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:These results are contrary to the long-held belief
 that monarch butterflies were most vulnerable to distur-
 bance on the wintering grounds since they congregate in a
 small area at high densities (Brower et al. 2002, 2004).
Category: