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:1991) and empirical studies (e.g. Kruuk 1978; Kruuk and Parish 1982; Mills 1989; Mills and Gorman 1997; Geffen et al. 1992; Patterson and Messier 2001; Valenzuela and Macdonald 2002).
Category: