The Long Boardwalk at Bagno Ławki is the signature site, running out across open marsh. This is the place for aquatic warbler — a globally threatened species with a large share of its world population here — along with displaying great snipe at dusk in the right meadows, and the constant sound of marsh birds in spring.

The Biebrza river itself, canoed over one or several days, gives the best overall feel of the valley, with beaver lodges, otters, kingfishers and the possibility of elk swimming across ahead of the boat.

The wet meadows and willow scrub hold elk — Poland's largest population, and best seen at dawn from the dyke roads — along with roe deer, wild boar and the wolves that follow them.

The Red Marsh and the northern basin hold the raptors: greater and lesser spotted eagles, montagu's harrier, short-eared owl in vole years, and white-tailed eagles over the river.

Late April to early June is the peak — flooding, display, and the specialist warblers. Autumn brings migration and rutting elk; winter is stark and largely empty.

Practicalities

Goniądz and Osowiec are the practical bases, around three hours from Warsaw. Local guesthouses cater to birders and know the sites.

Rubber boots are not optional. A local guide is worth it for the specialist species — aquatic warbler and great snipe both need knowing exactly where and when to stand, and the great snipe leks in particular are sensitive and should only be visited with someone who knows the protocol.