Seasonal declines of clutch size and other components of reproductive performance are very common in birds (Hochachka 1990). Multi-brooded passerine species show a decrease in egg size from the first to later broods because of physiological constraints on the laying female bird (i.e., the decreases in intrinsic quality and fat reserves) (Perrins 1996; Kvalnes et al. 2013) or because smaller eggs are adequate for survival under favourable environments (Yampolsky and Scheiner 1996). In a double-brooded Desert Finch (Rhodospiza obsoleta) population, Yosef and Zduniak (2004) suggested that the parents are more capable of fledging young successfully during more stable summer conditions, leading to less variable eggs and smaller clutch sizes in the second brood period. Our results indicated that contrary to common seasonal trends and physiological constraints, female Saxaul Sparrows laid significantly more eggs per clutch and did not decrease egg size during successive broods. In the study area, the early period in the breeding season (April‒May) was characterized by lower temperatures, less precipitation, inconsistent and extreme weather conditions, and even sand storms (Liu and Yang 2006). The second part of the breeding season (June–July) was typically a much more stable period (the CV of temperature during the first brood was significantly higher than that during the second brood). We suggest that the Saxaul Sparrows living in this extremely arid environment allocate more breeding resources to the second brood, which is produced under relatively favourable conditions. This strategy resulted in a better reproductive output (i.e., the fledging rate of the second brood was significantly higher than that of the first brood).
Birds producing a relatively large final egg adopt a “brood survival” strategy, whereas birds adopting a “brood reduction” strategy produce a smaller final egg (particularly birds with large clutches) (Slagsvold et al. 1984). Among different bird species (Enemar and Arheimer 1999) and even within a single species (Dolenec et al. 2011), the two strategies may vary with breeding conditions. For example, in a population of Tree Sparrows in northwestern Croatia (Dolenec et al. 2011), the female birds were found to adopt a “brood survival” strategy for the second clutch (the deviation of the final egg size from the mean egg size of a clutch, %D value = 1.58), while they adopted a “brood reduction” strategy for the first (%D value = − 1.58) and third clutches (%D value = − 1.07). The common understanding is that under favourable environmental conditions, egg size within a clutch increases with egg laying order (Enemar and Arheimer 1999; Hargitai et al. 2005; Dolenec et al. 2011). According to our results regarding intraclutch variations (Fig. 1) and the relative size of the final egg, the female Saxaul Sparrow adopts a “brood reduction” strategy for both the first (%D value = − 2.62) and second clutches (%D value = − 0.92). This strategy is compatible with its habitat. In desert environments, where weather conditions are often mutable and extreme, birds face unpredictable abiotic conditions and cannot guarantee the survival of the last egg, which hatches asynchronously in disadvantageous circumstances. However, once conditions change in a relatively stable direction, such as the second brood period, the Saxaul Sparrow may tend to eliminate discrimination among eggs within a clutch (the second brood exhibits a significantly higher %D value than the first brood).
Individuals producing larger eggs are forced to lay smaller clutches, a relationship that seems intuitively obvious (Smith and Fretwell 1974). However, a literature survey indicated that egg size is generally (in 40 of 63 studies and species) unrelated to clutch size (Christians 2002). Under favourable conditions, such as warm weather, a large clutch with large-sized eggs is the best combination for high fledging success and productivity in Ficedula hypoleuca (Järvinen and Väisänen 1984). In our analysis, we failed to find a negative relationship between egg size and clutch size. Our further analysis revealed that above a clutch size of four, five, or six eggs, there was also no significant correlation between clutch size and egg size, which is similar to the results obtained from a study of another passerine species, Ficedula albicollis (Hargitai et al. 2005).
We found that the laying date was an important factor affecting clutch size and that it also significantly affected hatchability and fledging rates, especially during the first brood period when the climate conditions were unstable. A negative relationship between laying date and clutch size is common in altricial species (Crick et al. 1993; Gil-Delgado et al. 2005). In many cavity-nesting species, it has been reported that the number of eggs per clutch often decreases as the breeding season progresses (Goodenough et al. 2009). One potential cause of the decline in clutch size observed in many avian populations is that food availability decreases as the breeding season progresses and that late breeders exhibit a lower fledging mass and lower reproductive success (Siikamäki 1998). Another interpretation is that younger females (Järvinen 1991) or lower-quality individuals (Christians et al. 2001) may lay fewer eggs and lay later in the season. However, the situation was different in our study; the number of eggs per clutch clearly increased when the laying date was late in both the first and second brood periods. We suggest that during the first brood, a breeder laying early faces a capricious climate and that breeding activities are often interrupted or their products are destroyed by unexpected sandstorms, intense cool weather or even snowfall. Environmental conditions, including food abundance, tend to become favourable as the brood progresses, so females that start clutching later will lay more eggs to achieve greater reproductive output, and the finding that the hatchability and fledging rate of the first brood increased as the season advanced supports the hypothesis that a relatively later laying date will result in a higher reproductive success.
Among environmental factors, ambient temperature is well studied and is considered to affect the availability of food resources for female birds and thermoregulatory requirements during egg formation (Haftorn 1986; Magrath 1992; Nager and Noordwijk 1992; Hargitai et al. 2005). We did not detect any effect of ambient temperature on clutch size or egg size, but ambient humidity exhibited a positive relationship with clutch size during the first brood. It is well established that precipitation is the main limiting factor affecting productivity in desert ecosystems. Higher ambient humidity during egg laying may signal to a sparrow that good precipitation conditions will occur in that season.
It is common for female body quality to present a significant positive relationship with egg size (Christians 2002). Our data did reveal a significant correlation of female body size with egg size but not with clutch size. The two sexes share the responsibility of feeding the young in this species. Our results suggested that the female birds laid more eggs in a clutch when they paired with male birds exhibiting high quality. The hatchability of the Saxaul Sparrow depends mainly on egg fertilization, and 80.2% of unhatched eggs were unfertilized (our data). It makes sense that in the Saxaul Sparrow, the male’s body size presents a positive relationship with hatchability.