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:…bird species), it is males that suffer from higher mortality when the parental effort is increased; this may be because males tend to adjust their investment according to brood size while females are working on their maximum level irrespective of the current brood size (Santos and Nakagawa 2012).
Category: