While the writer may believe them to be true, opinions are a product of the writer's biases and personal system of beliefs. While many opinions make good sense and may win a reader's approval, they must still be classified as mere opinions if there is no factual evidence supporting them. Opinion may, in fact, be entirely correct, but generally it still should be viewed with less trust than facts. Some statements contain both fact and opinion. For example, research has demonstrated that animals living in crowded cages show more aggressive behavior than those living in less crowded cages.
Propaganda is information presented in order to influence a reader.
Many authors consciously use propaganda techniques in order to convince their readers of their special point of view. A close look at the author's background or some of the motivations and editorial policies of the source of the publication may provide clues about what types of propaganda techniques might be used. Scientists seem to be linking more and more observations with their antecedent causes. There may be other hidden factors underlying the relationship. However, no experiment has conclusively established that temperature is a causative factor of bird migration.