Following the appearance of the recent Royal Tern in Western Ireland I had a timely visit to Guinea, West Africa, last week, where coastal estuarine systems are currently packed full of Royal Terns.
The online speculation following its discovery was that the bird was of North American origin Thalasseus maximus maximus (its arrival had coincided with a series of extensive and fast moving Atlantic depressions), however no one had provided any sound reasons for it not being an African Royal Tern T. m. albididorsalis. This post is designed to be of reference to those aiming to ascertain the origin of vagrant late-summer Royal Terns.
Dermot Breens superb images of the Irish bird (here) clearly show a smart bird with virtually a full black cap, however still showing a dark secondary bar and faint dark tail tips.
The following images, though taken in much stronger light provide a good comparison of plumages at this time of year in West Africa.
Key features that are worth looking at in each of these images include:
- Outer tail feather length
- Tail feather colouration (dark suffusion), especially outer tail feathers
- Extent of dark on crown (rear crown crescent)
- Extent and intensity of dark secondary bar
- Presence of dark greater covert and lesser covert bars
- Primary colouration
- Bill size and shape may also be an important guiding feature between the two races.
The extensive dark crown of the Irish bird certainly stands out as being different from the birds illustrated below, however the Irish bird is also known to have a leg injury which could have caused a delay in moult, and hence a retained dark cap. The Irish bird shows a relatively strong dark secondary bar and faint dark tail tips indicating it is probably a second-summer bird.
It would be interesting to hear from birders in the US if birds typically retain such dark caps into August or if this bird really is exceptional.