First of all I hope that your planned site has different objectives to Birdforum, else you will be competing for membership and you may struggle
A big problem will be to get people to submit their sightings everyday when there are so many already doing this ie Birdcall, Birdline, Birdforum etc...
I should think you will have to come up with an entirely new idea to attract sufficient members to keep the site going.
You may be better off trying to stick to one county initially and then expand the area covered, else utilise the information from other sites and newsgroups to build up birding maps.
I already try to judge the movement of migrants across the country in my head from viewing various sites, in the hope they may be heading my way. It works surprisingly well in so far as I have a good idea what to keep my eye ope for next time I go out.
I find the problem with using bird club data is that it is all historical, by at least a couple of months, but if you have records for five years plus, you could try to establish trends and move on to making predictions of new arrivals and departing migrants.
As has been touched upon, the more narrowly defined you make your website, the fewer the number of members you will have.
Steve in his 'subtle' manner hit the nail on the head. Reciprocal links will tremendously boost the number of visitors, but then it's up to you to keep them !