With 11 points, there are a maximum of 3 cross sensors available but with some lenses or teleconverters you might have just a single cross sensor. It depends on the most open aperture of your lens. for example with 11 points you have only 3 sensors that work at all at f/8, while with 51 points you would have 11 working sensors.
For birds in flight, unless you have a medium or fast lens (f/5.6 or faster) you pretty well need to stick with the center sensor. Even with a medium or faster lens, there are relatively few cross sensors. That does not mean the others won't work with good light - just that your misses are going to be more frequent.
I really wonder why Nikon does not allow a menu setting to use cross sensors only or to display cross sensors in a different color from regular line sensors. I'd also like to see the option to only use sensors that work with a given lens. If I used cross sensors only, AF accuracy should improve.
If you have a D600 or D7100, its much more important to be aware of these issues as the number of sensors is much more limited. The center sensor is always good, but other sensors can have more limitations.