You are quite right to ask, separating these three is difficult and I am not convinced there is a single or combination of features which will guarantee accuracy. One problem is that the majority of deaths occur in newly fledged so that the features on the skull are not as clear and the sizes are smaller. Looking at the top view (128) of your skull, the attachments of the muscles round the eye socket don't quite meet in the midline making this a young individual. If I have measured the 50p correctly then this skull is about 83mm long, without the beak sheath, so this is a slight underestimate. That is a little long for a black Headed gull (particularly a young one) but would fit with Kittiwake or Common Gull.
There is an exceptionally useful site for identifying skulls here:-
I have taken the liberty of copying three images and creating a montage,
Look at the muscle attachments between the eye sockets and you will see that all three meet in the midline; these are adult skulls. So I excluded Black Headed Gull on size, 83mm is too big for a juvenile.
Now look at the light area at the back of the skull, particularly on the Kittiwake, this is also a muscle attachment and is almost hidden on the Common Gull but easily seen on the Kittiwake and your skull.
There is another problem these comparisons rely on taking the photos from an identical angle and with a very similar lens, neither of these we have any control of here so there comparisons may be invalid.
So my best suggestion is Kittiwake.
Interesting. I always thought the depressions on the top of the skull were to house salt glands.
I have a kittiwake that is 91mm long and came from an adult in which these depressions don't meet, I've other Kittiwakes in which they do meet. They meet in all my black headed gulls, even those that are so juvenile as to be seperate bones.
Now that I look more closely the bit sticking out behind the eyes sockets in my kittwakes sticks out further than in the other two species, this can be seen in your photos too. Interestingly 5mm to 5.5mm in the Kittiewake, 4mm in the common gulls and only 3mm in the black headed (although it is hard to measure). It seems not to stick out very much in the skull in question.
Perhaps we could ask David to send some measurements of his skull?
Thank you Richard, somewhere in my past I do recall salt glands but had completely forgotten about them. I have always thought that they were smaller in juveniles and that they became deeper and closer to the midline as the bird matures, am I wrong in thinking this?
I have had a look through my Black headed gull and I agree that these depressions meet in all of them, even known juveniles. Simillarly with Kittiwake some meet and some do not, like your skulls. It is noticeable when looking at the two together how much more robust the kittiwake skulls look compared with Black Headed gull. A primary feather would be very useful.
It seems that the size of the salt glands (and the size of the depression they dwell in?) varies by age, body weight and the salinity of the water available to the animal... My black headed gulls are all from the same place but my kittiwakes and common gulls are from various locations.
I agree, with black headed gulls being less robust but my kittiwakes and common gulls are pretty similar. I'm not sure how robust the skull in question here is.
Thanks so much for all the information Richard and John.
Which measurements would you find useful?
I can tell you that the skull is 74mm long and 23mm wide at it's widest point.
Richard, I wasn't sure which part you were referring to when you mentioned the bits behind the eye socket. Could you clarify this please and I can post the measurements.
The only one I have of the species group in question that matches that size in my collection is Black Headed Gull. I've tried to mark the bit I mean with a red line here on my size matching skull; I couldn't get the ruler to stay lined up for the photo. Do you know where the skull came from? Good luck!