Whale, dead rat, cat or pigeon: Which animal is the best spy?

Hvaldimir, a beluga thought to have been a Russian spy, is dead. Which other spy animals have spooks used before?

Whale, dead rat, cat or pigeon: Which animal is the best spy?

Whale, dead rat, cat or pigeon: Which animal is the best spy?

Whale, dead rat, cat or pigeon: Which animal is the best spy?

Whale, dead rat, cat or pigeon: Which animal is the best spy?

Whale, dead rat, cat or pigeon: Which animal is the best spy?