One of my favourite Spots used to be Miller Creek Conservation Area. Take Carnegie Ave, which becomes the Centre Line of Smith, to the 7th Line and turn left (west). The entrance is at the bottom of a hill just past a farmhouse. It always used to be poorly marked and probably still is.
It's a large wetland with an old railway line running through it. You follow the rail line south, with a few detours into fields. Last time I was there (a few years ago) the fields were becoming quite forested and the water level in the marsh was low, making for very little open water. It's worth checking out in any season. It was the most reliable place I knew for American Bittern.
The zoo can also be productive: the tall conifers and the road that runs along the top of the riverbank.
Also, check eBird. It will show you the local areas that have been identified as "hotspots".
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