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 experiment may be difficult as two different studies observed that severe bottlenecks are present in the colonization process of the chick gastrointestinal tract when an inoculum is used that contains more than one strain of C. jejuni (Hendrixson and DiRita, 2004; Grant et al., 2005).
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