Around here, anyway. It means they have laid their eggs, and do not wish to advertise their presence. It's not just Robins, it's all of the songbirds.
All the same, for the next several weeks the crows will be swooping everywhere to steal the songbirds' eggs and nestlings. They know how to find them.
In other breaking news, our wrens are back this morning. Welcome home, little friends.
Have not seen a warbler yet but our trees have not leafed out yet -still just tree flowers. Not ornamental, just yellowish tree blossoms spewing pollen.