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.

What causes the definition of a cormorant's 'roost' to shift geographically throughout the year?
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