Birds of prey have long been regarded as a very powerful species, but they have one possible constraint: their beaks.

Bird species have played a huge role in the development of the theory of evolution as the differences of beaks among finches in the Galapagos Islands inspired Charles Darwin.

Many studies have also shown how beaks had changed over millions of years, including when a group of researchers were able to revert the beak of a chicken embryo to a snout of a dinosaur.

Further, the bird beaks are believed to be according to adaptation, that is, they develop based on the type of diet they eat. For example, by having toothless beaks, the ancestors of modern-day birds were able to munch on seeds that helped save them from mass extinction.

However, the shape of beaks of birds of prey is slightly more peculiar as it is more likely dependent on the shape of its skull than food.

"Our results show that in birds of prey such as eagles and falcons, the shapes of the skulls change in a predictable way as species increase or decrease in size. The shape of the beak is linked to the shape of the skull, and these birds can't change one without changing the other," said Dr. Jen Bright, the study's lead author.

The European research involved obtaining 3D images of different landmarks of skulls coming from 147 species of raptors. Using a technique called principal components analysis and Procustes superimposition, they determined variations of the shapes of the skulls of the birds, how they compared with the actual size of the skulls, and the kinds of food they consume.

"We show that beak and skull shapes in birds of prey ('raptors') are strongly coupled and largely controlled by size. This relationship means that, rather than being able to respond independently to natural selection, beak shapes are highly constrained to evolve in a particular way," said the study.

On the other hand, other bird species such as finches may have experienced an "explosive evolution" mainly because they didn't have the same constraints birds of prey like vultures and hawks had.

The researchers, who are now planning to perform the same tests on other types of birds, want to make it clear that the study doesn't discredit Darwin's theory of evolution, but that it's one of the best ways to explain the varied species of birds today.

The study was published in the Proceedings of the National Academy of Sciences on April 28.