What causes the definition of a cormorant's 'roost' to shift geographically throughout the year?
Answer
Large-scale seasonal migration between northern breeding areas and southern wintering grounds
The migratory patterns of populations, such as Atlantic coast Double-crested Cormorants moving south around September and returning north in spring, dictate where suitable roosts are located seasonally.

Related Questions
What primary activity most dictates where a cormorant spends its resting hours?Why are cormorant feathers not fully waterproofed by preen oil?What proportion of a cormorant’s day is often dedicated to resting activities, including digestion and feather drying?Which species is specifically known for its willingness to incorporate man-made structures like utility poles into its resting routine?During the breeding season, where do Pelagic Cormorants frequently establish roosts or nesting sites?What environmental condition in northern latitudes causes cormorants to push south?What is the energetic trade-off that governs a cormorant’s decision on where to settle for the night?What physical impact can a large congregation of cormorants have on a roost site like a few trees or a stretch of shoreline?In areas like North Carolina's sounds during winter, where are large communal roosts of Double-crested Cormorants often observed?What causes the definition of a cormorant's 'roost' to shift geographically throughout the year?Which other colonial waterbirds might compete with Double-crested Cormorants for desirable nesting or resting islands?