We have that the first graph shows that the wagtails take more calories per second when they eat dung flies of length 7mm, since that is the peak. Thus, it would make sense for wagtails to eat these flies. The second graph shows that this is indeed the case, namely most birds choose this length of flies, especially given the number of available flies. This showcases natural selection; birds that chose other lengths had a disadvantage since they needed more time to get their required calories.
