Two researchers would work together on each bird walk, with one researcher observing through binoculars and the other recording the number and species of all birds observed.
(one researcher; observing; through binoculars and the other recording the number and species of all birds) (the other; recording; the number and species of all birds)
one researcher the other
observing recording
through binoculars and the other recording the number and species of all birds
the number and species of all birds