A scoring scheme to assess the reliability of bird hybrids

I explain this new scoring system and apply it to Tinamous.

In 2015, I published my very first paper in the journal Ibis where I introduced the Avian Hybrids Project (the website you are reading right now). Apart from launching this website, I also estimated the percentage of bird hybrids. Using the Handbook of Avian Hybrids of the World by Eugene McCarthy and the online Serge Dumont Bird Hybrids Database, it turned out that ca. 16% of bird species has hybridized in the wild. Although these sources provide supporting references for the documented hybrids, the reliability of these references has not been systematically assessed. That is why I decided to develop a simple scoring scheme to determine the reliability of the data sources supporting hybrid records. This system was recently published in the journal Ornithology Research.

The scoring scheme is based on three criteria: (1) the observation of a putative hybrid with photographic evidence or a detailed description, (2) thorough morphological analyses in which the putative hybrid is compared with potential parental species, and (3) genetic analyses of the putative hybrid with reference material from potential parental species. The first criterion encompasses sightings in online databases, such as eBird, as well as scientific papers describing the observation of a presumed hybrid. The second criterion requires analyses in which the hybrid specimen is quantitatively and/or qualitatively compared with the probable parental species. And the third criterion includes genetic analyses that confirm the hybrid origin of an individual or report recent gene flow between hybridizing species.

To express the varying levels of confidence that each of these criteria provide, they will be weight differently in the final score for a putative hybrid, namely one point for an observation, two for a morphological analysis, and three for a genetic test. The final tally of these three criteria (ranging from 0 to 6 points) will indicate the level of confidence for a particular hybrid combination. Got it? Let’s apply it to some bird hybrids!

Tinamou Hybrids

This scoring system developed quite naturally while I was writing a short review on tinamou hybrids. To structure my thinking and keep track of the papers I was reading, I constructed a table with the three criteria explained above. The tinamous (Neotropical family Tinamidae) turned out to be an ideal group to test the scoring scheme. Only a handful of hybrids have been reported and the evidence supporting them is easy to interpret. My analysis revealed one well-documented case and three doubtful records that require further investigation. All information is summarized in the table below, providing a quick overview of the reliability of potential tinamou hybrids.

Crypturellus boucardi × cinnamomeus
In the Handbook of Avian Hybrids of the World, McCarthy (2006) mentioned one well-documented case of hybridization in tinamous, namely slaty-breasted tinamou (Crypturellus boucardi) and thicket tinamou (C. cinnamomeus) which interbreed in Honduras. Two hybrid specimens were briefly described in Monroe Jr. (1965).

Eudromia elegans × formosa
In his zoogeographic analysis of the South American Chaco avifauna, Short (1975) suggested that elegant crested tinamou (Eudromia elegans) and Quebracho crested tinamou (E. formosa) may be in contact but indicated that their interactions are unknown.

Nothura boraquira × Nothoprocta cinerascens
Short (1976) discussed the morphological resemblance between white-bellied nothura (Nothura boraquira) and brushland tinamou (Nothoprocta cinerascens), but offered no clear evidence for hybridization.

Rhynchotus rufescens × maculicollis
Finally, when describing the distribution of the red-winged tinamou (Rhynchotus rufescens), Short (1975) mentioned that the subspecies pallescens seems to intergrade with two other subspecies (rufescens and maculicollis). The latter one has recently been elevated to species rank based on a distinctive song. However, these species do not seem to overlap in distribution and occur on different elevation levels.

Table summarizing the evidence supporting different tinamou hybrids. From: Ottenburghs (2021) Ornithology Research.

Additional Evidence

In addition to the references supporting certain hybrid records, I also took into account the distribution of the parental species and their divergence time. If the putative parental species do not overlap in distribution, the hybrid record can be considered less reliable (although you have to consider escaped or introduced individuals). Similarly, an extremely old divergence time between the parental species can cast doubt on a proposed hybrid.

For the tinamou hybrids, the information on distribution and divergence times did not add much. With the exception of red-winged tinamou x huayco tinamou, all parental species overlapped in distribution (see maps below). And I could not find a reliable estimate of divergence time due to a lack of genetic studies. I had to rely on a maximum date that corresponds to the split between the subfamilies Nothurinae and Tinaminae (17 million years ago). This date is within the range of divergence times at which bird species still have the ability to hybridize (on average 21 million years).

Distribution maps of putative parental species for four hybrid combinations. From: Ottenburghs (2021) Ornithology Research.

Picking the Journal

A final note on the journal where this study appeared. I decided to submit my manuscript to Ornithology Research (previously known as Revista Brasileira de Ornitologia | Brazilian Journal of Ornithology), because they focus on Neotropical species. Moreover, it was also a small act of rebellion against the current academic climate. The scientific world is so focused on developing your cv by publishing in high-impact journals and collecting citations. Smaller journals are often neglected, even though they reach an important audience. I hope my paper will inspire local researchers to work on avian hybrids.

References

Ottenburghs, J. (2021) An evidence-based overview of hybridization in Tinamous. Ornithology Research. Early View. https://doi.org/10.1007/s43388-021-00049-y

Featured image: Pale-browed Tinamou (Crypturellus transfasciatus) © Nick Athanas | Flickr

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