Parental investment by birds is limited by the habitat environment, and a male parent increases its effort to reproduce in birds that live in high-altitude areas.

A study of the reproductive behaviour of the Saxaul Sparrow (Passer ammodendri) and the Isabelline Shrike (Lanius isabellinus) was carried out at the Gansu An'xi Extremely Arid Desert National Nature Reserve in northwest China to determine the reproductive input of passerine species in desert habitats.

In Saxaul Sparrows, compared to the female parent, the male parent exhibited a significantly higher frequency of nest-defense behaviour (chirping and warning) during nesting, hatching and feeding periods. In addition, in comparison to the female parent, the male parent exhibited almost equal frequencies of nesting and incubation but fed nestlings significantly more times. Similar to the male sparrows, the feeding rates of the male Isabelline Shrikes were significantly higher than those of the females. The hatching rate and fledging rate of the Saxaul Sparrow on average in this study were 81.99 and 91.92%, respectively, while those of the shrike were 69.00 and 96.53%, respectively.

These two different passerine species living in the same desert environment exhibited the same trend in their reproductive investments. Adapting to desert environments is a strategy that may have evolved in passerines where male parent birds put more effort than females into reproduction to ensure high reproductive output.

As one of the reproductive strategies adopted by bird species, variation in investment in egg production and its influencing factors are important and well-studied subjects. Intraclutch changes in egg size associated with laying order may reflect a strategy of "brood survival" or "brood reduction" adopted by female birds in different situations.

We conducted field studies on the breeding parameters of the Saxaul Sparrow (Passer ammodendri) in Gansu Province, China from 2010 to 2017, to clarify the factors affecting the egg investment and reproductive performance of this passerine species.

Our results revealed significant differences in clutch size, egg size and the fledging rate between the first and second brood of Saxaul Sparrows and suggested that this typical desert species allocates more breeding resources to the more favourable second brood period, leading to greater reproductive output. Female body size presented a positive relationship with egg size, and male body size presented positive relationships with clutch size and hatchability. The females that started their clutches later laid more eggs, and hatchability and the fledging rate also increased with a later laying date in the first brood period. With successive eggs laid within the 5-egg clutches (the most frequent clutch size), egg size increased for the first three eggs and then significantly decreased.

Our results indicate that female Saxaul Sparrows increased egg investment because of good quality of paired males and good environmental conditions. The intraclutch variation of egg size suggests that this species inhabiting an arid environment adopts a "brood reduction" strategy.