I always shot print film so I'm the wrong guy to ask. For print film, you worry more about shadow detail since there are features of the development process the inhibits the highlights from washing out. For slides, you worry more about, i.e., expose for, the highlights. I happened to remember that you had asked about slide film so my comments were aimed that way.
One easy way to view all this is that for slides the number of stops to go from very white to very back is about 5. This is called the latitude of the film. A simple meter will try to make the entire picture average to middle gray. If you were to base your exposure purely on the white duck (get really close so the white feathers filled the viewfinder) the duck would come out gray and the rest of the picture would be too dark. Green grass is often a good source of middle gray for most lighting, so you could meter on that and just accept the exposure. A safer this to do (to protect the highlights) it to meter on the white feathers and open up a stop or two depending on how white you want those feathers to come out. Of course, I'm explaining this to you in front of a lot of people who probably understand it better than I do. Print film has a wider latitude (more stops form black to white) so you don't have to be as careful. Digital is a lot like slide, so I have to think more about that now. Which is why I look a lot at the histograms. . .
