Are morphological adaptations sufficient to explain the varieties of lifestyles of all the extant animals? No. Behavioral adaptations are equally significant, but they almost always get second billing, usually far below morphological adaptations.
Although behavioral traits may be more difficult to quantify than morphological traits, they are probably as variable as morphological traits. If a variable behavioral trait has a genetic basis, then it too will be subject to selection.
Careful observations of any animal, especially in its natural habitat, will make it clear that the contributions of behavior and morphology are not necessarily separable during the day to day survival of an animal. Therefore, behavioral and morphological traits must have evolved concurrently.
Consider the hypothetical case of a land bird that doesn't normally go into water, for example, a chicken, that was born with webbed feet like those of a typical water bird*. I suppose such a morphological anomaly could result from a mutation. However, it would be highly unlikely that such a chicken would behave like a duck, go into a lake, duck its head under the water and attempt to pick up edible things from the bottom.
Birds with webbed feet acting funny
All of the morphological adaptations of ducks (and those of other water birds) are essential, but not sufficient, for their survival in the water. They also need behavioral adaptations.
Disclaimer: I jotted these preliminary ideas down late last night shortly before I fell asleep. If they are not entirely coherent, take into account the late-night factor and go easy with your criticism.
*On the Internet, there is, in fact, a news report about a chicken with webbed feet as well as one about a duck with chicken feet, but they both lack details and could well have been hoaxes.