Tree frog choruses of Zeeuws-Vlaanderen
European tree frog (Hyla arborea) — c. 2,000 calling males in the Dutch part and ~1,000 across the Belgian border; marsh frogs, natterjacks in the dunes. West Zeeuws-Vlaanderen: Retranchement, Cadzand, Sluis, Aardenburg, Netherlands.
Best time: 1 May – 31 May (May). Where: Retranchement – fortress ramparts and moats, Aardenburg – polder pools on the Sluis–Aardenburg route, Wallen van Sluis (Het Zeeuwse Landschap) (Netherlands). Time of day: Males start as the light goes and run for a few hours — best from about 30 min after sunset to midnight on evenings when the air stays above 10 °C.
- You can go on your own
- Free
- No binoculars needed
- Sound 5/5
Open it in the interactive calendarSunrise and sunset listening times for the day you pick, and the map.
When is the best time?
Peak: 1 May – 31 May. May, on warm evenings; calling from mid April to early June, weaker calling again in late summer
Active from 15 Apr to 10 Jun (April to June) in a typical year.
Time of day
Males start as the light goes and run for a few hours — best from about 30 min after sunset to midnight on evenings when the air stays above 10 °C; a sunny day beforehand makes the difference.
- Evening chorus (warm nights): 20 min after sunset → 00:00
What will you hear?
A rattling, surprisingly loud 'kè-kè-kè-kè' chorus from hundreds of tree frogs around the old fortress moats and pools — audible across the polder — with marsh frogs and nightingales behind; the ramparts of Retranchement alone hold a few hundred.
Binoculars?
No binoculars needed. A pond-side concert heard from the road; a red torch to see the 3 cm frogs on reeds, no binoculars needed.
Where to go
- Retranchement – fortress ramparts and moats — Village on the Zwin edge; public paths round the ramparts; a few hundred males. Map
- Aardenburg – polder pools on the Sluis–Aardenburg route — Cycle route Cadzand–Retranchement–Sluis–Aardenburg: frogs audible along the whole way on a good evening. Map
- Wallen van Sluis (Het Zeeuwse Landschap) — Town walls and moats of Sluis, public footpaths. Map
How to go on your own, and what it costs
Base in Cadzand, Retranchement or Sluis; cycle or walk the lanes between the villages after sunset; the Retranchement ramparts and the Wallen van Sluis have public paths. Free, no hours.
Cost: Free.
Field-recording notes
The classic approach is by bicycle at dusk from Cadzand through Retranchement, Sluis and Aardenburg, stopping wherever the chorus is strongest — all public roads and paths, no permits. Choruses are at ponds a few metres from the lane; traffic is light but the Belgian border roads carry some. Tree frogs are strictly protected: no handling, no entering the pools, no playback (it disrupts calling). Pick an evening after a warm sunny day; a cold front kills it instantly. Het Zeeuwse Landschap counts the population by ear each spring.
Sources
- Vroege Vogels (BNNVARA) – Boomkikkers in Zeeuws-Vlaanderen doen het goed (2020; Het Zeeuwse Landschap counts)
- RAVON – Boomkikker (calling period, call)
- Het Zeeuwse Landschap – Wallen van Sluis
Similar spectacles
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More: all spectacles in the Netherlands · frog and toad choruses · the Netherlands in May