why is the laburnum tree called as the engine of goldfinch's family?

The laburnum tree was called the engine of the goldfinch's family because the tree is like a machine which starts up as the goldfinch alights on the tree to feed its family. The noises made by the young ones of the bird is like that of a machine coming to life. As the goldfinch feeds its little ones,it actually signals the act of giving fuel to the machine which helps it to start up just like the tree does.