I still like this idea but I've thought up a few issues that have to be considered...
Firstly there's no way to moderate or edit messages, meaning if a newby decides his juvenile GSWs are middle spotted woodpeckers, or someone decides their 100 waxwings in their gardens are cedars because they've done a Google Image Search then there's no way of ridding it of stringy reports without deleting the user (which is a bit heavy handed).
More importantly the same goes if someone irresponsibly says "I've just found a goshawk nest! It's in the tallest spruce at grid ref SKxxxxxxx!", or "a golden eagle just flew towards a large grouse estate!", or advertises a rarity on private land without landowners' consent or some other inadvisable scenario.
Secondly there's no way of filtering results, so anything near to you may be buried by rares in far flung corners of the country. Perhaps services set up with county-wide news would be more useful? It could even fill gaps that the news services leave behind - that is species so common they don't deserve a mention on Birdmap, for instance, but locally would be fairly significant and worth seeing for some (for example a yellow wagtail or flock of red-legged partridges in my end of Sheffield).