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

Abstract

Egg size in birds may be positively correlated with hatching success (Jarvinen and Vaisanen 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, Jarvinen and Viisanen 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 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 (Vifnuela 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 (Ryden 1978, Pierotti and Bellrose 1986). Finally, fluctuating environmental conditions at the time of egg laying associated with low temperatures (Ojanen et al. 1981, Jarvinen and Ylimaunu 1986, Magrath 1992b) or with changes in food availability (Jarvinen and Vaisanen 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