作者:Daniel Charles de la Hey*
School of Science and the Environment, Bath Spa University, Newton Park Campus, Newton St. Loe, Bath, BA2 9BN, UK.
* Corresponding author: Tel: þ44 (0)7736674370. Email: danny_delahey@yahoo.co.uk
Supervisor: Mr G. M. Smith, School of Science and the Environment, Bath Spa University, Newton Park Campus, Newton St. Loe, Bath, BA2 9BN, UK.
本研究的调查对象为英国Shapwick Heath国家自然保护区里生活的水獭,旨在探究鸟类性食物对这些水獭的重要性。研究时间为2006年七月到2007年一月。我们采集水獭的粪便,并用显微镜检查其中是否有鸟类的羽毛。如果可以的话,我们还会通过这些羽毛鉴定鸟的物种。我们也记录了粪便里的鱼鳞出现情况,但没有定量分析。41%的样本里都存在鸟羽,相比之下,1981年在Slapton Ley保护区的数据为4.6%,而1975年在Somerset Levels的数据为4.7%。我们发现2006年七月的粪便中的鸟羽出现比例是最高的,占到了61%。其中,鹤形目鸟类占比最高。白骨顶 Fulica atra L. 是最常被水獭捕食的鸟类;有15份样本里都存在白骨顶的羽毛。绿头鸭 Anas platyrhynchos 为3,普通鸬鹚 Phalacrocorax carbo 为2,针尾鸭 Anas acuta、黑水鸡 Gallinula chloropus 和小䴙䴘 Tachybaptus ruficollis 均为1。有两份样本都同时包含有白骨顶和绿头鸭的羽毛。过去研究中都没有发现如此高的鸟类性食物比例,因此我们认为鸟类性食物是水獭主食鱼类的替代品。同时也希望能有其他研究者参与到有关Shapwick Heath保护区内鱼类资源和水生生态系统的研究。
本研究的结论和另一个欧洲地区的研究相似[3],不过在另一篇研究里,掠食者的新目标是小龙虾而不是鸟。Shapwick Heath保护区内的水獭食谱中鱼类占比比我们查阅其他文献[8]后预想的还要低。那些文献中,水獭食谱中鱼类占比达90%,而在本研究中平均却只有67%。鱼类占的比例会随时间变化。2006年八月,鱼类占比达80%,但到十二月就降到了20%。食谱中剩下的部分就靠鸟类或其他猎物来代替。十二月不仅是鱼类占比最低的月份,也是样本数量最少的,总共只有6份样本,这可能会导致很大的误差。水獭食谱中鸟类的占比则要比同类研究所示的要多。Chanin[3]和Mason and Macdonald[6]的研究显示,鸟类占比为10.3%,但我们的研究数据是这个的四倍。Webb[4]调查了Somerset Levels的水獭食谱,发现鸟类占比是4.7%。
Ferraras and Macdonald[18]调查过貂对水鸟的捕食作用。他们发现貂捕食白骨顶的频率远远超过了黑水鸡。他们认为,产生差异的原因是两种鸟类夜栖行为有所不同。白骨顶爱在水边缘栖息,而黑水鸡则会藏在草丛深处。黑水鸡常在浮木顶端筑巢,离水面较远[20]。如果这个行为可以帮助他们避免被貂捕食,那么他们逃避水獭的原因可能也是如此。
一些鸟类把水獭视为对自身和鸟巢的威胁。我们在2006年八月就见证了一群红嘴鸥驱赶水獭的场景。当我们在采集粪便的时候,也常看到红嘴鸥在芦苇丛上忽上忽下地飞行。即使水獭被驱赶到水里,远离芦苇丛后,红嘴鸥依然会穷追不舍。Clode et al.[22]研究过红嘴鸥和燕鸥的围攻行为,发现不同情况下鸟类对掠食者有不同的反应。成鸟通常在威胁者头顶盘旋,看起来就像喂食幼鸟的行为。而当他们感到自己的蛋或幼鸟受到威胁时,他们会表现出攻击性。他们会尝试不停地俯冲攻击掠食者,企图把威胁赶走。直接的攻击可能会导致自己受到伤害,因此红嘴鸥选择比较安全的围攻。围攻行为不是先天行为,是通过经验学习的[22],因此保护区内的水獭可能在过去会捕食红嘴鸥,因此被红嘴鸥视为威胁。尽管我们没有粪便证据表明水獭会捕食红嘴鸥,但红嘴鸥的行为显示他们可能也会被水獭捕食。
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