Clutch size represents the trade-off between current breeding and future reproduction of parents and between the quality and number of offspring (Simith and Fretwell 1974; Winkler and Wallin 1987; Zhao et al. 2002b). Lack (1947) suggested that passerine birds should produce the largest number of offspring that can be successfully fed to breeding age by the parents. Therefore, he proposed that in most species, clutch size will ultimately be determined by the average maximum number of young the parents can raise, where the common clutch size or the average clutch size represents the maximum number produced (Lack 1947). This hypothesis is supported by several studies (Lack 1948; Högstedt 1980; Murphy 1994; Zhang et al. 2003; Crick 2004). In our study, the most common clutch size was five eggs (8/11 in 2008 and 10/15 in 2009). The average clutch size was 4.90 ± 0.57 and 5.20 ± 0.26 in 2008 and 2009, respectively (Table 1), with the average clutch size slightly larger in 2009 than in 2008; however, no significant difference was found between the 2 years (t = −1.304, df = 23, p = 0.205) (Table 1). The results indicate that five eggs represent the optimal productivity of the Black Redstart.
Females can vary their egg-size investment according to environmental circumstances, their own breeding condition and the quality of their mate (Hargitai et al. 2005). A few species begin to hatch eggs at the end of egg-laying and all the young then hatch together; while others have adopted a pattern termed “hatching asynchrony” (Clark and Wilson 1981), in which eggs begin to hatch before all the eggs are laid so that at least some of the young hatch before the others, often on successive days. The degree of asynchrony varies among species and sometimes within species (Viñuela 2000). Hatching asynchrony has generally been associated with the “Brood-reduction hypothesis” first advanced by Lack and Lack (1951) and Lack (1954). Several studies have suggested that size hierarchy would be more pronounced if the female investment in the final egg was decreased (Lack and Lack 1951; Lack 1954; Slagsvold et al. 1984). Therefore, when food availability is insufficient to raise all offspring, the smallest chick and the last-hatched, generally will starve quickly to increase the survival opportunity of its siblings (Lack 1954). In contrast, the “brood survival” strategy means females would reduce the disadvantages within a brood through more investment in the last-hatched offspring, by increasing the egg size with the laying order to ensure that more offspring can survive (Clark and Wilson 1981; Slagsvold et al. 1984). The Black Redstarts breeding in our study area seem to have adopted the “brood survival” strategy since a significantly positive correlation exists between egg size and laying order in that the volume of the eggs increased with the laying order and the final egg was generally larger than the others. The size hierarchy among nestlings within a brood was further decreased because the Black Redstarts begin to incubate at the end of egg laying and all young then hatch together.
Organisms may have a certain limited amount of time or energy available to expend and natural selection acts on the allocation of time or energy in a way that will maximize the contribution of a genotype to subsequent generations (Cody 1966). How to allocate this energy has led to the emergence of trade-offs between the benefits and risks under the guise of limited resources or capabilities. For example, is it the size of the clutch that determines the volume of the egg, or is the volume reduced because of the capacity of the female to reproduce limited? Our study on the Black Redstart indicates that egg size is negatively correlated with clutch size (r = −0.274, p = 0.002), which suggests a trade-off between these two life-history traits, i.e., the volume is smaller with a large clutch and vice versa. The investment of the female in these two interrelated characteristics is limited, which affects the emergence of this phenomenon. The females, therefore, have to make a trade-off between clutch size and egg size.
What leads to such a trade-off? One possibility is that females adjust their reproductive strategy according to their own situation; on the other hand, the trade-off may be triggered by environmental stress, with adjustments being made to accommodate offspring fitness. Williams (1994), after reviewing available research on the variation of egg size within species, noted the absence of unequivocal data to date in support of a positive relation between egg size and offspring fitness in birds. However, in his review, he did find a few studies that provided more consistent evidence of a positive relation between egg size and the fitness of offspring early in the chick-rearing period (Williams 1994). Therefore, he suggested that the most important effect of variation in egg size might be in determining the probability of offspring survival in the first few days after hatching (Williams 1994). Large eggs appear to have an advantage in harsh environments (Smith et al. 1995; Fox and Czesak 2000). On the Tibetan Plateau, elevation affects the breeding ecology of birds (Lu et al. 2007, 2008, 2010; Du et al. 2012, 2014) through the strong effects of temperature and rainfall, which also affect plant distribution and food availability. In our study area, temperature and rainfall were lower in 2008 than in 2009, suggesting that the climate in 2008 was harsher than in 2009 (Table 3). The disadvantages of low temperature and less precipitation would adversely affect the survival of the nestlings. The Black Redstarts adopted the strategy of laying larger eggs, which would improve the fitness of the nestlings under the harsh environment to recover the costs conferred by the environment (Smith et al. 1995; Fox and Czesak 2000). Therefore, because large eggs are produced in response to the harsher environment, a reduction in clutch size becomes a more favorable strategy, which can increase the input of each egg, whereas reducing the number of offspring can ensure that the offspring have a higher survival rate for future generations. Maybe under such harsh conditions, an increase in the number of eggs can lead to low quality in the entire clutch, with the final number of offsprings less than normal as more eggs might not be successfully hatched or offspring could not successfully fledge because of low quality. This circumstance remains to be further demonstrated. As for our study area, the environmental conditions were superior in 2009 and the advantage of laying large eggs could not be supported. Rather than wasting energy on fewer offspring, birds can increase the number of offspring, which will benefit from improved conditions so that a large clutch size and smaller eggs represent a good choice.