It is a beautiful name: "uguisu-mame" literally means "bush warbler beans" and the bush warbler is the bird that in Japanese poetry announces the spring - but reality is more prosaic: ordinary peas are boiled down with some salt, and lots of sugar or mizu-ame (glucose syrup).
[The Japanese Bush Warbler - Photo Wikipedia]
Even less than prosaic is the reason why these boiled down peas are called "bush warbler peas:" it is because they resemble the droppings of the bush warbler! Bon appetit!
The "uguisu" or bush warbler is a bird which appears frequently in Japanese poetry. Its distinctive breeding call ("Hooo-hokekyo") can be heard throughout much of Japan from the start of spring. In poetry the bird is associated with plum blossoms, although in fact its distinctive song is usually not heard until well after the plum blossoms have scattered.
The beauty of its song led to the alternative English name "Japanese Nightingale," although the Japanese Bush Warbler does not sing at night as the European nightingale does.
In addition, as squeaking wooden floorboards reminded the Japanese of the low chirping of the bush warbler, these floors were called "uguisubari" - such floors were on purpose designed to squeak, as one often reads in order to warn of the approach of ninja, but I believe it was more for aesthetic effect. Examples can be seen in many old temples in Kyoto as Chion-in, as well as in Nijo Castle. In English such floors are usually called "nightingale floors."
Those bush warbler droppings were, by the way, in the past used as a cosmetic, as they seem to contain an enzyme that works as an agent that whitens the skin and helps remove wrinkles. And therefore the association with something dirty did not exist, so that even boiled green peas could be compared with such cosmetic droppings.
Back to the peas! These sweet, boiled peas are used as such, as a side dish, but they also form an ingredient in Japanese sweets (wagashi) or even in bread. There is, in fact, a great variety of uses. Often the link with spring is stressed, or in other words, only in spring green peas are sold as "uguisu-mame."