecki et al., 1997; Barta & Szekely, 1997). The greatest variation in egg volume and shape is probably due to the variance accumulation arisen from computational formulas. Both parameters are calculated in such a manner that they inherently contain cumulated variance of input variables. The phenomenon of greatest within-clutch variation recorded for larger clutches may originate from the fact that in most bird species females allocate exogenous resources in egg production. For larger clutches the egg production period is longer. Fluctuating environmental conditions influence egg dimensions resulting in greater within-clutch variation. As the environment, especially feeding conditions, influence mainly egg volume, for this feature relationship between clutch size and within-clutch variation could be shown. The repeatability estimates for egg parameters of jackdaw ( ) are similar to values calculated for other passerine birds (Boag &Van Noordwijk, 1987; Banbura, 1996). There is lack of data concerning repeatability of jackdaw egg dimensions from other populations. We believe that this kind of information is very useful for estimating the amount of variation that could be associated with environmental influence. Our results and conclusions are drawn from the sample of eggs from a museum collection. The use of such kind of data has many limitations and drawbacks. In some cases there is lack of data, e.g. sample size had to be reduced due to unknown clutch size. Whole clutches were taken by collector and there is no possibility to estimate breeding success or other reproductive parameters. Nevertheless, we consider the studies on museum collections are very useful, particularly on such species like jackdaw, where acquisition of field data is difficult or even impossible. Specimens collected in museums, if acquired, prepared and handled properly, can provide large amount of good quality information. Acknowledgements We thank Daniel Grula and Jaroslav Ilek for help during our work in museum laboratory. Special thanks go to Tomáš Jászay for help and enabling studies in Šarišské Museum. The research was supported by grant UAM PBWB 2/99. References Banbura, J Zmienność wewnątrzpopulacyjna rozmiarów jaj dymówki Hirundo rustica [Intrapopulation variability of egg measurements in the Barn Swallow Hirundo rustica]. Wyd. U l, Lódź, 64 pp. Banbura, J.& Zielinski, P Within-clutch repeatability of egg dimensions in the Black-headed Gull Larus ridibundus. J. Ornithol. 131: Banbura, J.& Zielinski, P An analysis of eggsize repeatability in Barn Swallows Hirundo rustica. Ardeola45: Barta, Z.& Szekely, T The optimal shape of avian eggs. Funct. Ecol. 11: Becker, W. A Manual of quantitative genetics. Academic Enterprises, Pullman, Washington, 192 pp. Boag, P. T.& Van Noordwijk, A. J Quantitative genetics, pp In: Buckley, P. A. & Cooke, F. (ed.) Avian genetics, Academic Press, New York. Chesney, M. C The effects of poisoning by alpha-chloralose on the breeding success of a colony of Jackdaws Corvus monedula in Perthshire. Bird Study 33: Chylarecki, P Decomposing selection on avian egg size, p. 82. In: Proc. IVth Congr. Europ. Soc. Evol. Biol., Montpellier. Chylarecki, P.,Kuczynski, L.,Vorgin, M.&Tryjanowski, P Geographic variation in egg measurements of the Lapwing Vanellus vanellus. Acta ornithol. 32: Coulson, J. C Egg size and shape in the Kittwake (Rissa tridactyla) and their use in estimating the age and composition of populations. Proc. Zool. Soc. Lond. 140: Dwenger, R Die Dohle, Corvus monedula. Ziemsen Verlag, Wittenberg Lutherstadt, 148 pp. Falconer, D. S.& Mackay, T. C. F Introduction to quantitative genetics. Longmann, Harlow, UK, 464 pp. Fisher, R. A The relation between variability and abundance shown by the measurements of the 214

5 eggs of British nesting birds. Proc. Roy. Soc. Lond. B 122: Heeb, P Intraclutch egg-mass variation and hatching asynchrony in the Jackdaw Corvus monedula. Ardea82: Meier, T.&Drent, R A re-examination of the capital and income dichotomy in breeding birds. Ibis 141: Murphy, E. C.& Haukioja, E Clutch size in nidicolous birds. Curr. Ornithol. 4: Ojanen, M Significance of variation in egg traits in birds, with special reference to Passerines. Acta Univ. Oulu 154 A, Biol. 20: Perrins, C. M Eggs, egg formation and the timing of breeding. Ibis 138: Schifferli, L The effects of egg weight on the subsequent growth of nestling Great Tits Parus major. Ibis115: Sokal,R.R.&Rohlf, F. J Biometry. Freeman, New York, 887 pp. Soler, M. 1988a. Egg size variation in Jackdaw Corvus monedula in Granada, Spain. Bird Study 35: Soler, M. 1988b. Colour and shape of the eggshells of the Jackdaw, Corvus monedula, a quntitative study. Gerfaut 78: Soler, M. &Soler, J. J Latitudinal trends in clutch size in single brooded hole nesting bird species: a new hypothesis. Ardea 80: Soler, M.&Soler, J. J Effects of experimental food provisioning on reproduction in the Jackdaw Corvus monedula, a semi-colonial species. Ibis 138: Van Noordwijk,A.J.,Keizer,L.C.P.,Van Balen, J. H. & Scharloo, W Genetic variation in egg dimensions in natural populations of the Great Tit. Genetica 55: Williams, T. D Intraspecific variation in egg sizeandeggcompositioninbirds:effectsonoffspring fitness. Biol. Rev. 68: Received August 2, 2000 Accepted December 15,