Sea Birds – aeas精读 (Y10-12)
Among the species of seabirds that use the windswept cliffs of the Atlantic coast of Canada in the summer to mate, lay eggs, and rear their young are common murres, Atlantic puffins, black-legged kittiwakes, and northern gannets. Of all the birds on these cliffs, the black-legged kittiwake gull is the best suited for nesting on narrow ledges. Although its nesting habits are similar to those of gulls that nest on flat ground, there are a number of important differences related to the cliff-nesting habit.
The advantage of nesting on cliffs is the immunity it gives from foxes, which cannot scale the sheer rocks, and from ravens and other species of gulls, which have difficulty in landing on narrow ledges to steal eggs. This immunity has been followed by a relaxation of the defenses, and kittiwakes do not react to predators nearly as fiercely as do ground-nesting gulls. A colony of Bonaparte’s gulls responds to the appearance of a predatory herring gull by flying up as a group with a clamor of alarm calls, followed by concerted mobbing, but kittiwakes simply ignore herring gulls, since they pose little threat to nests on cliffs. Neither do kittiwakes attempt to conceal their nest. Most gulls keep the nest area clear of droppings, and remove empty eggshells after the chicks have hatched, so that the location of the nest is not given away. Kittiwakes defecate over the edge of the nest, which keeps it clean, but this practice, as well as their tendency to leave the nest littered with eggshells, makes its location very conspicuous.
On the other hand, nesting on a narrow ledge has its own peculiar problems, and kittiwake behavior has become adapted to overcome them. The female kittiwake sits when mating, whereas other gulls stand, so the pair will not overbalance and fall off the ledge. The nest is a deep cup, made of mud or seaweed, to hold the eggs safely, compared with the shallow scrape of other gulls, and the chicks are remarkably immobile until fully grown. They do not run from their nests when approached, and if they should come near to the cliff edge, they instinctively turn back.
在夏天使用加拿大大西洋沿岸的海风悬崖进行交配，产卵和抚育幼仔的海鸟种类中常见的是大西洋海鹦，黑腿三趾鸥和北方塘鹅。在这些悬崖上的所有鸟类中，黑腿三趾鸥最适合在狭窄的壁架上筑巢。虽然它的筑巢习性与平地上筑巢的海鸥相似，但是有许多与悬崖筑巢习惯有关的重要差异。 筑巢在悬崖上的优点是它可以从狐狸身上获得免疫力，这种狐狸无法扩大陡峭的岩石，也无法从乌鸦和其他种类的海鸥那里获得免疫力，这些海鸥难以登陆狭窄的壁架来偷鸡蛋。这种免疫力之后一直是放松防御，而三趾鸥对掠食者的反应几乎与地面筑巢的海鸥一样猛烈。一群波拿巴的海鸥响应掠食性鲱鸥的外观，通过一群喧嚣的警报呼叫飞行起来，然后是协同的围攻，但是三趾鸥只是忽略了鲱鱼鸥，因为它们对悬崖上的巢穴构成了很小的威胁。 Kittiwakes也不试图隐藏自己的巢穴。大多数海鸥保持巢区没有粪便，并且在雏鸡孵化后移除空的蛋壳，因此巢的位置不会被丢弃。 Kittiwakes在巢的边缘排便，保持它的清洁，但是这种做法，以及它们离开巢穴的倾向充满了蛋壳，使它的位置非常显眼。 另一方面，在狭窄的壁架上筑巢有其自身特有的问题，并且三头肌行为已经适应于克服它们。雌性三趾鸥在交配时坐着，而其他海鸥则站立，所以它们不会过度平衡并从壁架上掉下来。巢是一个深杯，由泥或海藻制成，与其他海鸥的浅刮相比，可以安全地抓住鸡蛋，并且雏鸡在完全长大之前非常不动。他们走近时不会从巢穴中跑出来，如果他们靠近悬崖边缘，他们会本能地转身。