Ruby,
In case you are still looking for information, I was in the Algarve during a few days in the first half of October 2002. Here is some information on a set of sites resonably close to Tavira that may be of use.
Quinta do Lago is somewhat surreal, being a hyper-posh resort that couldn't contrast more with the small Portuguese villages you drive through to get there. Very reminiscent of upmarket American towns. Once you have recovered from the shock, walk east along a marked trail (Pied Flycatcher) to the Sao Lourenco Lake, a water body on the edge of a golf course. Although man-made, it has become rather semi-natural. Purple Gallinule was easy to see with a bit of patience; I had 8 there as well as Kingfisher and White Stork and the usual array of waterfowl.
A bit further east in the woodland along the trail were Azure-winged Magpies. If you continue even further east, you arrive at the western edge of Ludo Farm, a wonderful labyrinth of saltpans. In the past there were access restrictions there but apart from a couple of private signs around a small block of buildings access was open throughout.
Birds encountered there included flocks of Greater Flamingo, Marsh Harrier, several Kentish Plover, 2 Hoopoes, 2 late Red-rumped Swallows, White Stork, 90 Azure-winged Magpies gathering to roost, Woodchat Shrike, 2 Common Waxbills, Hoopoe, Subalpine Warbler, Purple Gallinule, Cettis's Warbler and 3 Caspian Terms on the river as well as several species of common waderss and ducks.
The western section of Quinta do Lago, including the pool at the end of Avenido do Ayrton Senna were comparatively unproductive, though I did see 3 Azure-winged Magpies in the scrub at the western end of the saltpans.
Another site that is near the above are the Montenegro waterworks. Situated adjacent to the airport (right behind the car park of the rental companies), they are nonetheless difficult to find. From the N125 drive into Montenegro, turn right at the cross and continue all the way down to the airport. Here a dirt track is sandwiched between some arable land behind a fence, a small river and the airport fence. Follow this down to the end - beware, though, it is not a good track. Although this spot has yielded some good birds in the past, it was unproductive when I was there in the evening. A few Curlew Sandpiper amongst the Dunlin, two Grenshanks, 3 Sardinian Warblers and a couple of Common Waxbills is all I saw.
There are other good sites within reach (though outside your 50km), such as Castro Verde about an hour and a half to the north in Alentejo region (close views of Great and Little Bustards, Stone Curlew, Azure-winged Magpie, Red Kite, Hoopoe, etc.). Note though that nobody speaks English up there but it is worth visiting to see the low intensity farming practised there and associated wealth of birds.
If you need additional information on the above or other sites, let me know.
Cheers,
Andre