Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:This well-established rhythm of swimming on ebb tides is the basis for the spawning migration (Forward et al. 2003, 2005), in which ovigerous female blue crabs migrate seaward from estuaries to coastal areas where larvae are released.
Category: