Assessing the reliability of hybrid records.
A few months ago, I published a review paper in the Journal of Ornithology where I provided an overview of recent attempts to quantify the incidence of avian hybridization on the species level and on the individual level. Estimates on a species level vary between 10% to almost 20% of hybridizing bird species, largely depending on the choice of global species checklists with differing decisions on the taxonomic status of certain (sub)species. However, the hybrid records in these estimates have generally been taken at face value. Indeed, detailed assessments of certain hybrids in the Handbook of Avian Hybrids of the World and the Serge Dumont Bird Hybrids Database revealed that some cases are questionable (see for example this blog post on a putative woodpecker hybrid).
Recently, I discovered that the records of hybrids between Black Noddy (Anous minutus) and Lesser Noddy (A. tenuirostris) were unreliable. Another reminder to always read the original papers.
Three Noddies
In the Handbook of Avian Hybrids of the World, Eugene McCarthy provides two sources to support hybridization between Black Noddy and Lesser Noddy. The first paper concerns a Short Communication in Emu in which the authors assess whether three Noddy species – Black Noddy, Lesser Noddy and Brown Noddy (A. stolidus) – all breed at Ashore Reef in the Eastern Indian Ocean. At the end of the paper, they briefly discuss the possibility of hybridization.
It is possible that some hybridisation is occurring within noddies at Ashmore as some birds are reported to be of a size and with characteristics intermediate between stolidus and the smaller noddies. A photograph of insufficient quality for reproduction appears to show one of these birds with many brown feathers in the otherwise black wing and mantle plumage, a grey nape and neck, and a longer more narrow bill than stolidus. However, Cramp et al. (1985) state that the stolidus of Western Australia north to Indonesia are smaller, blacker, and more slender of bill, perhaps warranting sub-specific recognition. A thorough taxonomic study of the Ashmore noddies is required.
The authors mainly indicate the possibility of hybridization and highlight the occurrence of some intermediate individuals (but note that it concerns the Brown Noddy, not the two species listed by McCarthy). Not very convincing evidence for hybridization.
Museum Specimens
The observations in the Emu-paper prompted Bill Bourne to write a short piece for the Sea Swallow. He remembered that “the collection in the Natural History Museum at Tring (to whom I am indebted for assistance) does indeed contain two small noddies from the Indian Ocean with both much larger bills and darker lores than is usual there.” He reexamined these specimens, but could not provide a definite answer about the likelihood of hybridization.
These specimens are difficult to interpret, and may either imply that birds of the Pacific type are liable to stray west along the equatorial current systems into the Indian Ocean, where the occurrence of some intermediate specimens (Higgins and Davies 1996) suggest that they may then hybridise with the small, pale local population, or alternatively that the two forms may breed alongside each other much further west than Ashmore Reef (Stokes and Hinchley 1990), in which case they should clearly continue to be regarded as distinct species. These birds deserve closer scrutiny in the field.
Taken together, these two papers do not provide clear evidence for hybridization between Black Noddy and Lesser Noddy. It is certainly possible that the specimens mentioned by Bill Bourne are hybrids, but more research (preferably using genetic data) is needed to confirm hybridization. Currently, I would not consider this hybrid record reliable.
As I mentioned at the beginning of this blog post: Do not take the records in the Handbook of Avian Hybrids of the World at face value, but always check the original papers.
References
Bourne, W.R.P. (1997). The smaller Noddies of the Indian Ocean. Sea Swallow, 46, 79-80.
Ottenburghs, J. (2023). How common is hybridization in birds?. Journal of Ornithology, 1-8.
Stokes, T., & Hinchey, M. (1990). Which Small Noddies Breed at Ashmore Reef in the Eastern Indian Ocean?. Emu-Austral Ornithology, 90(4), 269-271.
Featured image: Black Noddy (Anous minutus) © Sirrob01 | Wikimedia Commons