I agree. I think birds rely more on sight than smell. Many birds can actually see in ultra violet, so the colors you and I see are much brighter to birds.
I know that turkey vultures have a highly developed sense of smell.As for songbirds?I am not sure.I would use aluminum foil or plastic owls to deter them like the others have said.
A lot of activity in the near by area can deter them. When I lived in Virgina Beach my porch was a haven for birds. However I noticed the more I was on the porch the less likely they would return the following year. Personally I like as many birds as possible near by. I know whenever I lived near birds nest I never seem to have many bug problems like wasps and bees and yet where there are no birds, I seem to get wasps nest and bees. Not sure if its just a coincidence but at least the birds have never try to attack me.