Abstract:The avian eggshell provides a protective barrier for the developing embryo, its traits such as eggshell thickness and porosity are known to be ecologically important. This paper studied characteristics of eggs and eggshells of 7 passerine species breeding in alpine meadow, Qinghai-Tibetan plateau: Horned Lark (Eremophila alpestris), Oriental Skylark (Alauda gulgula), Citrine Wagtail (Motacilla citreola), Tree Sparrow (Passer montanus), Rosy Pipit (Anthus roseatus), Twite (Carduelis flavirostris) and Black Redstart (Phoenicurus ochruros) from May to August in 2013, to explore the potential correlation between those characteristics across species and how different species adapt to the alpine environment from the aspect of eggs. For each egg, we weighed its fresh mass, then measured its length and width to an accuracy of 0.01mm, followed by dividing the whole egg into three circular observation regions (the pointed end, the equator and the blunt end), a sample the area of 0.25 cm2 was taken from each region, and calculated the mean of three samples’mass of each egg, thickness was also measured on three shell samples. The shell samples were processed and then placed under a digital microscope, the pores were counted and recorded respectively. Pores were photographed to measure diameters. We then investigated differences in egg size, egg mass, shell thickness, pore density, pore diameter and porosity between different female body masses, nest types and clutch sizes, and examined the influence of these traits on incubation period as well as regional differences of shell thickness across species, indicating the primary effect of heredity across species. We found significant interspecies variation (except between Oriental Skylark and Horned Lark) in all traits except in egg elongation and pore diameter, suggested that in two closely related species that have a similar nest environment, there was a convergence in their egg’s evolution. We conducted Linear Regression and found that shell thickness, egg size were unrelated to pore density (P = 0.11,P = 0.09),egg mass and egg size were positively related to shell thickness (r2 = 0.46,P < 0.001;r2 = 0.44,P < 0.001); egg size, egg mass and eggshell thickness were positively related to female body mass (r2 = 0.66,P = 0.03;r2 = 0.92,P < 0.01), suggesting that across species, heavier females lay larger eggs with thicker shells; Linear Regression of 6 species (we were unable to obtain Rosy Pipit’s clutch size data) showed eggshell thickness and porosity were negatively related to clutch size (r2 = 0.64,P = 0.056;r2 = 0.87,P < 0.01), suggesting besides the hereditary side, different calcium content of food sources across species might also affect shell thickness, also supported that larger clutch size needed to reduce porosity to keep the rate of water loss and nest humidity remain constant; Linear Regression of 6 species (we were unable to obtain Tree Sparrow’s nest – cup volume data) showed porosity was unrelated to nest - cup index or nest type (P = 0.49,P = 0.435), this result may be due to small sample size or the intercorrelation of egg mass and egg size. Principal Component Analysis showed egg surface area and total pores per egg explained 87% of interspecies variation of porosity, in contrast to our expectation, egg size and porosity were failed to explain interspecies variation of incubation period (P = 0.77). However, it could be expected that differences in average shell thickness across all regions were mostly insignificant due to the unique shell structure of Passeriformes and porosities were significantly lower than the predicted porosities as the adaption to alpine habitat. We also found that porosity were significantly lower than predicted porosity in all 7 species (P<0.001). To conclude, our findings indicated that, hereditary factors such as body mass and clutch size decided most of interspecies eggshell traits variation, but different species might apply the same adaptation like reduced porosity to the cold and hypoxia environment. In addition, egg (egg size and porosity) alone could not determine incubation period among our studied species, with combined effects of incubation behavior might explain more of species – specific incubation period.