The avian bill is a textbook example of how evolution shapes morphology in response to changing environments, as can be seen from this plate from 1928:
Diversity of shapes and sizes of bills (#51 and 52 are non-avian examples).
Traditionally, and erroneously, avian bills have been viewed as dead structures, perhaps not surprising if viewed only from still images:
The Toco toucan (Ramphastos toco), the largest member of the toucan family, possesses the largest bill relative to body size of all birds. This exaggerated feature has received various interpretations, from serving as a sexual ornament to a refined adaptation for feeding.
Bills of seed-specialist finches in particular have been the focus of intense study demonstrating how climatic fluctuations acting on food availability drive bill size and shape. The avian bill also plays an important but under-appreciated role in body temperature regulation, and therefore in energetics. Birds are endothermic and rely on numerous mechanisms for balancing internal heat production with biophysical constraints of the environment. The bill is highly vascularized and heat exchange with the environment can be high. This heat exchange may impact how birds respond to heat stress, substitute for evaporative water loss at elevated temperatures or environments of altered water availability, or be an energetic liability at low environmental temperatures.
As a result, in numerous taxa, there is evidence for a positive association between bill size and environmental temperatures, both within and between species.
Species mean Relative bill size (normalised to body mass) is a positive function of the minimum habitat temperature. Show here is a simplified cartoon of family means. Note how the toucans fall way above the line.
Bill size is both developmentally plastic with respect to temperature, and evolutionarily adaptive in response to temperature. Understanding the evolution of variation in bill size however, requires explanations of all potential mechanisms, and is the subject of much of my laboratory’s research, in collaboration with colleagues from around the world.
For a review on the subject, please see Tattersall, Arnaout, and Symonds, which will hopefully be published later this year.
Sources
Tattersall, G. J., B. Arnaout, and M. R. E. Symonds. 2016. The Evolution of the Avian Bill as a Thermoregulatory Organ. Biological Reviews Accepted July, 2016, with minor revisions.
Burness, G., J. R. Huard, E. Malcolm, and G. J. Tattersall. 2013. Post-hatch heat warms adult beaks: irreversible physiological plasticity in Japanese quail. Proceedings of the Royal Society B-Biological Sciences 280: 20131436
Greenberg, R., V. Cadena, R. M. Danner, and G. J. Tattersall. 2012. Heat loss may explain bill size differences between birds occupying different habitats. Plos One 7:e40933.
Symonds, M. R. E., and G. J. Tattersall. 2010. Geographical variation in bill size across bird species provides evidence for Allen’s rule. American Naturalist 176:188-197.
Tattersall, G. J., D. V. Andrade, and A. S. Abe. 2009. Heat exchange from the toucan bill reveals a controllable vascular thermal radiator. Science 325:468-470.
Tattersall Lab (TEMP) 