Why is the Water Rail Rallus aquaticus a very scarce breedernorth of 61 deg N?

Along a transect from just north of Uppsala (c. 60 deg ?N) to Lulea (c. 65 deg 30′ N), the eastern coast of Sweden, we searched for Water Rails Rallus aquaticus. Rails were found at four of twenty-four visited sites, the northernmost site at 61 deg 08′ N. The reason for the absence of rails further north we assume to be a transition of the marsh vegetation from one dominated by dense Phragmites australis and Carex elata with a layer of shallow water above the bottom to one dominated by Potentilla palustris and increasing amount of Sphagnaceae and Polytrichaceae mosses without any water above the vegetation mat. The former vegetation, high and dense but with open space at the bottom, provide both shelter and nest sites and permit the rails to move easily without being exposed to predators. The latter vegetation is too dense and too low for the rails to be able to move without exposing themselves to predators.

G.H.J. De Kroon & M.H.J. Mommers, ORNIS SVECICA 13: 47-52, 2003