The effect will always refer to the phenomenon we want to explain, and the cause will refer to the explanation proposed to have led to that effect. In this case, the passage states that "many people want to explain this decrease as..." so we see that people are trying to explain the decrease of songbirds. Therefore, the decrease of songbirds is the effect and the explanation for this effect will be the cause.
Does this make sense? Please let us know if you have additional questions!
