OK I will state the obvious, which I suspect will be wrong. But I assume that whereas we can not tell, they can and are not hangin' out with the fake on the right. Of course it could be the opposite.
I doubt there is duck-hunting in that neighborhood park so unclear why there would be a decoy...
I'm intrigued by all of your answers! If you haven't clicked into the full res photo, you might want to do that before you read the answer.
In person, it's about as hard to see the details on the ducks as it is in the photo, so if it was hard for you to tell which one was fake, it was for me too. The far left one (lightest one) was easy to tell because as long as you look carefully, it has no eyeball. So when the wind blew harder, you could sometimes see the wooden underside and tell for sure it was fake.
But! I have never seen an unaccompanied female Mallard which the lone one on the right was. So I thought why doesn't one or both of the males go over to the real one? Also, males don't usually like to be that close to each other. I watched them for a long time and they were bumping each other quite a bit. Then I realized the males were fake too.
So then, I thought oh that must be why the one on the right is alone. Because the three aren't even real. Happy that I figured it out, I went around the lake, but when I came back, all of them were still in the same formation. I looked suspiciously at the lone one for a while and yep, it was also fake. All of them! Fake!