Ecology 6th Edition by Charles Krebs

Exercise 11

Three species of crossbills in northern Europe (see figure 5.16, page 78) tend to irrupt together. But two species concentrate on larch and spruce cones, which mature in one year, while the third species feeds on pine cones, which mature in two years. Poor flowering seems to occur at the same time in pine, spruce, and larch. How can you explain this puzzle? Suggest an experiment to test your hypothesis, and compare your ideas with those of Newton (1972, p. 239).
Reference Figure: 5.16