Unfortunately, you've left it pretty late for a Mourning Warbler to be easy to find. We've had 2 males singing steadily all spring at Riverwood Conservancy in Mississauga (Burnhamthorpe and the Credit River near Mississauga Road), being readily seen and photo'd from the main path, but that's finished now more or less. On the Sat bird walk, one of the males and his mate and one fledged juvenile all came out to see us on the Red Deer Trail, but that was a lucky chance. When they aren't singing territory any more, they're hard to spot. This is at least the 3rd year we've had a male singing at the 600 m marker on the red deer trail, though, so you could try next June if you want. Just keep an eye on ebird and ONTbirds to see if he's back.
