Herons.

Each pair of nests belongs to one pair of herons. Draw a flight path between the two nests of every pair. Paths run through neighbouring cells, never cross, and every cell of the marsh is used by exactly one path.

A path steps from one cell to the next, up and down or left and right, never diagonally. It never runs beside itself either: no cell of a path may touch another cell of the same path except the one before it and the one after it. Two nests with the same number are the two ends of one path, and no cell is left over when the marsh is finished.

When every pair is joined and every cell is used, the marsh is finished and the game notices by itself.

Press on a nest and drag, and the path follows your finger from cell to cell; drag back along what you have drawn and it rubs out again. Dragging over your own path shortens it to that cell, and dragging into a cell another path uses takes that cell away from it. If you would rather not drag, tap a nest and then tap one neighbouring cell after another; tap the same cell twice to stop. A tap on a cell of a path clears it from that cell on.

Nothing lights up while you play: finding a mistake is part of the puzzle. Check first tells you only how many cells are wrong and roughly where on the path. Press it again and it shows which. Hint names the rule and the place, and the second press draws that one cell for you. If something on the board is already wrong, the hint points there first. Hints and checks are free and counted, and shown when you finish. Clear empties the marsh; the clock keeps running and one Undo brings everything back.

Keyboard: arrows move the cursor from cell to cell, Enter starts drawing from the cell you are on and the arrows then extend the path, Enter again or Escape stops. Delete clears the path from the cursor on, U or Ctrl+Z undo, R or Ctrl+R redo, P pauses.

The clock runs from your first cell to the last one and pauses when you leave the tab or sit still for a minute, which also hides the marsh. Your progress, time, history and day streak stay in this browser, and in your account when you are signed in, so you can continue on another device.

A marsh finished with no hint and no check counts as clean; your history keeps count. Only the marsh of the day builds the streak, and after five days in a row one missed day is forgiven, at most twice in a month. When you finish, Share copies one line with the date, your time and how much help you took. It never copies the marsh or your paths, so nothing is spoiled for anyone.

Monday and Tuesday are easy (five by five, three or four pairs), Wednesday and Thursday medium (six by six, four or five), Friday and Saturday hard (seven by seven, five or six) and Sunday is the challenge: eight by eight with up to eight pairs. The week strip under the marsh takes you to the other days; the archive goes back a year, and the practice sets have eight marshes of each level with no date attached.

Difficulty is measured, not guessed. Every marsh is solved by a program that only knows named rules, in three layers: what one nest or one cell settles on its own, what follows from which pair can still reach a cell and from a part of the marsh being cut off, and the hard ones, where a step is tried out and ruled out because it ends in a contradiction. Easy and Medium never need that third layer, so they never ask you to try anything out; Sunday leans on it the most.

Start in the corners. A nest in a corner has only two ways out, and a corner cell with no nest on it has only one way through: both of its sides must carry the same path.

Every cell of the marsh is used, so a cell with only one free side left has its path decided for it. That one rule alone opens most of an easy marsh.

A path never runs beside itself, so a cell next to a path that is already there is closed to that path, even when it looks free.

When a corner of the marsh can only be reached by one pair, every cell of that corner belongs to it, whatever the shortest route between the two nests looks like.