I would not recommend Niagara at all without a scope or car.
Queenston docks to adam beck is several kilometers
Adam beck to the Whirlpool is another 4km or so
The whirlpool to the falls area is another 5 or 6 km
Fort erie, where the fish crows, black-headed gull and kittiwake are is another 30 km or so from there.
And most of the birds are very far away, Queenston boat docks and a small part of the upper falls area are the only place where there's even a chance of seeing any numbers of gulls close, and they likely won't include the species you want.
Time of day doesn't matter, but the number of birds drops significantly on weekdays, probably because of dumps being open.
If you go, I'd say its likely that you'll get to the falls area, see maybe 6 or 7 gull species at most and get bored within a couple of hours, with no way of getting to any of the other spots.
