Sometimes birds can be sleeping and yet appear dead so with him been still on his nest, personally, I wouldn't worry too much yet but, assuming he is dead, there could be a good number of reasons for this both natural and man made. Birds are highly complex in their inner workings and organs and a bird could easily perish due to a number of factors such as air sac infection or a problem with the food supply?
mink are usually a big problem for all water birds and unfortunately, they have a particular keen taste for eggs and chicks, but the parents wouldn't be still on the nest if that was the case here. (Mink should be reported and I believe Ian has put on this forum a mink report form in the other wildlife tab.)
Man made reasons include that it could be tangled in fishing line and not been able to feed. But either way, any suspicious activity must be reported to the police or rspca but I don't think that is the case here. Hope this helps.
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Which bird is ideal for keeping cakes in? I asked. The answer: a Bun-tin.
http://www.flickr.com/photos/135715507@N06
Having only recently taken up birding, last night I took out a new pair of binoculars onto Worthington Lakes to see how they performed. The binoculars were fine but I was shocked to see that a Great Crested Grebe I photographed on the nest only 3 days earlier, was still on the nest but dead. I recall seeing glimpses of 3 eggs on the nest when I first saw it too.
I ask myself how this could have happened? It really is very sad indeed. Is this likely to be by human hand?
What is the procedure in these circumstances, should any authority or organisation be notified?