What is the line-by-line meaning of the poem "The Lost Dances of Cranes" by Juliet Wilson?

Juliet Wilson's poem "The Lost Dances of Cranes" reflects on the changeover from endangered species to the development of cities via construction cranes. In this poetic analysis, lines from the poem will be in italics and commentary in regular print.

"The Lost Dances of Cranes"

From the very beginning, in the title, the poet signals that the poem is about something from the past that is no longer available in the present. The title is unified by some of the sounds of the words (consonance), such as the /s/ of "lost," "dances," and "cranes." 

Your fields are empty now.

Part of the reason the whooping crane is on the verge of extinction comes from the drastic reduction of wetlands due to the human desire to expand land on which things can be built. 

Only your ghosts dance

Dancing is part of the mating rituals of whooping cranes. Since so many birds have been lost, they would be only memories—ghosts. 

While cranes of another kind
Dance cities into being.
The poet connects construction cranes to the birds, which makes sense, since they resemble each other in lankiness. The dance here doesn't yield eggs and more birds but, rather, the cities that are ironically destroying the birds. 

All that remain of you are 
A fading crackle of your energy
And some grainy video footage
By using the word "you," the poet personifies the birds. The birds are fading quickly and the poet mentions what we have now—memories and video. This stanza sets up the next stanza with the specific things it mentions. 

That people in the new cities 
Will watch to marvel
At the wonders the world

This stanza captures an essential irony of modern living: we who live in cities think that we are getting the "real thing" by watching nature videos. In fact, a video is a worse than poor substitute for the actual birds. It is worse because it fools us into thinking that we are fully appreciating the natural beauty of our world through nature documentaries. 

A poem like this implicitly challenges its readers to do something besides watching television about the losses of the natural world.