Could you please explain what this sonnet means?

It is called March by Susie Harrison. The sonnet is below:

With outstretched whirring wings of van-dyked jet,
Two crows one day o'er house and pavement pass'd.
Swift silhouettes limned 'gainst the blue, they glass'd
Smooth beak and ebon feather in the wet
Of gaping pool and gutter, while, beset
By nestward longing, high their hoarse cry cast
In the face of fickle sun and treacherous blast,
Till all the City smelt of violet.
Then through that City quick the news did run.
Great wheels were slacken'd; belts were stopped in mill,
And fires in forges. Long ere set of sun
Dazed men, pale women sought the open hill--
They throng'd the streets. They caught the clarion cry--
"Spring has come back--trust Spring to never die!"

Spring has sprung and everyone is thrilled.