What and mango birds sing in the trees in midsummer?

"Mango-Birds Singing in the Trees in Summer" comes from the old version of the TV series Princess Zhu Huan. In the play, Wei Zi and Kangxi went out for sightseeing in the name of traveling incognito, passing through mountains and grasslands. The emperor suggested that everyone have a picnic on the ground. But where can I eat in the middle of nowhere? Two attendants suggested going hunting, but Erkang dissuaded them. At this time, Wei Zi proposed to borrow vegetables from a nearby farmer. However, the borrowed food is crude and looks bleak. Therefore, the talented Wei Zi renamed them.

You said I cooked fried spinach, I said you ate red-billed green Ying Ge, you said fried dried beancurd with green vegetables, I said an egret flew over the quiet swamp, you said spinach was wrapped in fried eggs, and I said mango birds sang in the trees in midsummer. In short, these dishes are not vegetables, but "our mulberries here have green threaded branches" and "the grass in your north is as blue as jade".