The study also uncovered some evidence for hybridization.
Morphology matters. Most species descriptions are based on careful morphological analyses. But morphology cannot always be trusted when drawing taxonomic lines between species. Take the redpoll finch complex, for example. This group of songbirds currently contains three species: common redpoll (Acanthis flammea), hoary redpoll (Acanthis hornemanni) and lesser redpoll (Acanthis cabaret). Despite obvious morphological differences – as any birdwatcher can tell you – these species have largely undifferentiated genomes. From a genetic point of view, these three species could thus be lumped together. Similar results have been reported for bean geese, warblers and crows. Alternatively, species might be morphologically indistinguishable, but drastically different on a genetic level. These cryptic species can thus only be identified with genetic techniques. The yellow-browed warbler (Phylloscopus inornatus), for instance, turned out to be composed of three cryptic species.
These examples clearly indicate that there is more to life than morphology. It is advisable to test classifications based on morphological data with genetic analyses. A recent study in the journal Zoologica Scripta performed this exercise for two East Asian scimitar babblers.
Morphology and Genetics
In 2006, two races within the rusty-cheeked scimitar babbler (Pomatorhinus erythrogenys) were elevated to species level. From that moment onward, you could check the black-streaked scimitar babbler (P. gravivox) and the grey-sided scimitar babbler (P. swinhoei) on your Asian birding list. This taxonomic decision was based on meticulous morphological analyses. The most conspicuous features were the coloration of the ventral and dorsal feathers. The black-streaked scimitar babbler has olive-gray dorsal feathers and orange-tawny ventral feathers, while gray-sided scimitar babbler can be recognized by its foxy-rufous and grayish coloration. But are the distinct plumage patterns also reflected in the genetic make-up of these species?
Chuanyin Dai, Feng Dong and Xiaojun Yang took a closer look at one mitochondrial and four nuclear loci to see whether morphology and genetics were congruent. Analyses of the nuclear markers pointed to two genetic clusters that corresponded to the morphologically described species. These findings were recapitulated by the haplotype networks which showed a clear separation between the taxa (see figure below). The researchers concluded that “the concordance of divergence between morphological, mitochondrial locus and nuclear DNA analyses should be viewed as the most convincing evidence supporting P. gravivox and P. swinhoei as distinct species.”
The genetic and morphological data were also confirmed by differences in vocalizations. The researchers did not test this feature explicitly, but describe an interesting incident during their fieldwork.
Besides, it is worth mentioning that in one case a pair of P. swinhoei showed no reaction to the playback of P. gravivox vocalizations during the period of sample collection in the field. When we collected samples in a location far from the contact zone (Jixi county), where the collected birds were P. swinhoei, we had mistakenly played the songs and calls of P. gravivox to attract a pair of targeted birds observed in a shrub. We noted that the birds nearby neglected this disturbance and did not exhibit any reaction at all. Approximately 15 min later, we checked the player and recognized this mistake and then played the songs and calls of P. swinhoei. The two birds came out the shrub immediately, and the male bird was caught by the mist net!
This suggests that there is some acoustic isolation between the two species, but more research is needed to confirm this observation.
A Mitochondrial Mismatch
All these lines of evidence point to two distinct species. The ultimate test would be to check whether these birds can produce viable offspring. If they are unable to hybridize, there will be no debate on this separate species status. However, it seems that reproductive isolation between the black-streaked and the grey-sided scimitar babbler is not complete yet. Analyses of the mitochondrial gene revealed one grey-sided scimitar babbler that falls right in the middle of the black-streaked scimitar babbler group. Most likely, this individual received this mitochondrial variant through hybridization. Whether we can find hybrids in the wild remains to be seen…
Dai, C., Dong, F., & Yang, X. (2020). Morphotypes or distinct species? A multilocus assessment of two East Asian scimitar babblers (Aves, Timaliidae). Zoologica Scripta, 49(3), 265-279.
This paper has been added to the Timaliidae page.