I've got siskins at my feeder every day. I had them on and of all winter, and starting in late March, I started getting a male and female pair visiting my feeder everyday. The same pair (likely) is still visiting, and likely have a nest nearby.
They do nest in Southern Ontario, and have nested as far south as Long Point as far as I know.
We've had young at feeders in the London area in June. Not sure if there are any July or August records for Middlesex County though.