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 trials with zebra ®nches, the numbers of sperm collected using false cloacas (Pellatt and Birkhead 1994) are similar to those produced by natural copulations determined by comparing the number of sperm in the seminal glomera of males that had or had not recently copulated (Birkhead et al. 1993).
Category: