The most limiting resource in urban areas seems to be daytime resting areas. Although built primarily to provide shelter for humans, cities also provide an abundance of shelter for non-human animals. Foxes can take advantage of many of these, occasionally even setting up dens in occupied houses. The most important factor affecting foxes in human areas seems to be human disturbance. It should be no surprise that fox densities are higher in quiet, owner-occupied housing areas or in isolated places like railway embankments than in busier urban areas.
Foxes also avoid feral dogs, probably because dogs prey on cubs as coyotes do in the wild.
Given that fox densities are higher in urban areas than in rural areas, it is not surprising that the territories foxes keep are smaller. What is surprising is that these ranges are fluid, and shift from month to month, or even week to week. Food availability is generally higher in urban areas than in rural areas, but that availability may fluctuate greatly among specific sections of the city. A fox that constantly explored new areas and re-explored old ones would tend to be more successful than a fox that kept a stable territory. Most studies on fox mortality are inconclusive.