OK. That's an instance where a habitat is made less suitable. But now, what about cases where an animal exhibits a clear choice between two suitable habitats. In cases like that, does the preference matter? Well, let's look at the blue warbler. The blue warbler is a songbird that lives in North America. They clearly prefer hardwood forests with dense shrubs, bushes underneath the trees. They actually nest in the shrubs, not the trees. So they're pretty close to the ground, but these warblers also nest in forests that have low shrub density. It's usually the younger warblers that nest in these areas because the preferred spots where there are a lot of shrubs are taken by the older, more dominant birds. And the choice of habitat seems to affect reproductive success. Because the older, more experienced birds who nest in the high density shrub areas have significantly more offspring than those in low density areas, which suggests that the choice of where to nest does have an impact on the number of chicks they have. But a preferred environment doesn't always seem to correlate with greater reproductive success. For example, in Europe, study has been done of blackcap warblers. We just call them blackcaps.

好了，这就是一个关于栖息地变糟糕的一个例子。但是现在，如果两个栖息地都很合适，那么动物会表现出怎样的选择呢？那么它们的偏好重要吗？让我们来看看蓝莺。 蓝莺是一种生活在北美的鸣鸟。它们很显然的偏好于有着稠密的灌木丛阔叶树森林。它们通常在灌木丛中筑巢，而不是在树上。所以，它们倾向于接近地面，但是这些蓝莺也会在有着稀疏灌木丛的森林里面筑巢。通常，年轻的蓝莺将会在这些地方，因为更好的地方已经被主流的成年蓝莺所占领。并且这些栖息地的选择貌似将会影响繁殖的成功与否。因为，更加成熟，更加有经验的鸟将会在高度稠密的灌木丛中筑巢，相较至于在低密度灌木丛筑巢的鸟，它们有着显著的更多的后代。这也就表明了在那里筑巢对于下一代的数量有着很大的影响。但是，一个动物偏好的环境并不总是与高的繁殖率相关。比方说，在欧洲，关于黑顶莺做过了一些研究，我们仅仅把它们叫做黑顶。