It is common wisdom that the shapes of bird beaks are closely linked to feeding ecologies.  Texts similar to the following are not difficult to find: “Changes in the size and form of the beak have enabled different species to utilize different food resources such us insects, seeds, nectar from cactus flowers as well as blood from iguanas, all driven by Darwinian selection.”  The historical context for such statements can be found in observations of finches made by Charles Darwin when he visited the Galapagos Islands.  Beak shape appeared to correlate with diet and feeding behaviour.

It is only in recent years that quantitative analyses of beak shapes have provided evidence to support Darwin’s theory of evolution by natural selection, although the number of such studies has been few.  Textbooks recount the finch beak analyses to support a general rule that feeding ecology drives the selection of bird beak shapes.  However, what has been called a “truism” has now been shown to be highly misleading.

A Press Release from the University of Bristol summarises the new findings as follows:

An international team of scientists from the United Kingdom, Spain and the US used computational and mathematical techniques to better understand the connection between beak shapes and functions in living birds. By measuring beak shape in a wide range of modern bird species from museum collections and looking at information about how the beak is used by different species to eat different foods, the team were able to assess the link between beak shape and feeding behaviour.

Professor Emily Rayfield, from the University of Bristol’s School of Earth Sciences, and senior author of the study, said: “This is, to our knowledge, the first approach to test a long-standing principle in biology: that the beak shape and function of birds is tightly linked to their feeding ecologies.”

Guillermo Navalón, lead author of the study and a final year PhD student at Bristol’s School of Earth Sciences, added: “The connection between beak shapes and feeding ecology in birds was much weaker and more complex than we expected and that while there is definitely a relationship there, many species with similarly shaped beaks forage in entirely different ways and on entirely different kinds of food. This is something that has been shown in other animal groups, but in birds this relationship was always assumed to be stronger.”

Darwinism has had a long tradition of extrapolating small changes in morphology to major changes affecting the complexity of organisms and their body plans.  So strong has been the Darwinian attachment to the concept of natural selection, that specific cases involving minor dimensional changes have become generalised to explain all bird beaks as the product of adaptive evolution.  They think it is self-evident that natural selection is the key to understanding the connection between beak shape and feeding behaviour.

A team of scientists have been testing the hypothesis that “beak morphology predicts feeding ecology in birds”.  The data set involved 175 species, from 94 families and 38 orders of birds.  They found that “diet accounts for less than 12% of beak shape variation”.  This means that the hypothesis has “little explanatory power”.  The scientists write: “Our results suggest that adaptation to dietary composition is not as fine-tuned as generally perceived, and there is not a close to one-to-one mapping of beak shape on feeding ecology.”

It is to be hoped that textbook authors will take these findings on board, and make the appropriate revisions.  These should include the following.

  1. Present the Galapagos finch data in the context of microevolution, with reversible adaptations to changing environments;
  2. In the absence of an evidence base, avoid the temptation to extrapolate microevolution to explain all the diversity in living organisms;
  3. To recognise that scientists have only recently been able to lay aside the dogmatism of Darwinism, and realise that much research remains to be done. The lead author of the research is quoted as saying: “Really, we’re just starting to scratch the surface, and a lot more research is needed to fully understand the drivers behind beak shape evolution.”

David J. Tyler (8 February 2019)

 

Sources

Navalón, G., J. A. Bright, J. Marugán‐Lobón, and E. J. Rayfield. 2019. The evolutionary relationship between beak shape, mechanical advantage, and feeding ecology in modern birds. Evolution, Online 8 Dec 2018, doi:10.1111/evo.13655.
https://onlinelibrary.wiley.com/doi/abs/10.1111/evo.13655

Abstract: Extensive research on avian adaptive radiations has led to a presumption that beak morphology predicts feeding ecology in birds. However, this ecomorphological relationship has only been quantified in a handful of avian lineages, where associations are of variable strength, and never at a broad macroevolutionary scale. Here, we used shape analysis and phylogenetic comparative methods to quantify the relationships among beak shape, mechanical advantage, and two measures of feeding ecology (feeding behavior and semiquantitative dietary preferences) in a broad sample of modern birds, comprising most living orders. We found a complex relationship, with most variables showing a significant relationship with feeding ecology but little explanatory power. For example, diet accounts for less than 12% of beak shape variation. Similar beak shapes are associated with disparate dietary regimes, even when accounting for diet‐feeding behavior relationships and phylogeny. Very few lineages optimize for stronger bite forces, with most birds exhibiting relatively fast, weak bites, even in large predatory taxa. The extreme morphological and behavioral flexibility of the beak in birds suggests that, far from being an exemplary feeding adaptation, avian beak diversification may have been largely contingent on trade‐offs and constraints.

Van Wassenbergh, S., and Baeckens, S. Evolution of shape and leverage of bird beaks reflects feeding ecology, but not as strongly as expected. Evolution, 2019. DOI: 10.1111/evo.13686
https://onlinelibrary.wiley.com/doi/abs/10.1111/evo.13686

Abstract: Is feeding ecology the main driver of beak diversification in modern birds? Taking a broad‐scale interspecific comparative approach, Navalón et al. (2019) found a relationship between feeding ecology (diet and feeding behavior) and beak morphology (shape and leverage), although much of the observed variation remained unexplained. This low explanatory power may suggest that variation in the multitude of non‐feeding functions of the beak also influences its evolution.

Bird beaks did not adapt to food types as previously thought
Date: January 22, 2019
Source: University of Bristol
https://www.sciencedaily.com/releases/2019/01/190122115043.htm

Summary: A new study has shed some new light on how the beaks of birds have adapted over time.

Images

Darwin’s finches (Credit public domain https://en.wikipedia.org/wiki/Darwin%27s_finches)
Large Ground Finch (Credit A.Davey https://www.flickr.com/photos/adavey/5043961543)