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:For example, elk may form smaller groups when exposed to wolf predation risk in relatively closed habitats (Creel & Winnie 2005), but form larger groups in open habitats where wolf densities are higher (Gower et al. 2009).
Category: