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:…used routinely to survey for squid of commercial interest (Jefferts et al. 1987, Goss et al. 2001), but it has been debated whether toothed whales are able to detect squid (Fristrup & Harbison 2002) using echolocation clicks of shorter duration and broader bandwidth (Au 1993, Madsen et al. 2004).
Category: