I was pruning a client's oak and realized I hadn't heard any chickadees all morning. This used to be peak nesting time, but now it's just quiet. My guess is the insect population crashed, so the birds moved on. It's a bad sign for tree health too, since birds eat pests.
Timing might matter, but where downtown did you see the sparrows? Were they nesting in trees or on ledges? I've seen birds pick quiet corners even in busy places. What time were you there? It's interesting how they manage to build nests with all the noise around.