I should point out that it’s quite a few years since I visited this area, so some of the information below may be out of date, but here goes:
Lac de Maine is in south west Angers: from memory, it is signposted off the N23. There is a good campsite (“Camping Lac de Maine”) and a hypermarche next to the lake. There is also a large pyramid-shaped recreation centre on the NW shore, and lots of parking.
From here, follow footpath south along the western side of the lake – some decent habitat along here, with reeds and willows. You will soon reach the river Maine: turn right (SW) and follow the track along the river until you reach the Pont des Pruniers. Cross this and you enter a large area of seasonally-flooded meadows. This is an excellent area for Corn Crake (best looked for in the evening, when you should be able to hear several calling males).
Fontevraud Abbey is a very well-known tourist spot about 15km SE of Saumur. From Saumur, take the D947 along the south bank of the Loire (worth stopping to scan the river where you can), turning south at Montsoreau. The abbey is a couple of km south of here. You should be able to find Rock Sparrow without difficulty as you walk around the grounds – I saw several sitting on the buildings.
There is some good heathland just SW of Fontevraud, but it’s in a military area so access may be restricted. From Fontevraud, take the D162 west towards St Cyr-en-Bourg, then turn south onto the D93 to Breze and Epieds.
For Little Bustard, Montagu’s Harrier etc drive south from Saumur along the D347 to Montreuil-Bellay (which has a good camp site). Stay on the D347 as it turns SE towards Loudun, and continue for a few kms until you see a minor road heading north signposted to Meron. Take this road, and after a few hundred metres, turn right (west) along a dirt track into an area open grassland. Stop and scan here.
A good area for woodland birds is around the Hippodrome along the road between Verrie and Marson, about 5km west of Saumur.
