The "Surfplas" is the largest lake of the Reeuwijksche Plassen and is a very important area for waterfowl. Especially in winter, large numbers of waterfowl can be found here of which Eurasian Wigeon is the most numerous. Numbers can go up to 100.000 individuals! Other species that can be present in large numbers are Common Pochard, Tufted Duck and Common Teal. Small groups of Red-crested Pochard can be found near the shore in the central part of the lake. Above the deeper parts of the lake, groups of Common Goldeneye and Goosander are present. Also Smew can be present in small numbers. Often, scarce species can be found within these large groups of waterfowl, like Common Scoter, Velvet Scoter and Ferruginous Duck. Besides ducks, swans en geese make use of the area as resting habitat at night. Sometimes the large numbers of waterfowl can be restless when predators are present, like Osprey.
Besides ducks, geese and swans, many other waterfowl are present in the area. Good numbers of Black-necked Grebe can be present near the shore in the central part of the area. Also Little Grebe can be found here and sometimes Red-necked Grebe and Slavonian Grebe (Horned Grebe) are discovered. Divers are sometimes found between the large numbers of waterfowl, like Red-throated Diver and Great Northern Diver (Common Loon). In the central part of the lake, near the shore, small parts are fallen dry and attracts waders of all kinds, like Dunlin, Curlew Sandpiper and Common Greenshank. Flocks of Northern Lapwing, Eurasian Curlew and Ruff can be seen passing overhead towards the surrounding pastures.
Gulls and terns make use of the area to rest and forage for food. Large groups of Black Tern gather here in early spring just before leaving towards their breeding grounds inland. Sometimes, a single Whiskered Tern or White-winged Tern can be seen within these groups. Common Tern uses the area to hunt and take their prey towards their breeding grounds on the islands in surrounding lakes. Black-headed Gull and Common Gull als breed on these islands and rest in large groups on the "Surfplas". Small numbers of Mediterranean Gull also breed between them and can pass overhead. In early spring, small numbers of Little Gull can be seen above the lake. Large gulls rest on the wooden pillars in the lake and sometimes Great Black-backed Gull, Caspian Gull and Yellow-legged Gull can be found here.
In several surrounding areas, large reedbeds are present and being used by many species of reedbirds. One of the most peculiar species is the Cetti's Warbler, which can be found in larger numbers around the lake. Many common species like Sedge Warbler and Reed Warbler breed here and sometimes Grasshopper Warbler and Savi's Warbler can be heard. If you're lucky, you'll hear a Water Rail or see a Great Bittern.
The surrounding pastures are used by large flocks of geese, like Barnacle Goose and Greater White-fronted Goose. Sometimes, a Red-breasted Goose can be found within these large flocks. The pastures and ditches also attract Great Egret and waders, like Ruff and Black-tailed Godwit.
The area is easy accessable on foot and by bike. You can make a circular walk around the lake of about 7 km. You can park the car on the large parking lot on the western side of the lake. There's also a restaurant, showers and changing rooms for swimmers and surfers. The circular road is completely paved, except for a small part in the southwestern part. On the eastern and southern side of the lake a birdwatching hide is present from where the waterfowl is easy visible.