Testing prototypicality

Radford et al. (2009: 181) offer the ingenious ‘technically’ or ‘strictly speaking’ test as a way of establishing prototypicality in such cases:

(a) Strictly speaking, a penguin is a bird.

(b) Strictly speaking, a robin is a bird.

(c) Technically, a whale is a mammal.

(d) Technically, a trout is a fish.

While all of the above sentences are grammatically well formed in English, there’s something slightly odd about (b) and (d), which seem to labour the obvious, because robin and trout are prototypical hyponyms of bird and fish respectively, while penguin is not a prototypical bird, nor whale a prototypical mammal.