震旦鸦雀在镶嵌型芦苇收割生境中的巢址选择

大规模的芦苇收割会对在该生境中繁殖的鸟类产生严重影响。为降低芦苇收割对鸟类繁殖的负面影响,许多地区的湿地管理者逐渐采用了镶嵌型的芦苇收割方式。但是,这种新的芦苇管理方式对鸟类繁殖期生境选择的作用机理尚不清楚。震旦鸦雀是一种主要分布在我国东部、完全依赖芦苇生境而生存的珍稀鸟类,已被IUCN定为全球性近危物种。大规模的芦苇收割导致其适宜生境丧失,被认为是该物种面临的一个主要威胁。选择山东黄河三角洲国家级自然保护区为研究地区,采用AIC信息准则法,从巢址微生境、巢域斑块和全模型3个水平上对可能影响震旦鸦雀巢址选择的生境因子进行了逻辑斯蒂回归分析,并重点探讨芦苇镶嵌收割对其巢址选择的影响以及不同水平的相互效应。结果发现,震旦鸦雀的巢址选择受巢址微生境和巢域斑块尺度双重水平的影响,但巢址微生境因子的影响要大于巢域斑块因子。在巢址微生境水平上,影响震旦鸦雀巢址选择的因素有:1.5—2.5 m旧芦苇的密度、2.5 m以上新芦苇的密度、香蒲密度和巢的可见度;在巢域斑块水平上,影响因素包括芦苇生境类型、旧芦苇斑块面积的比例和植被盖度;在综合模型中,影响因素有1.5—2.5 m旧芦苇的密度、2.5 m以上新芦苇的密度、香蒲密度、巢的可见度和植被盖度。综上,震旦鸦雀倾向于在植被盖度较高、具有一定比例(18.3%)旧芦苇面积、1.5—2.5 m旧芦苇密度较高、可见度较低的生境斑块中筑巢。研究中,78.7%(n=141)的震旦鸦雀将巢筑在新旧芦苇混合的微生境中,其中36.9%的巢址附近旧芦苇的比例在一半以上;53.2%(n=111)的巢筑于新、旧两种芦苇茎秆之上,22.3%的巢的支撑芦苇中旧芦苇所占比例在一半以上。研究结果表明,旧芦苇在震旦鸦雀巢址选择和筑巢过程中发挥了重要作用。由于大范围的芦苇收割在微生境水平上可降低1.5—2.5 m旧芦苇的密度,在巢域斑块尺度上可降低旧芦苇斑块面积的比例,因此建议,在震旦鸦雀的繁殖地,未来的芦苇收割应采用镶嵌收割的形式,并至少应保留面积在20%以上的结构较好的旧芦苇斑块,以供震旦鸦雀选择巢址和繁衍后代。

Nest-site selection of reed parrotbills in the mosaic reed harvesting habitats

The impact of an ongoing loss and fragmentation of reedbeds on reed-inhabiting passerine birds has received much attention in recent years. In addition, global commercial reed harvesting is another important anthropogenic factor threatening the existence of passerine birds. According to previous studies, large-scale winter reed cutting has greatly reduced the number of reed-inhabiting passerine birds across the world. Thus, a new reed harvesting method called ''mosaic reed cutting'' has been introduced in many natural reserves to reduce the impact that reed harvesting has on the reed-inhabiting birds. However, the precise impact of the mosaic reed cutting on the habitat use by passerine birds, especially the nest-site selection, has not been well studied. Reed parrotbill (Paradoxornis heudei) is an endemic reedbed-inhabiting bird that is native to East China, and it is currently listed as "near threatened" mainly because of the loss of the wintering and breeding habitats due to the ongoing over-harvesting. In this study, we chose the Yellow River Delta Nature Reserve as a study site to assess the effect of mosaic reed cutting on the nest-site selection and habitat use by reed parrotbills. As habitats can be destroyed by reed harvesting on different spatial scales, we measured 17 variables to determine the nest-site selection of reed parrotbills in the microhabitat (eight variables), territorial patch (nine variables), and full model scales. In total, 141 nests were found and the nest-site characteristics of all the nests were measured during 2008 and 2011-2012. One unused control plot was randomly chosen for each nest and the corresponding habitat variables were also measured. We investigated the nest-site selection of reed parrotbills using binomial logistic regression that was based on two different levels, microhabitat nest-site and nesting patch. The relevant factors for each level were selected based on the Akaike''s Information Criterion. The results showed that the nest-site selection of reed parrotbills depended largely on the scales of both microhabitat and nesting patch. However, microhabitat appeared to exert a stronger impact than the nesting patch. In the microhabitat, the nest-site selection positively correlated to the density of dry reed with stems 1.5-2.5 m high, but it was negatively affected by the density of green reed with stems over 2.5 m high, the density of cattail (Typha orientalis), and the visibility of the nest. In nesting patch, the nest-site selection was positively associated with the percentage of dry reed-patch area and vegetation cover, as well as the reed habitat type. However, by combining the two levels, the ultimate factors included all of the above, except for the percentage of dry reed-patch area and reed habitat type. Therefore, reed parrotbills preferred to nest on higher-density dry reed with stems 1.5-2.5 m high, in the microhabitat with lower nest visibility, higher percentage of dry reed-patch area, and habitats with higher vegetation coverage of nesting patch. Thus, 78.7% (n = 141) of birds built their nests in mixed habitats that included both dry and green reed stems and the percentage of dry reeds exceeded 50% in 36.9% of the nests; 53.2% (n = 111) of birds built their nests on substrates formed by both dry and green reeds, and the percentage of dry reeds in these substrates exceeded 50% in 22.3% of the nests. Thus, we concluded that the dry reed left during the mosaic reed harvesting is critical to the nest-site selection and nesting of reed parrotbills, in which both the density of dry reed stems in the microhabitat and the percentage of dry reed-patch area in nesting patch can be significantly reduced by extensive reed harvesting. We further recommended that future reed management should ensure that the reedbed retains at least 20% of dry reed to provide a sustainable nesting habitat for reed parrotbills.