Abstract A positive correlation between egg size, early growth and nestling survival has been frequently reported in the ornithological literature. Albeit of interest, most of these studies did not determine whether the relationship between egg size, early growth and nestling survival was confounded by the quality of rearing conditions. How- ever, this is of importance in order to assess the extent to which a life-history trait like egg size causally affects fit- ness. In a colony of the alpine swift Apus melba, we cross-fostered complete clutches between nests to deter- mine the relative contribution of egg size and rearing condition on nestling growth and survival. In foster nests, nestlings that hatched out of larger eggs were signifi- cantly heavier at birth and at the age of 10 days; at 25 days, however, the relationship was no longer signifi- cant. The likelihood of a chick surviving from birth to 25 days of age was not correlated with its original egg size, but with the size of the eggs laid by its foster par- ents. This experiment therefore lends support to the hy- pothesis that in the alpine swift the relationship between egg size and nestling growth and survival is mainly due to a covariation between egg size and parental care rather than to a direct contribution of egg size. Keywords Apus melba · Coloniality · Cross-fostering experiment · Egg volume · Parental quality Introduction In a wide range of bird species, egg size is positively correlated with hatchling size, nestling growth and survival (Williams 1994). Two hypotheses can explain how such correlations can arise. First, since larger eggs usually contain a larger yolk sac, embryos have access to larger amounts of energy during early development (Birkhead and Nettleship 1982). At least in the first days after hatching, nestlings issued from larger eggs may therefore grow more rapidly and achieve a higher sur- vival (Nisbet 1973). Second, egg size may be correlated with parental or territory quality so that nestlings origi- nating from larger eggs may be in better condition be- cause parental care was of higher quality (Reid and Boersma 1990; Bolton 1991; Amundsen et al. 1996). For instance, more experienced females may have improved foraging abilities allowing them to simultaneously lay larger eggs and provision offspring at a higher rate. The relative importance of the two hypotheses can be deter- mined only through an experimental approach where eggs are swapped between randomly chosen nests (Williams 1994; Bernardo 1996). Indeed, cross-fostering experiments permit the separation of the potential co- variation between egg size and parental, territory or nest site quality from the effect of egg size on nestling growth and survival (Amundsen and Stokland 1990; Reid and Boersma 1990; Bolton 1991; Meathrel et al. 1993; Smith et al. 1995; Blomqvist et al. 1997; Hipfner and Gaston 1999; Styrsky et al. 1999; Hipfner 2000; Risch and Rohwer 2000). In the present study, we investigated the contribution of egg size and parental care to offspring growth and survival after having cross-fostered complete clutches between al- pine swift Apus melba nests. Swifts are altricial, and hence post-hatching parental care is crucial to ensure a high breeding success. We therefore predict that the eggs out of which nestlings hatched (hereafter referred to as ‘original-egg volume’) make a minor contribution to nes- tling growth and survival. In addition, if we assume that egg size reflects the quality of parental care, as shown in five other bird species (Amundsen and Stokland 1990; Reid and Boersma 1990; Bolton 1991; Magrath 1992; Risch and Rohwer 2000), we also predict that offspring growth and survival should correlate with the size of the P. Bize ( ✉ ) · H. Richner Department of Zoology, University of Bern, Division of Evolutionary Ecology, Baltzerstrasse 6, 3012 Bern, Switzerland e-mail: pierre.bize@esh.unibe.ch Tel.: +41-31-6313021, Fax: +41-31-6313008 A. Roulin Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK Oecologia (2002) 132:231–234 DOI 10.1007/s00442-002-0980-y POPULATION ECOLOGY Pierre Bize · Alexandre Roulin · Heinz Richner Covariation between egg size and rearing condition determines offspring quality: an experiment with the alpine swift Received: 4 March 2002 / Accepted: 12 May 2002 / Published online: 15 June 2002 © Springer-Verlag 2002