J. Avian Biol. 42: 370373, 2011 doi: 10.1111/j.1600-048X.2011.05202.x # 2011 The Authors. J. Avian Biol. # 2011 Nordic Society Oikos Received 23 March 2010, accepted 1 February 2011 A sum of its individual parts? Relative contributions of different eggshell regions to intraclutch variation in birds Lenka Polac ˇikova ´, Mark E. Hauber, Petr Procha ´zka, Phillip Cassey, Marcel Honza and Toma ´s ˇ Grim L. Polac ˇikova ´ (lenka.polacikova@gmail.com) and T. Grim, Dept of Zool. and Lab of Ornithol., Palacky´ Univ., tr ˇ. Svobody 26, CZ 77146 Olomouc, Czech Republic. LP also at: Zool. Garden of Brno City, U Zool. zahrady 46, CZ 63500 Brno, Czech Republic and, Centre for Adv. Sudy (CAS), NO 0271 Oslo, Norway.  M. E. Hauber, Dept of Psychol., Hunter College of the City Univ. of New York, New York, NY 10065, USA and School of Psychology, Victoria Univ. of Wellington, PO Box 600, Wellington 6140, New Zealand.  P. Procha ´zka and M. Honza, Inst. of Vertebrate Biol., Acad. of Sci. of the Czech Republic, Kve ˇtna ´ 8, CZ 60365 Brno, Czech Republic.  P. Cassey, School of Earth and Environm. Sci. Univ. of Adelaide, SA 5005, Australia. The rejection of eggs of brood parasites in several species of hosts is based on cues only at the blunt pole (BP) and not at the sharp pole (SP) of the foreign and own eggshell. We investigated whether intraclutch variation is confined to a specific egg pole in species where the extent of intraclutch variation in the overall egg appearance is known to positively covary with either egg rejection rate or the probability of being parasitized. For the two poles separately, we analysed intraclutch variation of eggshell brightness and blue chroma. We quantified intraclutch variation as the standard deviations of these colour metrics, instead of their coefficients of variation which would represent a statistically flawed approach. Pooling measurements of brightness across the whole egg surface led to statistically non-significant results and masked positive correlations of BP brightness with egg rejection or parasitism risk, respectively. In contrast, patterns of blue chroma were important across the whole egg. Thus, the traditional whole egg ‘averaging’ approach may mask biologically important effects of intraclutch variation when the variation and potential signalling functions of egg appearance are confined to a specific egg part (brightness). However, analyses based on only BP and SP eggshell region specific data may also lack the power to detect effects of phenotypic traits that do not vary between egg poles (blue chroma). We advocate the use of a combination of region-specific and whole-eggshell based colour metrics and manipulations in cognitive, perceptual, and ecological studies of foreign egg rejection. Avian eggs are known to differ greatly in their appearance between species and higher taxonomic levels (Kilner 2006). Yet, typically, intraclutch variation in egg appearance is low  the eggs laid by individual females look remarkably similar within the clutch (Cassey et al. 2009). Functionally, low intraclutch variation in birds’ egg appearance might play at least two adaptive roles. First, in avian hostparasite coevolutionary systems, reduced colour variation within clutches could facilitate the rejection of foreign eggs laid by con- and/or heterospecific brood parasites (Soler and Møller 1996, Moska ´t et al. 2008). Second, egg colour variation within a clutch might affect the probability of being parasitized by brood parasites (Polac ˇikova ´ et al. 2009). Irrespective of the exact function, similarities of egg traits in previous studies of hostparasite coevolution were studied using mean trait (colour, maculation, reflectance spectra) values per each host egg when evaluating egg similarities within clutches. Yet, in most small passerines, maculation or speckling is concentrated at the blunt egg pole (hereafter BP) whereas the sharp egg pole (hereafter SP) is usually covered with very few or even no spots (Kilner 2006, Polac ˇikova ´ and Grim 2010). Thus, averaging heterogeneous regions across the whole egg surface might be misleading both with respect to characterising egg phenotype and understanding the salience of each egg region’s appearance regarding the discrimination of own vs foreign eggs by hosts (Polac ˇikova ´ et al. 2007, Honza and Polac ˇikova ´ 2008). Indeed, recent studies showed that various avian species use only cues at BP, but not those at SP, when rejecting foreign eggs (Lahti and Lahti 2002, Polac ˇikova ´ et al. 2007, 2010, Polac ˇikova ´ and Grim 2010). Therefore, it is necessary to address whether phenotypic variation between eggs is confined to specific parts of eggshells. Investigating each egg pole separately, we asked whether the averaging of the measurements across the whole egg surface (as done in all previous studies) is sufficient to identify the role of intraclutch variation in egg appearance. We studied species in which intraclutch variation across the whole egg had already been shown to: 1) influence behavioural responses to experimentally added conspecific 370