We buy bulk citrus before Christmas from a local school for a fund raiser. Almost always have some left over and make marmalade or just juice them for breakfast. This year my wife cut a few up to put out for the birds to see if an Oriole would come by (We see them periodically, although it's a bit early). She was out there one day (warm winter day which we are having a lot more of these past few years) and saw one of the bees sitting on a piece of grapefruit and told me about it. I thought I would throw a few on the compost heap and leave the lid off and see if on a warm day they would come and feed. Today is only 45 degrees, but I saw a few hardy girls feeding. I wanted to post some pics, but can't quite figure that part out yet.
Re: vitamin C
I will try a few oranges and see if they like them.