Hi Joe,
Me and best mate went to St.Mary's in June. We got one of those flimsy hand drawn map guides of the island (when we bought our Scillonian tickets) which turned out to be of great use.
We did a clockwise perimeter walk of a good portion of the island then started working our way in so as to cover a lot of habitat as that's exactly what there is there, a lot of different habitats.
Obviously you've got the journey over for a lot of seabirds, the coastal path takes you over the heathland up on to the moor, then over towards the beach and all the rocky islands, Old Town, The Garrison, The Golf Course, Hugh Town, then we cut inward and actually in to the woodland nature reserve, then round some of the farmland. We went in the wrong season for birding really but you can tell why it becomes a haven for migrants. There's so much different landscape, sheltered coves, little isolated areas of woodland, heath, farm etc.
There's also plenty of articles on the net that tell you what's been seen and which areas of the island are supposedly best for certain birds. We got mainly Warblers, usual Thrushes, Stonechats, Mipit's, and the odd Wader.
I can't remember seeing a sightings board (that's not to say there wasn't one) we relied on the Cornish birding sites...when we got 3G!
Good luck

Thanks John and Pete, I'll certainly look out for that bus and will keep an eye out for a bird report board.

Many thanks,

Joe

Basically rare birds have turned up all over the island(I remember seeing a Yellow Billed Cuckoo in the centre of Hughtown in a garden),any bit of cover/weedy field/hedgerow/short grassy areas such as golf course are worth checking,the Garrison itself has turned up a few goodies over the years,Parula Warbler,Black and White Warbler as just two of the birds I've seen up there,I'm sure there are many more.The whole island is walkable,or a good option is to get the bus to the top of the island(assuming it still runs)and work your way back to the Garrison.Seawatching is best off Penninis Head,could well still be a few Cory's/Great Shears/Storm Petrels knocking about this time of year.