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:The upper frequency limit of the neurons' capability to follow envelope modulations was 320 Hz and 380 Hz in the European starling and the mynah bird (a closely related species), respectively (Hose et al., 1987; Knipschild et al., 1992).
Category: