Why were you born when the snow was falling?
You should have come to the cuckoo’s calling,
Or when grapes are green in the cluster,
Or, at least, when lithe swallows muster
For their far off flying
From summer dying.
As you can see from the full context, the writer is lamenting that the subject of the poem was born in the Winter, and not in the Spring [ the cuckoo’s calling], the Summer [ when grapes are green in the cluster], or the Autumn [ when lithe swallows muster]. Swallows are small birds which gather together [ie "muster"] in the Autumn in the UK, before flying back to Africa for the Winter. The adjective "lithe" is used to describe them as thin, supple, and graceful.