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:While beaked whales and large delphinids produce clicks with comparable energy flux source levels (Madsen et al. 2004, Zimmer et al. 2005, Au et al. 2007), they may hunt in quite different ambient noise environments.
Category: