I think the easiest place to see Corncrakes on the Uists in May is at the RSPB Balranald reserve on North Uist. Go for an evening walk around the headland there (good for otters) and then aim to get back to the visitors centre (where the car park is) and just stand and listen. They keep the grass long in the garden to make it attractive to Corncrake.
If it's early to mid May Balranald is a hot spot for migrating skuas, including Pomarine and Long-tailed if you get very strong WSW winds. Rubha Ardvule on South Uist is good for sea watching too. The long beaches running north and south from there are good for waders as is the "ford" near the Hebridean Jewelry shop at the top of South Uist (needs to be a high tide for this).
Other good spots:
- the river and beaches either side of the Church at Howmore on South Uist - usually Great Northern Divers offshore well into May with possibility of both Red-throated and Black-throated Divers too, Corn Buntings here also.
- the road past Loch Druidibeg to Loch Skiport takes you into Golden Eagle country, could be Red-throated Divers on Loch Drudibeg.
- take a day trip from Vatersay onto Barra to go and see the beaches on Eoligarry - including the beach where the planes come to land.
- the view from the Pollacher stone across to Barra is superb and it's a good location for otters too and the Pollacher Inn can be good fun on a lively evening
- my best otter spot is on North Uist where you walk from the Langass Hotel toward the stone circle - go slightly past the stone circle and sit and watch on an incoming tide for an hour or so.
- the "Committee" road across the middle of North Uist to Solas is another good spot for raptors including golden eagles
- and if you want to go slightly further afield go onto Berneray and walk the length of the beach on the west coast