A field biologist who studies the behavior of birds in a rainforest most likely collects data through observation.
Rainforests are the areas with rich diversity of plants as well as animal species. The trees of rainforests are so long, broad and thick that they completely cover the canopy. They are also recognized by highest rainfalls. Rainforests are considered to be the oldest in the in the Earth's ecosystems.
Observation is the act of carefully watching any action or scenario in order to gain information. Observation can be turned into a formal activity when any data is collected through it for educational or experimental purposes.