I put a feeder out a few months ago and have kept it stocked up with
fat balls.

For months, almost the only visitors (that I saw) were robins. But over
the last month or so, I've not seen any robins at all and the only
visitors have been sparrows.

Sorry for the newbie question but where have the robins gone? Are they
nesting somewhere? If so, they surely still need to eat! And why the
sudden arrival of sparrows? Where were they while the robins ruled the
roost?