Largest native bird in Wisconsin is the Whooping Crane, but you don’t see many of them.
Second is the Sandhill Crane, they’re not an uncommon siting.
I have seen an ostrich (or perhaps emu) in the Kettle Moraine. I presume it escaped from a local farm.
No idea what you saw, though.
Now, if it were yellow, we’d have an identification.
I actually went and sat parked in my car for a while where I saw them, until I decided that the odds they would cross the same path again at some arbitrary time while I was there were pretty low.
Then I started thinking about, if I did exaggerate their height, by how much. So, I saw the legs of the closes bird above th hood. I gotta figure that means legs at least 15", maybe more like 18". As to the total height – well, maybe that’s what I got most wrong. The head height was anove my eye level sitting in my car’s driver seat – I did have to look up at it. I’d guess my ass was also about a foot off the ground, maybe a little more, and my seat back is quite upright. Another guess is that my eyes would be about 2 and a half feet above the seat. So my eye were maybe a little over 3 and a half feet. So it is possible that I significantly overestimated the height of the bird. I guessed it was taller than my car, but it was not at such proximity that I could be sure – for instance, my view of its head wasn’t actually obstructed by the roof. So maybe its total height was 4 feet or a bit more rather than the six I guessed the first time. I was in a still-moving car, having to dodge left to avoid the birds, and then back to the right again to get out of the way of an oncoming car. I didn’t come to a complete stop during any of it.
And now I’m kicking myself for that. I had a very good camera in the car practically right next to me and I never once thought about using it until after I got home. #$#$$@)U!!
There are ponds in Owens Park. So it’s very possible you saw herons or cranes. And they can get quite large, even while immature. Even egrets can get big.
I just found this page claiming to show the only five species of wild turkey that exist, and I’m pretty certain it wasn’t any of the. The one coming closest by body shape is the Gould’s turkey, which is native to Central America. I haven’t found anything specifying how tall they might get.
Just found this pretty funny story about terrorist turkeys in Owen Park, which is right next to where I saw these birds. And these are definitely not the birds I saw.
Well, so maybe it was an immature crane of some kind? Don’t they go through a moult where they lose their baby fuzz and adult feathers come in? Could that explain the lack of feathers in places? 
Uh huh, I was definitely thinking of something else. Oops.
Well, maybe. Maybe they go though some kind of annual or seasonal plumage change as well.
Nah.
It was a Wisconsin Kangaroo in drag.
Do they really have emu and ostrich farms in Wisconsin? They are native to Australia and Africa, respectively. I can’t imagine that they’d do very well in -20° temps. Maybe in a zoo, but on a farm?
I have heard about ostrich farms in WI, though I haven’t heard of emu farms. Presumably they need heated barns in the winter, but that’s just a guess.
An Emu escaped in time to show up in the Christmass bird count 
Cite: an earlier post by me Can’t find it, so it is possible apocraphal - but it COULD happen
Brian
Sandhill fits for everything you’ve said except for “black”.
Most Sandhill cranes are in groups of three this time of year, they are very tall, and look sort of egret-ish, I suppose.
Are you completely convinced of the black coloration?
Yes, unless it was an extremely dark solid color blue (except for the tail). There were trees behind them, and they were in the shade of the trees, so I didn’t see them in direct sunlight.
Emus are farmed in NH, ME & VT and it’s hard to argue we don’t have harsh weather in the upper northeast.
It sounds like Big Bird and Big Foot had big fun in the barn.
It can get pretty damn cold in parts of Australia, and Emus aren’t just found in the tropical regions; they’re found in regions that get pretty decent snow too. I’d bet they don’t need extra heat, just a shelter maybe.
Hrm, can’t find anything matching that, yet, but I did find this site; maybe it will help me find the white bird I see around here during the winter. 