An important part of the natural values in The Netherlands is based on the fact that the country has a unique geographical position in temperate lowland Europe at the mouth of the rivers Rhine and Meuse. This creates a number of interesting gradient situations e.g. between saltwater and freshwater systems, between eutrophic and oligotrophic systems, and between tidal, streaming and stagnant waters. This position offers excellent conditions for a wide variety of wetland systems: river and clay marshlands with open water and macrophyte vegetation like reedlands, peat marshland with bogs, fens and mires, as well as estuaries with saltmarshes. This marshland diversity resulted in a very diverse avifauna with many species occurring in high densities. The diversity and quantity of these wetland ecosystems, however, is severely threatened and consequently the number and the distribution of typical marshland bird species decreased (Den Boer 2000). Acidification, euthrophication and desiccation, cultivation and unfavourable management practices all contributed to a decline of total area and an increasing degree of fragmentation of the remaining habitat for marshland birds (Figure 1.1). The Dutch Nature Policy Plan, published in 1990 (NPP 1990), mentioned fragmentation as one of the most important threats to biodiversity. A nation-wide ecological network of nature areas was proposed as the solution for this problem. It is vital for the success of this strategy that the implementation is underpinned by ecological knowledge of the underlying processes in this network system.The process of habitat fragmentation leads to landscapes with dispersed small populations within an inhospitable matrix. Small populations are likely to go extinct by stochastic demographic fluctuations. Consequently, the viability of these small populations depends on the likelihood that they will be recolonised by individuals from elsewhere. Crucial is whether the distances most individuals are likely to cover between years, as a result of the ...