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:After birds established a stable baseline KPR on a VI 56 schedule, each was instrumented with sensors and a 5-gm FM radio transmitter to continuously telemeter ECG and record HR with a physiological recording system as previously described (Thompson et al., 1966, 1968a,b).
Category: