Having more then one independent variable will give unreliable results. Hope this helps
More than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described.
Basically, there is a presentation of types of food and the amount of those foods over 50 years in this graph. This means the graph is showing amounts of certain foods over the time period. I'm guessing that basing off of the the previous text you're just noting a change in the breaks of birds based off of the diet available to them at certain times. The adaptations and natural selection will explain the changes because due to certain limited and available foods the diet of the birds change.
