When focusing on moving objects that are small it is best to switch over to spot focus and focus on the EXACT area you intend to focus on! Unless you zoom in to fill up the viewfinder frame, then center-weighted is okay in this instance as well.
Nikon's matrix metering - especially the 51 point area of the D7100, will focus on several areas at the same time. That is, for any given aperture setting, their is a certain depth of field associated with that given aperture setting. Thus, a number of focus points across the frame will light up and set at the same time.
This is no good if you just want a bird within the frame to be set as the actual focus point because areas in front as well as behind the bird are in focus. The depth of field for the aperture set could render parts of the bird to appear softer than intended - especially at apertures of F2.8 and F4! The depth of field is narrow and might not encompass the entire animal within the frame.
This also applies to portraiture shots - focus on the eyes and use either spot metering or center-weighted metering.
The default Matrix metering is best used for non-moving, and very large depth of field shots such as landscapes and architecture, etc.