Hilfield Park Reservoir (map; OS grid reference TQ157959) is a local nature reserve in Hertfordshire, sandwiched between Elstree Aerodrome and the A41 and M1 routes into (and out of) London.
The reservoir has a storage capacity of about 2,700 megalitres (600 million gallons), which is apparently enough to make 11 billion cups of tea. It covers an area of about 46 hectares (115 acres — or 65 football pitches, if you prefer) and is surrounded by 32 hectares (80 acres) of woodland, grassland and scrub.
Because access is restricted, the reservoir is notable among London-area sites for its lack of disturbance. Birders denied access can obtain distant views from two awkward viewpoints (see below). Access is available through membership of the Herts and Middx Wildlife Trust, whose annual review lists what can be seen at the site.
Hilfield Park Reservoir was constructed in the early 1950s as a reservoir for drinking water, although it is not currently used for that purpose. It was established as a local nature reserve in 1969 in an agreement between Three Valleys Water Services Plc (now Affinity Water) and Hertfordshire County Council. Management is undertaken by the Hertfordshire and Middlesex Wildlife Trust.
Fed by water from the chalk aquifer, the reservoir supports abundant aquatic weeds, including stoneworts. A lack of islands is compensated for, to an extent, by a couple of rafts, on which a few Common Terns nest, and a floating Sand Martin bank installed in 2008. Around the margins of the reservoir are large boggy areas where many species of marsh plants flourish, including reed mace, reed canary grass and various sedges. The reservoir is surrounded by woodland, rough grassland and invading oak and bramble scrub, which is all managed to encourage a diversity of species.
Download the Hilfield Park Reservoir 2011 Report
Birds In winter, the reservoir is a refuge for large numbers of ducks, particularly Mallard, Shoveler, Teal, Wigeon, Pochard, Tufted Duck and Goldeneye. Scaup, Common Scoter, Goosander and Red-breasted Merganser are also possible. Especially after easterly winds or in harsh conditions, the site can also harbour Divers and Red-necked, Slavonian and Black-necked Grebe. The site has a gull roost that can contain Mediterranean Gulls.
In spring, passage migrants include Black-necked Grebe and the occasional Red-necked or Slavonian Grebe. Ducks may include the odd Garganey, Common Scoter, Scaup and Red-breasted Merganser. Passage raptors include the occasional Osprey. Wader species pass through in small numbers and include Oystercatcher, Grey Plover, Little Stint, Dunlin, Whimbrel and Turnstone. Little Gull, Sandwich Tern, Arctic Tern, Little Tern and Black Tern are all seen on occasion. Passerines seen on the embankment include Larks, Pipits, Wagtails, Wheatear, Whinchat and the occasional Redstart.
In summer the site supports breeding Sedge Warbler, Reed Warbler, Whitethroat, Garden Warbler, Blackcap, Chiffchaff and Willow Warbler, along with Common Tern (about 10 pairs), various common waterbirds and Hobby. Buzzard have also bred in recent years.
A species list for recent years is available at the British Trust for Ornithology BirdTrack website.
Perhaps the site's greatest claim to fame is its discovery in 2011 of an Eastern Crowned Warbler — only the second to be found in Britain and the first to be caught and ringed. Other rarities have included Spotted Sandpiper (1956 and 2012), White-headed Duck (2005), Blue-winged Teal (1998), Green-winged Teal (1961) and Ring-necked Duck (2001)..
Mole - occasional mole-hills indicate presenceEdit
Rabbit - resident in small numbers
Grey Squirrel - resident
Fox - often seen, breeds
Weasel - occasional records
Badger - tracks found in 2011
Muntjac - one or two, has bred
Invertebrates More than 20 species of butterfly and more than 15 species of dragonfly and damselfly have been recorded at the site. The Ruddy Darter Dragonfly returned in 2008 after an absence of six years.
Directions From London, the site is most easily approached along the A41 Watford road. About 1km north-west of the A411 (Bushey–Elstree) roundabout, keyholders should turn into Hilfield Lane then after about 300m turn right into the water company car park, locking the gate behind them after signing the visitors' book.
Rarity news services often advise viewing from a footbridge over the M1 opposite Hilfield Lane but note that the view from here is now mostly obscured by tall trees.
Alternatively, southeast of Hilfield Lane, turn off the A41 into Dagger Lane and then left into Hogg Lane. Park just inside the Elstree Aerodrome site and take the public footpath south across a field (typically very muddy thanks to the resident horses) to reach a viewing platform (map TQ159961). This offers an adequate telescope view of two-thirds of the reservoir. Although distant, from here there is actually a better view of the reservoir's western edge (and tern rafts) than from the dam, and the muddy field and its hedges can often produce Yellow and White Wagtail, Wheatear, Little Owl, Redstart and even Whimbrel as well as numerous migrant thrushes and warblers. On the southern bank a channel in the reeds has been cut and aligned to provide views of Bittern from the platform for the hopeful. Note however that the viewing platform is particularly unsheltered and extra layers of clothing are always necessary even on mildly cold days.
Access Access to the site is restricted to members of the Hertfordshire and Middlesex Wildlife Trust who may borrow a key from the St Alban's headquarters (although escorted visits are occasionally possible through local wildlife groups). Once inside the site, visitors must climb a long flight of steps up the reservoir dam. Non-keyholders must risk losing their boots in the boggy approach to the viewing platform near the aerodrome. Wellies are recommended as the field becomes waterlogged after rain.
Facilities Apart from a single hide on the south shore of the reservoir, there are no facilities on site. Refreshments are available locally at the 3 Greens Restaurant at Elstree Aerodrome (open daily from 9am) (map). Somewhat more basic refreshments (along with a limited range of Winnie-the-Pooh characters) can be obtained from the Aldenham Country Park refreshment kiosk, which can be reached on foot from Dagger Lane or by car from Aldenham Road (map but note the £5 parking fee and do not turn into the approach road unless you want to pay). The kiosk is open daily from Easter to the end of October and at weekends throughout the year, weather permitting.
Two local hostelries serve good pub food. For birders, the better choice is the Fishery, on the A411 Elstree Road/Watford Road (map) because its “outside eating terrace” and its large windows offer views of waterbirds on Aldenham Reservoir (formerly described on the pub’s website as “Aldermaston Lake” and now referred to as "Elstree Lake"). And the staff are friendly too. The Fishery opens at 11.30am from Monday to Saturday and at noon on Sundays. Closer to Hilfield Park, but less appealing ornithologically (and also, in my opinion, culinarily, if that is a real word), is the Battle Axes, at the junction of Aldenham Road and Butterfly Lane (map), which opens daily at noon.
This information has been cobbled together from various sources by Andrew Haynes, who has eaten at the 3 Greens, the Fishery and the Battle Axes but has never actually had access to the reservoir, even though he was once briefly a member of the Herts & Middx Wildlife Trust (when he failed in an attempt to borrow a key to the site).