Comparative psychology is the study of animal behavior whose methods usually focus on the similarities and differences between different organisms. Not only are psychologists involved in comparative psychology but biologists, anthropologists, and scientists from many other fields of study. Heavy emphasis is placed on evolution and the relationship between organisms.
Because of evolution we can look at relationships between related species and compare and contrast them based on shared traits. Comparative psychologists also use modern animals behavior to draw conclusions about ancient and extinct species. Organisms with simple neural systems can be studied and information can be presumed to cross over to organisms with similar neural networks.
This way scientists can manipulate conditions and study behaviors in organisms to determine information about creatures that are more difficult to study in a lab. Mice, rats, birds, and insects are all common animals used in comparative psychology research.