Intraclutch Egg-Size Variation in the Eurasian Kestrel: Advantages and Disadvantages of Hatching from Large Eggs

Egg size in birds may be positively correlated with hatching success (Jfirvinen and Vfiisfinen 1983, Magrath 1992a), hatchling mass (Magrath 1992a, Nilsson and Svensson 1993), and nestling growth and survival (Schifferli 1973, Moss et al. 1981, Williams 1990).Hence, egg size could influence offspring fitness (Williams 1994).Egg size varies among and within clutches, the former accounting for most of the variation (Ojanen et al. 1979, Jfirvinen and V•isfinen 1983, Wiggins 1990).However, variation in egg size within clutches may have greater fitness consequences than that among clutches because young that hatch from smaller eggs compete with larger siblings and have a higher risk of starvation (Nilsson and Svensson 1993).Generally, egg size varies with laying order following patterns that are typical for a species or population; e.g.increasing (Howe 1976, Mead and Morton 1985), decreasing (Parsons 1972, Nisbet and Cohen 1975), or increasing up to the middle egg and decreasing thereafter (Arnold 1991, Williams et al. 1993).These patterns may change relative to food supply (Simmons 1994), year (Jover et al. 1993), or clutch size (Vifiuela 1997) and may result from physiological or nutritional constraints on the female and/or be adaptive strategies to improve breeding Success.Several constraints could cause intraclutch variation in egg size.For example, egg size may increase with laying sequence because females are "gearing up" physiologically for starting egg production (Parsons 1976).Last-laid eggs may be smaller because prolactin blood levels, which increase with the onset of incubation, could reduce the size of developing follicles (Leblanc 1987).Egg size also could decline with laying sequence because energy reserves for egg formation are depleted throughout laying (Ryd•n 1978, Pierotti and Bellrose 1986).Finally, fluctuating environmental conditions at the time of egg laying associated with low temperatures (Ojanen et al. 1981, J•rvinen and Ylimaunu 1986, Magrath 1992b) or with changes in food availability (Jfirvinen and V•is•nen 1983) may cause variation in egg size independent of laying order.At least three hypotheses have been proposed to explain the function of intraclutch variation in egg E-mail: mcnam3e@mncn.csic.essize.First, variation in egg size relative to laying order may have evolved to influence the degree of sibling competition.Howe (1976) suggested that an increase in egg size with laying sequence is an adaptation to counteract the effect of asynchronous hatching, whereas a decline in egg size with laying sequence would accentuate the effect of hatching asynchrony and facilitate brood reduction (Slagsvoid et al. 1984).However, this hypothesis does not consider the costs of producing and incubating large eggs.Second, if the incubation period is longer for larger eggs, then egg-size variation could lead to differences in incubation periods that either facilitate or hinder brood reduction.In this case, a smaller last egg may reduce the degree of hatching asynchrony (Parsons 1972).Finally, egg-size variation may be adaptive because females allocate nutrients differentially based on the probability that an egg in a given position in the laying order will produce an offspring (Ankney 1980, Leblanc 1987, Williams et al. 1993, Vifiuela 1997).I performed a feeding experiment to test whether food supply determines the pattern of intraclutch egg-size variation in Eurasian Kestrels (Falco tinnunculus).Moreover, I analyzed costs and benefits of large eggs in relation to laying position in order to test the two supposed adaptive functions related with increasing/decreasing hatching span.Methods.--Thisstudy was conducted on a population of kestrels in the province of Cuenca, central Spain (40ø08'N, 02ø18'W).The study area is an agricultural plain cultivated mainly with cereals and sunflowers.Kestrels breed in natural rock cavities and to a lesser extent in old corvid nests in trees.Pair formation and selection of nest sites begin in February.Females are fed by their mates during courtship, laying, and incubation, the main prey types being rodents, lizards, and birds.Egg laying begins in the middle of April, and clutches range from three to six eggs and yield, on average, about three fledglings (Aparicio 1998).Breeding pairs were found several weeks before the onset of laying.Half of the pairs found in 1990 were randomly selected to be fed with supplementary food.Feedings started at least 17 days before egg laying, i.e. kestrels were fed at least twice as long as the nine days that they normally require to develop the first egg (Meijer et al. 1989).Each pair was