At least the seagulls were happy.
CBS Chicago reports that many Lake Michigan angler was welcomed with this troubling site. On the far south side thousands of perch have been found dead or dying on the lake's surface. So many fish were dying that it nearly covered the 2,000 foot slip.
The good news is that the DNR is looking into the issue, although at the time of this writing, the cause is still unknown. However, it's not the first time there has been a massive, initially mysterious fish die-off. In the past, causes have included the spread of a parasite as well as a large movement of fish to a shallow area, causing an oxygen deficit.
I imagine several ducks, seagulls, and other fish enjoyed the nice meal this provided. I would not worry about the birds catching whatever killed these fish. While it is not impossible, it would have to be a pretty potent toxin to pass into the birds.