New research published in Malimbus, the journal of the West African Ornithological Society, has documented the differing bill and skull shapes of West African Crested and Royal Terns, amending previously reported size differences.
Until fairly recently, these two similar-looking terns were treated as conspecific, with separate African and American populations classified as the subspecies albididorsalis and maximus respectively.
However, a 2017 paper used DNA analysis to demonstrate that the two were distinct, with West African Crested Tern genetically closest to Lesser Crested Tern. The split was adopted by the IOC in 2020.
Analysing skulls from 27 West African Crested and 18 Royal Terns, Clive Barlow and his colleagues performed detailed measurements of both bill and skull metrics.
The skulls and bills of West African Crested Terns are overall smaller than in Royal Terns but average more attenuated, with the skull appearing longer. The upper rear of the skull also appears more vertical in West African Crested. Further slight differences in the nasal bone combine to give West African Crested a more streamlined and tapered appearance when compared to Royal.
The most strongly supported difference in measurements between the two species was in gonys height, with Royal having a significantly deeper bill. This partly explains previous observations that Royal has a 'heavier' bill.
The distal part of the lower jaw, from the angle to the tip, is more curved in West African Crested, which, with the less prominent gonys compared to Royal, contributes to its bill appearing more uniformly downcurved.
In contrast to information presented by recent studies, Barlow and his colleagues found that Royal Terns averaged slightly longer bills than West African Crested. Previously, it has been written that West African Crested has a longer bill, despite its overall slightly smaller size. Barlow et al write that this would be better summarised as "the bill in West African Crested averaged longer relative to other bill and body measurements than in Royal".
In summary, the paper demonstrates the subtle differences in head and bill shape between West African Crested and Royal Terns. Royal has a longer but proportionally deeper ('heavier') bill, which may consequently appear shorter relative to its bulkier head and body. Meanwhile, West African Royal has a shorter bill than Royal, but this is more slender and more uniformly tapered ('dagger shaped') and may therefore appear longer relative to its overall smaller body. This, the researchers say, presumably reflects differing ecological pressures such as prey type and size, diving depths and weight of prey they can carry.
Barlow, C R, Waine, J, White J, Collinson, J M, Garrett, K L & Tye, A. 2022. Bill and skull metrics of American Thalasseus maximus and West African T albididorsalis "Royal" Terns amend previously reported size differences and explain distinctive shapes. Malimbus 44: 101-116.