Diet shift induced rapid evolution of size and function in a predatory bird.

A predator's body size correlates with its prey size. Change in the diet may call for changes in the hunting mode and traits determining hunting success. We explored long-term trends in sternum size and shape in the northern goshawk by applying geometric morphometrics. Tetraonids, the primary prey of the goshawk, have decreased and been replaced by smaller birds in the diet. We expected that the size of the goshawk has decreased accordingly more in males than females based on earlier observations of outer morphology. We also expected changes in sternum shape as a function of changes in hunting mode. Size of both sexes has decreased during the preceding decades (1962-2008), seemingly reflecting a shift in prey size and hunting mode. Female goshawks hunting also mammalian prey tend to have a pronouncedly "Buteo-type" sternum compared to males preying upon birds. Interestingly, the shrinkage of body size resulted in an increasingly "Buteo-type" sternum in both sexes. In addition, the sternum shape in birds that died accidentally (i.e., fit individuals) was more Buteo-type than in starved ones, hinting that selection was towards a Buteo-type sternum shape. We conclude that these observed patterns are likely due to directional selection driven by changes in the diet towards smaller and more agile prey. On the other hand, global warming is predicted to also cause a decrease in size, thus these two scenarios are inseparable. Because of difficulties in studying fitness-related phenotypic changes of large raptors in the field, time series of museum exemplars collected over a wide geographical area may give answers to this conundrum.