Apart from the extended reach in an APS-C camera (as compared to a FF camera), the AF system of the said camera is also an important factor. Sometimes you are bounded by unfavorable lighting exposure, making it difficult to pin point your focus. Looking at many Canon bird shooters, preference of using a 1D4 (APS-H) over the 7D (APS-C) is obvious in the shooting field even though the crop factor is 1.3x vs 1.6x.
Just an observation.