A history of studies of visual accommodation in birds.

Since 1813, when Crampton first described the ciliary muscle of the avian eye, there has been little agreement on how birds are able to change the focus of their eyes. Numerous later studies on the eyes of a variety of bird species contradicted earlier findings or proposed new accommodative mechanisms. The resulting confusion persists today, and a number of significant works on the avian eye perpetuate many of the myths developed during the 1800s. There is little consensus on avian accommodation; the early literature contains many accurate descriptions of the mechanisms, along with elegant experimental evidence to support them. Much of the early literature, however, is in German and has remained obscure. Further, among the mechanistic descriptions of avian accommodation are many that are incorrect. The current confusion can be attributed in part to the fact that some birds have both corneal and lenticular accommodation. It is unclear to what extent different bird species employ both mechanisms, or depend on one mechanism or the other. These facts, together with the diversity of bird species, their range of visual requirements, and the numerous anatomical differences in their eyes, make it impossible to describe a single avian mechanism of accommodation. Our own experience in studying accommodation in the chick eye has led us to review the historical literature in an attempt to provide a new foundation for future studies on visual accommodation in birds. While in relation to the anatomical arrangements [of the bird eye], these have led our knowledge pretty much to a conclusion.... There is among all these works no real difference of opinion. However, in terms of the accommodative mechanism of the bird eye, at the present time there are many different views of what is going on. Here the circumstances are very similar to what happened a few decades ago in the study of accommodation generally, but particularly with humans, when Helmholtz made the remark. "There is no other portion of physiological optics in which one finds so many differing and contradictory ideas us in the accommodation of the eye, where we have only recently actually made observations on what previously was left to the play of hypotheses".