Yes, the best spots change with the seasons.
For winter waterfowl (and some years birds like Siskins and Redpolls) J.C. Saddington park where the Credit River empties into Lake Ontario is often good. What's seen can vary greatly day by day so it's worth visiting somewhat regularly. Winter water birds can include long-tailed ducks, white-winged scoters, bufflehead, red-breasted mergansers, greater scaup, and the usual mute swans, mallards, and gadwall. There is a "duck pond" in the east end of the park that in the spring/fall sometimes has more than just mallards and Canada geese. The alder trees with their cones along the "stream" leading to the duck pond are a hangout for siskins, goldfinches and redpolls during years when they move south. They haven't been reported that I've noticed yet this year. Sometimes a Snowy owl appears briefly in the area but it's not its preferred haunt. You can also walk from Saddington along the Lake up to another small park and back. Looking over into the goldenrod and grasses behind the old refinery fence, you can often see juncos, goldfinches, american tree sparrows etc in the fall.
In spring, during warbler migration in May, Lakeside Park can be good. In particular, there is a wooded area alongside a very degraded stream at the west side of the park. With patience, almost every kind of warbler can be found here just not all on the same day or even same week. Several birders make it a point to visit daily or every other day during the peak migration to see the one or two examples of each type of warbler that comes through. Scarlet Tanagers also move through the park. Later in the summer, birds are much scarcer here although there are Orioles nesting in the old willows.
In spring and summer, you may want to join the guided bird walks at Riverwood Conservancy (Burnhamthorpe at the Credit River). They are excellent although they can get extremely popular. The walks will give you insight on where to go back and watch more closely on your own. The park has many great birds come through on migration and some stay to raise young. For example, both cuckoos have been seen in the spring, indigo buntings nest here yearly, etc. In the winter, you're only likely to find the usual suspects: chickadees, both nuthatches, downies, hairies, red-bellied and pileated woodpeckers, sometimes American tree sparrows, juncos, goldfinches, mourning doves but they are all easy to find because they have a feeder trail along the paths. Red tailed and Coopers hawks nest in the park most years. Great horned owls are probably nesting in the park or very close to it.
Riverwood connects with Erindale Park just south of the Burnhamthorpe bridge, so you can actually walk along the Credit River from Dundas to the 403.
The Rattray Marsh is currently a bit difficult to bird watch in. The tree cover in the lower parts of the park was actually 80% Ash trees which have all died from Emerald Ash Borer. The dead trees are being removed which has also resulted in a lot of underbrush being cut down, some of which was invasive like Buckthorn. It' was not particularly quiet or easy to bird watch there this year although it has had some good birds coming and going on migration. It also has a pair of Pileated Woodpeckers that are seen fairly regularly, if briefly. Visitors are not permitted to walk into the marsh itself, nor can you canoe in it. No bikes. I still bird the area but I would hesitate to recommend it to others unless I knew how patient they were with tree-removal activities, dog walkers, etc.
It's well worth using the eBird hotspots feature to keep an eye on local park bird sightings. For example, here's the link to the Riverwood page
http://ebird.org/ebird/hotspot/L357145By clicking on a date, you can see the full list of all birds reported by that person at that time. (eBird can be a bit odd about what it shows on the main hotspot page)
Also, near the top of that page, you'll see a link for Hotspot Map.
If you click on it, you go to
http://ebird.org/ebird/hotspots?hs=L357145&yr=all&m=where you can see dots for other Hotspot locations in Mississauga and area. Clicking on a dot opens the recent observation lists for that spot. The colour codes tell you about how many species have been reported as seen at that spot.
I hope this helps a bit.
