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:Flagella and motility are required for promoting optimal commensal colonization of the chick gastrointestinal tract (Nachamkin et al., 1993; Wassenaar et al., 1993; Hendrixson and DiRita, 2004; Wösten et al., 2004).
Category: