Soon after the settlement of Iceland in the late 9th and early 10th century, an extensive system of turf walls was erected and was functional for about 200 years. After its collapse in the 12th century, few if any walls were built until the late 18th century. The remnants of this Viking Age fence system have been mapped in northeast Iceland, revealing enclosures with a characteristic geometry, broadly reflecting property boundaries. The fences are the product of a pastoral society, which then as now was based on dispersed farmsteads subsisting on animal husbandry. There was a strong seasonal component, most notably a need for harvesting and storing winter fodder. The short-lived fence system offers a rare snapshot of the division of a newly colonized landscape into functional units. This paper presents a framework for evaluating those units based on current thinking in behavioral ecology. The settlement of Iceland by the Viking Age colonists may be described by principles of habitat-selection theory, and the configuration of fences may be understood by factors governing division of spatial resources by territorial organisms. These factors include density, predictability and dispersion of resources, habitat geometry, intruder pressure, and economic defendability. Finer-scale fence patterns emerge from the interaction of territorial boundaries and a need to restrict livestock movements within the farmstead. Illustrative examples are drawn from the author’s own studies of settlement patterns and configuration of defended space in wild ducks and also from the medieval Book of Settlements, which describes the early colonization of Iceland.