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:Differences between the immune function of males and females have also been reported for birds (Romano et al., 2011), reptiles (Svensson et al., 2009) and fish (Hoeger et al., 2005).
Category: