Answer:
By testing different seed
By examining the poop of cardinal bird
Explanation:
Seed is one of the major constituents of diet of Cardinal bird along with insects, wild fruits , berries etc. Johnny in order to determine the best kind of seeds need to conduct a small observatory experiment where he could place different varieties of seed in front of the cardinal bird at the place of feeding. The seed whose quantity reduces the maximum among all , will be the most preferred seed by Cardinal.
Also he can analyse the poop of cardinal bird to identify the most preferred seed
