Does anyone understand this behavior? I don't think it was sharpening its bill -- it was digging the tip straight into the concrete, working pretty hard at it too, which I would think would tend to dull the bill; it wasn't stropping it sideways to sharpen. Dunno, can't rule it out -- maybe there's something about what it was doing that was serving to sharpen the hook at the tip. It just really didn't look that way. Going after minerals -- calcium? But the Everglades are limestone, and I remember seeing outcrops of the stuff fairly often. That's soft calcium carbonate, a lot easier to eat than concrete. Nevertheless, they've clearly been working at this spot for a while -- you can see that it's hollowed out. Grit for the crop? But I'd think you'd want larger pebbles; and those are laying around loose all over the place.

Maybe it's after trace minerals. Portland cement is listed as 3% Fe2O3, which probably makes this a pretty good iron source.