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:In a second experiment, ten Antarctic prions were rendered anosmic using an injection of zinc sulphate solution into the olfactory chambers (Bonadonna et al. 2001), and ten other birds, acting as controls, were sham-treated.
Category: