Why do you require such detailed data rather than just summaries?

The answer to this question is complex, but here is an example that might help explain this. Imagine that you have 12 apple trees in your yard producing a total of 120 apples. If each of the 12 trees had about 10 apples, you might conclude that it was a very bad year, but that all of your trees were healthy. If all 120 apples were on one tree, you would undoubtedly come to a different conclusion – perhaps hypothesizing that something was killing off your trees. Summary data for an entire orchard (or trail) would not allow us to make this distinction and yet it is a distinction that is critically important with respect to the future of the orchard. If we are to become good predictors of, say, bluebird population trends into the future, we need to know how many pairs are reproducing successfully, what their clutch sizes are, how much they vary from one individual or year to the next, and how the number of pairs fluctuates over time.