One of the ways by which to distinguish 27-year old Herring Gull ZDGA from other Herring Gulls (apart from the color ring that he is wearing of course), are the bare patches on the right side of the neck and face that appear when he is moulting to winter (basic) plumage.
The question is: can we treat this as a characteristic of gulls of a certain age?
As far as ZDGA is concerned, we know that he showed these patches in 2009 (his 24th calendar year), the year when I first encountered him. Unfortunately, no data containing references to his appearance exists of him before that time so we don’t know when this started to show. Going by the size of the bare patch though, I would expect that it would have taken a few years to develop.
Sightings are rare
My personal sightings of such individuals are few and far between. Apart from ZDGA, I have only seen 2 other individuals over the last 5 years showing similar characteristics.
Is it health related?
The fact that all my 3 individuals were observed in July at a time when they were actively moulting to basic plumage indicates to me that it is not a health issue. If it was, you would expect it to be seen throughout the year.
Also, ZDGA is a healthy individual and has been ever since I saw him, showing active and regular moult sequences each year, in line with other individuals. And come September, the bare patch is fully replaced by a nice set of new feathers.
I do believe therefore that these bare patches are typical of old Herring Gulls, and that they are at a minimum 18 – 20 years old, but possibly older.
I am aware though that not all old Herring Gulls show such bare patches: I’ve seen a photo of a known (color-ringed) Dutch 24-year old Herring Gull (so the same age as ZDGA in 2009) in July showing no bare patches at all.
But I do believe that when you see a Herring Gull showing such bare patches that it is roughly 20 years or older.
More examples needed
To confirm my theory, I would love to know about other observations of ringed Herring Gulls showing these characteristics, preferably in the form of photographs. If you know of any, please let me know.