高寒草甸常见雀形目鸟类的卵壳特征

Eggshell Traits of Several Passerine Species Breeding in the Alpine Meadow,Qinghai-Tibetan Plateau

作为胚胎发育的屏障,卵壳特征如卵壳厚度和气孔密度具有重要的生态学意义。本文研究了高寒草甸繁殖的7种代表性雀形目鸟类角百灵(Eremophila alpestris)、小云雀(Alauda gulgula)、黄头鹡鸰(Motacilla citreola)、树麻雀(Passer montanus)、粉红胸鹨(Anthus roseatus)、黄嘴朱顶雀(Carduelis flavirostris)、赭红尾鸲(Phoenicurus ochruros)的卵与卵壳特征,以探索在高寒缺氧环境下,不同科鸟类的适应性调节。结果发现,7种鸟类中,除小云雀和角百灵之外,其他鸟类的卵与卵壳特征大部分具有显著的种间差异(除卵壳厚度和气孔直径之外);对7种鸟类卵壳厚度、气孔密度、卵体积、卵重之间进行线性拟合,卵壳厚度、卵体积与气孔密度均无显著相关性(卵壳厚度:P=0.11,卵体积:P=0.09),卵重、卵体积与卵壳厚度呈显著正相关(卵重:r2=0.46,P<0.001;卵体积:r2=0.44,P<0.001);对7种鸟卵的数据与繁殖期雌鸟平均体重线性拟合,平均卵重、卵壳厚度的差异与雌鸟体重成正相关(卵重:r2=0.66,P=0.03;卵壳厚度:r2=0.92,P<0.01);对6种鸟(缺乏粉红胸鹨窝卵数数据)卵的数据与孵卵相关变量线性拟合,卵壳厚度及气孔率与窝卵数成负相关(卵壳厚度:r2=0.64,P=0.056;气孔率:r2=0.87,P<0.01),6种鸟(缺乏树麻雀巢杯指数数据)气孔率与巢杯指数或巢型无显著相关性(巢杯指数:P=0.49,巢型:P=0.435)。卵表面积和总气孔数解释了大部分气孔率差异(87%),卵重和气孔率与孵卵期无显著相关(P=0.77),气孔率显著低于预期气孔率(P<0.001)。这些结果表明,种间的遗传性(如成鸟的形态、窝卵数等)决定了大部分卵与卵壳特性的差异,但是为适应高寒低氧的气候特征,不同种鸟都具有相对厚的卵壳和低的气孔率,说明环境因素同样影响卵的进化。另外,单纯的卵特性(卵体积和气孔率)并不能决定孵卵期长短,亲鸟的孵卵行为同样具有重要的影响,角百灵虽然离巢频率相对较高,但是孵卵期却相对较短,这可能与其较高的气孔率有关。

The avian eggshell provides a protective barrier for the developing embryo,its traits such as eggshell thickness and porosity are known to be ecologically importance. This paper studied characteristics of eggs and eggshells of 7 passerine species breeding in alpine meadow,Qinghai-Tibetan plateau： Horned Lark（ Eremophilaalpestris）,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. 01 mm,followed by dividing the whole egg into three circular observation regions（ the pointed end,the equator and the blunt end）,a sample of0. 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. We 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.