The first step is to select a specific category used for training the VGG16 on ImageNet. Let's say that we take the category 20, which corresponds to the American Dipper bird shown in the following picture:
An example of American Dipper as seen on https://commons.wikimedia.org/wiki/File:American_Dipper.jpg
ImageNet mapping can be found online (https://gist.github.com/yrevar/6135f1bd8dcf2e0cc683) as a python pickle dictionary where the ImageNet 1000 class ID is mapped to human-readable labels.