I would strongly suggest that you obtain a copy of the relevant OS maps as soon as possible after you arrive, or before then if possible - the one covering the Porthcawl area is Sheet 170 (Vale of Glamorgan & Rhondda), and you may also want to get hold of Sheet 171 (Cardiff & Newport).
Spots to visit include:
1. Kenfig Nature Reserve. This is very near Porthcawl (maybe 4 miles), and is one of the best birding spots in the whole of South Wales! It should be signposted from the middle of Porthcawl, but any local map will show Kenfig Lake, around which the reserve is centred.
The lake is shallow, and pretty good year around for ducks, grebes etc - it can hold unusual species, although these are more likely in winter, but in September you would have a chance of something unusual. There are extensive reedbeds around the western and southern shores, with 2 blinds which give good views. Around the lake are area of meadows, scrub and sand dunes - not brilliant in the summer months, but could hold almost anything during migration - Kenfig probably turns up more rarities than the rest of South Wales put together.
Most birders work their way around the lake, checking the scrub along the way, or alternatively, you can take the very pleasant walk westwards to the sea via Sker House - good migrant opportunities, and there may be something of interest on the shore or on the sea.
As with most migration hotspots, it depends entirely on how lucky you are - you could see loads of stuff or absolutely nothing, depending on circumstances, but as it is so near Porthcawl, it's a very good place to start, and will let you see some commoner resident birds.
2. Eglwys Nunydd Reservoir. This lake is maybe 3 miles north of Kenfig, and is usually visited in conjunction with it - birds often commute between the two. Again, better in winter, when it often holds some uncommon ducks, loons etc, but worth a quick look whenever you pass by - might have some waders around the shores, especially at the top left hand corner as you go in.
3. Darren Woods. One of my personal favourite spots, and an excellent one for the more typical summer breeding birds of Welsh deciduous woodlands, such as Redstart, Pied Flycatcher, Wood Warbler etc. Better for these in spring . early summer, of course, but should be some still around in early September. It's about 15 - 20 miles from Porthcawl. To get there, drive north out of Porthcawl on the A4229 to the M4, where you go east, towards Cardiff.
Exit at Junction 36 (Sarn Services & Bridgend), and go north towards
Aberkenfig, Tondu and Maesteg. Follow the A4063 towards Maesteg,
until you get to the village of Llangynwyd. Here, look for a minor
road off to the right towards the village of Pont Rhyd y Cyff, but don't
go all the way there. You are looking for a pub on the left called
the Tyler's Arms, where a small road goes off to the left. There
is a stream here which can be good for Grey Wagtail and Dipper. You
can either park at the Tyler's Arms, or try driving up this minor road
- right at the top there is just about room to pull the car over on the
last bend, but make sure you don't block access to the farm here.
From the apex of the bend a track leads off into some very nice woodland
- you can walk for a couple of miles here through lovely
habitat. If you can't park here, you will have to return to the Tyler's Arms, and walk back up - maybe a mile each way.
4. Ogmore Estuary. Good spot about 10 miles east of Porthcawl. Go east on the A4106 for 3 miles, then turn east onto the A48. Continue until you reach the western outskirts of Bridgend. Turn right here onto the B4265 towards St. Brides Major and Ogmore, then turn right again in the village of Ewenny onto the B4524. Continue past Ogmore Castle, past the entrance to a sewage treatment plant, and look for a grassy parking area on the right shortly after. This is right by the river, which although not the most scenic of spots can offer very good birding, depending on tide conditions - shorebirds, ducks etc, plus chance of Rock Pipit. If you continue towards Ogmore, just as you reach the sea, you reach a more official car park on the right - can also be good here, although may be a lot of people around. Keep your eyes open for Chough here - VERY rare bird in Glamorgan, but one or two birds have been hanging around this general area for a few years and are often seen in this car park.
5. Dunraven Park. 2 miles down the coast from Ogmore. Follow the B4524 along the coast to Southerndown, where you turn right onto a minor road to the park. Park in the beach car park, and walk along the clifftops into the park. I haven't really worked this area properly, but can hold decent birds (another place for the Chough!), and great views.
6. Nash Point. A good migration hotspot c. 8 miles south east of Ogmore - one of my favourites. From Southerndown, continue on the B4524 to St. Bride's Major, where you turn right onto the B4265 towards Wick. Before you get there, (after only c. 1.5 miles), turn right on a minor road to Monknash. Continue through that village to the next village of Marcross, where you turn right to Nash Point lighthouse and car park. A lovely spot, with a good mix of habitat - the small wooded valley on your right before the car park is always worth exploring, as are the dunes and scrub in front of you, and you should also walk down to the lighthouse, and follows the path along the clifftop beyond, checking the fields and scrub. Can hold pretty much anything on migration, depending on weather conditions.
7. Further afield, another of my personal favourite spots, the
Glamorgan Canal at Whitchurch, is actually in Cardiff - maybe
35 miles from Porthcawl,
but almost all on motorways. Drive north from Porthcawl on A4229 to the M4, and turn eastwards. Continue along this to Junction 32 (Cardiff North).
Turn off here, and follow the road around, passing the exit for the main road into the centre of Cardiff (A470). Take the next turn to the left, signed for Whitchurch (A4054). Pass Whitchurch Hospital on your right and continue until you reach a small roundabout. Take the 4th exit off this roundabout (Felindre Road), and continue past Felindre Hospital. Go through a really small roundabout, after which the road bends around to the right, where there is a small parking area. Park here and walk along the canal towpath - you can follow it for a couple of miles. There is a side track to the left part way along which goes through some fields, and gives access to a couple of blinds overlooking some wet areas. This is a very good site for some commoner woodland and water birds, and always seems to turn up
something nice - Kingfisher is regular here, and things like Green and Great Spotted Woodpeckers, Bullfinch, Siskin, Redpoll, Treecreeper, Nuthatch, tits
etc are often about.
8. If you fancy a trip a little further afield, the new reserve at Goldcliff east of Newport is well worth a visit - it's still only about 50 miles from Porthcawl, so less than an hour's drive. TO get here go along the M4 as if you were going to Cardiff, but go further along, and turn off at Junction 24. Follow the roundabout around to the right, and take the A455 around the edge of the large town of Newport. You will soon reach a roundabout where you turn left. Go through the next roundabout, and then left at the next, towards Nash and Goldcliff. There are two sections of this large (for Wales!) reserve that are worth visiting. The first is reached by turning left in the village of Nash towards the Uskmouth Power Station. Turn into the new car park on the left just before the power station, and follow the network of tracks and dikes between ponds and reedbeds to the foreshore. Good birding possibilities right through this area.
The other site is reached by continuing straight ahead in Nash until
you reach the village of Goldcliff. Go through this small village,
and at the end of the village you will see a wide farm track on your left.
Keep going past it, looking for a small road off to the right to a small
grassy parking area. Be careful; here - the road goes over a humpback
bridge, so watch your ground clearance, and be careful where you park -
I've got bogged down here after rain. Also, don't block the farm
entrance. Follow the path for a very short distance to come viewing
areas over the newly created scrapes, which can have some excellent birds
any time from August onwards - this is the latest rarity hotspot in South
Wales, and despite only having been open for maybe 3 or 4 years, has already
amassed a very impressive list of rarities. North American waders
feature very strongly on this - I once had Buff-breasted Sandpiper and
Pectoral Sandpiper in the same scope view at the
same time here! It's also a good spot for Little Egret, a recent colonist, and could have rarities such as Spoonbill if you are lucky.