Ecomorphology of Cinclodes ovenbirds
Cinclodes is a rapidly-evolving and ecologically diverse group of South American songbirds that may represent an unusual case of adaptive radiation, a phenomenon that is typically associated with organisms that inhabit island chains, like Darwin's finches. The Cinclodes case is unusual in that they span a vast range on a continent, rather than living on islands. The classic studies of adaptive radiation have focused on changes in morphology (body shape) among species, and the functions that each unique morphology confers to its bearer. So, following tradition, we analyzed the evolutionary patterns of morphological diversity among 13 Cinclodes species.
Big Cinclodes, little Cinclodes
A) We found that the two primary groups initially diverged strongly in body size leading to a large-sized clade and a small-sized clade. Two species are exceptional as they appear to have evolved a large-size within the small clade and small-size within the large clade. We speculate that these may represent cases of island gigantism and ecological character displacement.
B) We also found some minor differentiation among species in wing shape and foot morphology. Our findings suggest that although speciation and ecological divergence has been rapid in the genus Cinclodes, they are not as morphologically diverse as one might predict for an adaptive radiation. There's more to the Cinclodes story than this, though. Keep scrolling!
Stout-billed Cinclodes (Cinclodes excelsior). Photo credit: Francesco Veronesi via Wikimedia Commons.
Isotopic niches support the resource breadth hypothesis
A primary focus of ecological research is to understand what limits the geographic distributions of species. The distribution of resources that species depend upon is likely a limiting factor. Indeed, it’s intuitive that species that occupy large geographic ranges (or broad geographic niches) might be exposed to, and take advantage of, a broader array of food sources than other (or broad dietary niches), more geographically restricted species (e.g.: narrow geographic niches). This is a central tenet of the resource breadth hypothesis. However, this is not necessarily the case. A broadly distributed species could specialize on one particular food source that also happens to be widely distributed. Or, conversely, a species with a narrow geographic range could sample from many or all of the resources available to it. We tested the resource breadth hypothesis in a small group of South American ovenbirds (genus Cinclodes). We analyzed the stable isotopes in feathers of 12 species of Cinclodes from the collections of several natural history museums to assess the breadths their geographic niches (hydrogen and oxygen isotopes) and dietary niches (carbon and nitrogen isotopes). We also considered relationships between the species’ niches and their body shape.
Figure from Rader et al. 2017.
A) Geographic ranges of Cinclodes, redrawn from (Remsen et al. 2003). B) Carbon and nitrogen isotopic niches, and C) Oxygen and hydrogen isotopic niches of Cinclodes are depicted as standard ellipses. We used the area of these ellipses as a measure of niche breadth. For descriptive purposes we outlined the carbon and nitrogen isotopic space occupied by the food webs that Cinclodes can potentially use. The island space represents only island vegetation fertilized by seabird guano.
We found that the geographic niches and the dietary niches among Cinclodes were positively related. That is, that the Cinclodes species with broad distributions also had the most diverse diets. Our finding provides support for the resource breadth hypothesis. We also found that the Cinclodes with the broadest geographic niches, which are known to be seasonal migrants, had more pointed wingtips. This difference in wing shape may help to reduce the energy expenditure of long-distance migratory flight in these species.
Figure from Rader et al. 2017.
A) Isotopic niche widths varied among Cinclodes species, with C. patagonicus and C. oustaleti displaying the broadest geographic ranges and dietary niches. Points are scaled relative to niche width values. 95% confidence intervals are contained within the points.
B) Phylogenetically-controlled generalized least squares analyses indicated that dietary niches among Cinclodes were correlated with geographic range. This result is robust to the assumption of phylogenetic covariance.
Our study not only lends credence to idea that resource availability and geographic range are linked, supporting the resource breadth hypothesis, it highlights the usefulness of stable isotope analysis as a tool in the exploration of ecological niches. We have also illustrated the importance of scientific collections in ecological studies.
Cinclodes patagonicus is a particularly interesting species. It's a songbird that makes its living catching and eating seafood during the winter, with all of the salt that it comes with. This is remarkable, because songbirds, in general, are really terrible at dealing with high salt loads. Then, during the summer, C. patagonicus migrates up into the Andes to feed in freshwater streams. Via Wikimedia commons.
Adaptive radiation of Cinclodes
The Cinclodes lineage includes species that differ dramatically in their ecology. For example, habitats in which Cinclodes are found range from open grassland of the high Andes to the marine intertidal zone, and two species of this genus (C. nigrofumosus and C. taczanowski) are arguably the most marine-adapted of all passerine birds. Others appear to conduct seasonal migrations. Further, the genus contains ecological generalists as well as specialists. Cinclodes not only experienced remarkably high rates of divergence, but that rapid evolution was accompanied by remarkable phenotypic (both morphological and physiological) and ecological differentiation.
Our results demonstrated that Cinclodes patagonicus's migratory habits leads to broad geographic distribution, and a broad dietary tolerance. My Master's advisor, Carlos Martínez del Rio, and one of our collaborators, Pablo Sabat demonstrated that C. patagnoicus's unique salt tolerance comes from their incredible kidneys. Via Wikimedia Commons.
Further thoughts
Adaptive radiation is traditionally thought of as an accumulation of body-shape differences among a rapidly-diversifying lineage. The Cinclodes case demonstrates that morphology is only part of the story, though. Carlos, Pablo and collaborators' elegantly demonstrated that some Cinclodes species have specialized kidneys that allow them to consume salt-heavy diets, which is rare among songbirds. Also, Cinclodes span a range of elevations from sea level to the highest reaches of the Andes mountains. This implies that the high elevation species are adapted to the challenges of high-elevation life. Clearly, if we were so myopic as to only study morphology, we would have missed much of the rich story that is the Cinclodes radiation.