Reproductive investment of males is greater than that of females in Saxaul Sparrows
The frequency of warning and chirping behaviours in the male sparrows was significantly higher than that in the female sparrows from the nesting period to the nestling period, and together, these two actions could be called field protection, which means that field protection is mainly the responsibility of male individuals in the pairs of birds.
During the nesting period, the nest-building job was mainly performed by males of most social monogamy birds. Behaviours of the Passer birds which live and reproduce in some moisture- and climate-stable areas show similar trends that the nests were built almost by the males alone. For instance, the nests of Spanish Sparrows (P. hispaniolensis) are almost always constructed by males (Summers-Smith 1989), and the male House Sparrows (P. domesticus) are responsible for nesting and building nests alone before finding a mate in Calgary, Canada (McGillivray 1983). Unlike these species, the couples of Desert Sparrows (P. simplex) that live in desert region of Turkmenistan construct nests together (Sopyev 1965), and the Saxaul Sparrow as shown by our results that the nesting work was also carried out by the male and female parent birds together, and the frequency of nesting actions was essentially no different. This may be a common feature of the reproductive investment of birds living in desert environments during nesting periods.
Related studies have shown that House Sparrow, Spanish Sparrow, and Dead Sea Sparrow (P. moabiticus) females are responsible for egg incubation (Gavrilov 1963; Summers-Smith 1989). Although both female and male Desert Sparrows are involved in incubation, the hatching frequency times are two-fold greater for females than for males (Sopyev 1965). In contrast, in the Saxaul Sparrow, both parents participated in egg incubation, and there was no significant difference between the females and males in terms of the frequency and duration of hatching. Moreover, our results showed that in comparison to the female Saxaul Sparrows, the male Saxaul Sparrows not only devoted more time and energy to warning, preening and chirping behaviours during the hatching period but were also involved in egg hatching and were no different from females, which means that male Saxaul Sparrows invested more in the family in this period. In our study area, the temperature varies greatly in the breeding season, and it is often windy and sandy. The early morning temperature is often below 13 °C, and the average temperature of embryonic development of eggs is 28.89 °C (measured in this study). Therefore, rotating female and male hatching behaviours could ensure that the incubation temperature is not too low and may partly account for the hatching rate of Saxaul Sparrows reaching 81.99% (measured in this study), which is far higher than that of Desert Sparrows and Dead Sea Sparrows living in desert habitats (their hatching rates were 44% and 42%, respectively) (Summers-Smith 1989).
In the nestling period, in comparison to the female Saxaul Sparrows, the male Saxaul Sparrows also had higher reproductive input, which was embodied in the fact that males possessed a significantly higher frequency of warning, preening, chirping and waiting behaviours than females. In particular, in comparison to the females, the males provided more food to nestlings. Due to the apparently high feeding input by the male parent Saxaul Sparrow, there was a higher fledging rate for this species (91.92%) than for other species. In contrast, only 68% of the Dead Sea Sparrow nestlings fed by the female parent fledged (Yom-Tov and Ar 1980), and when female and male Spanish Sparrows and Desert Sparrows provide prey for offspring together and equal in frequency, 62.3% (Gavrilov 1963) and 76% (Sopyev 1965) of the nestlings fledged, respectively.
Male feeding rate was higher than the female feeding rate in Isabelline Shrikes
As one of the most critical stages in the whole reproductive process of altricial birds, feeding is directly related to the survival and health of nestlings. In addition, the feeding inputs can partly reflect the reproductive investment of male and female parent birds. Previous studies of feeding input differences between females and males in some socially monogamous birds have shown that, in general, the average feeding rate of females was higher than that of males or generally equal. For instance, the feeding frequency of the male Savannah Sparrow (Passerculus sandwichensis) accounts for only 24–40% of the total feeding frequency, which is significantly lower than that of females (Freeman-Gallant 1998). Another study of Great Tit (Parus major) showed that there was no significant difference in the feeding rates between females and males (Bengtsson and Rydén 1983). In addition, in the Loggerhead Shrike (Lanius ludovicianus), a species of shrike living in Central and North America, males and females also have similar feeding rates (Miller 1931; Gawlik et al. 1991). However, in our study, there was a significant difference (t = − 8.20, df = 135, P < 0.001) in the feeding rates of female and male Isabelline Shrikes that bred in the same area as the Saxaul Sparrow. Specifically, male feeding rates were approximately twice as high as those of females, and perhaps because of this, the fledging rate of the Isabelline Shrike nestlings was 96.53%, with a 69.00% hatching rate.
With the growth of nestlings, the feeding rates of males decreased and those of females increased at the same time. After the last breeding period, the male and female parent birds had similar feeding rates. One bird in a pair would increase its breeding input for compensation when the other in the pair reduced its role in caring for nestlings, when the nestlings were cared for by both parents (Wright and Cuthill 1989; Houston et al. 2005). Therefore, with a decrease in the male feeding rate, the female bird increases its feeding rate to meet the growth needs of their nestlings. Another reason for the change in male and female feeding rates in the daytime may be related to their division of work during the nestling period. Specifically, when the newly hatched squabs were naked with no down, their female parent always covered the nest to keep them warm or keep the sun off them, so the female parent did not spend much time and energy catching prey for the squabs. Along with the organs of the nestlings gradually developed, and their feathers gradually plumped with increasing nestling age (Xu 2002; O’Connor 2008; Zhang 2009), they no longer needed the female parent to be in the nest to provide these and the female could spend more energy and time on feeding behaviours.
With the increase in brood size, both the male and female Isabelline Shrike parent birds increased their feeding rates, which means that the parents played a role in the prey available to support more fledglings. This strategy could maximise their present reproductive benefits.
Males passerine birds living in harsh desert environments may increase their reproductive investments
The effort parent birds put into feeding is affected by the ecological environment of the breeding area (Aho et al. 2009). Two studies on parent-breeding input were carried out in two areas with extremely different environments for the House Sparrow. One study showed that the male feeding rates were lower than female feeding rates in temperate climate areas with sufficient food resources (Bédard and Meunier 1983), and the other study showed that there was no significant difference in feeding input between male and female parent birds breeding in places with less food (Hegner and Wingfield 1987). When the environment of a breeding area is poor, such as at high altitudes, parents, especially males, increase their feeding input to subsequent generations to compensate for the negative influence of environmental factors on reproduction in birds and mammals (Badyaev 1997; Wynne-Edwards 1998). In birds, the average feeding frequency of male parent birds breeding in high-altitude areas is 55.6% of the total average feeding frequency, which is significantly higher than that in low-altitude areas (Ghalambor 2001). Another study of reproductive investment in the Grey-capped Greenfinch (Carduelis sinica) showed that the total male input increased as the elevation of the breeding region increased during the breeding period (Badyaev 1997). In conclusion, the reproductive success rates of birds that breed in high-altitude regions are mainly dependent on the investments by males (Lyon et al. 1987; Badyaev 1997). In our 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 sandstorms (Liu and Yang 2006). Deserts are known for their droughts, gales, changing climates and extreme weather, which make them severely unstable ecological environments. Therefore, in comparison to the female Saxaul Sparrows, the male Saxaul Sparrows provided higher amounts of input during the three periods of reproduction, while the male Isabelline Shrikes possessed higher feeding rates than those of the females, which could be a common characteristic and an adaptation to the harsh desert environment. However, these two passerine birds are very different in terms of their life history characteristics and habits. Therefore, we think that male parent birds increase their reproductive investment and that this could also occur in other passerine birds breeding in hash desert regions. This is a reproductive strategy for birds to adapt to these adverse and changing conditions and ensure sufficient reproductive output.