Thanks to all three of you for your input.
I checked with the observer and images 4 & 5 were taken just seconds apart, so almost certainly show the same bird.
@SteveClifton I didn't even consider any of the marsh terns for the second bird, as it was apparently seen well out to sea. I checked on eBird and there's just a single sighting of Whiskered Tern in Madeira, photographed back in 2012. Are there any other possibilities?
Sorry about the delay in getting back to you. The apparent rarity of Whiskered Tern in Madeira puts a different slant on things, and the fact that the bird was photographed at sea adds another layer of difficulty/unlikelihood.
I presume these are the only images of this bird?
Firstly, to address Carery's point about the similarity to 2cy Common (or even 1st cy Common). Of course the resemblance to Common Tern struck me. In fact, when I trawled through the thousands of images of Whiskered Tern on E-bird, there are understandably quite a few which are mis-identified Common Terns of various ages. I've juxtaposed images of the bird against both species, and I still see this as closer to Whiskered than Common. Yet I'm also trying to be realistic, and open to be proven wrong!
The lighting doesn't make assessment easy, and the fact that there are only two pictures, neither of which give a good impression of the bird's upper-parts.
I see a 1st calendar year bird here (unless I've 'read' the images wrong) because all of the flight feathers appear to be of the same generation. I would expect a 2cy Common (or indeed 2cy Whiskered) to show a moult contrast in the primaries at this time of year - see attached images showing 1cy Common Terns x2 next to the subject bird. The second image shows two different 2cy Common Terns (both taken in September). Note the obvious moult contrast-they have two different generations of primaries.
I also see a bird with an apparent all black bill and rather short tail. 2cy Common Terns in particular tend to have longer tails than 1cy birds, but this bird's tail looks distinctly short with a rather shallow notch/fork, even in comparison with the 1cy Common Terns. I fully accept btw that this isn't the best angle to judge this feature, nor is it ideal to assess wing length/shape in comparison with the other 1cy Common Tern that is in a similar pose. Slight changes in the position of the wing can alter impressions, but still, I'm seeing a bird which gives the impression of having a combination of:
- A proportionally large head compared to the Common Terns, with a rather front-heavy, neck-less appearance
- Short & broad-looking wings (especially the 'hand')
- short tail with shallow fork
- An all-dark bill that looks fine for Whiskered Tern (but maybe the harsh lighting is playing a part here in hiding colours?)
The plumage features mostly look ok for Whiskered Tern (e.g. the weak carpal bar and shape of the black crown), with the possible exception that most juv Whiskered Terns have bolder scapular markings than this bird. I also thought that the bill was a little on the small side for Whiskered (many of them have noticeably heavier bills) yet I also found images of many Whiskered Terns that had similarly small/slim bills, very similar to this bird's.
It's an interesting bird for sure, and it raises quite a few questions in my mind. As you've guessed, I'm having a hard time ruling Whiskered Tern out (and Common Tern in as a possibility).
It might not be possible to prove with 100% certainty but I think this warrants further discussion, unless someone can persuade me otherwise...
