When it comes to Black-headed Gulls (Chroicocephalus ridibundus), one plumage aspect which has intrigued me for some time is a black mark on the outer web of P9* on what appears to be a minority of sub-adult and adult individuals.* In order to refer to individual primaries, primaries are marked 1 to 10: starting with the innermost feather Primary 1 (P1) and ending with the outermost feather Primary 10 (P10).
The feather shaft divides the feather into 2 parts called webs: the inner web (the part towards the body) and the outer web (the part away from the body).
Generally speaking, gulls are born with a (dark) brown plumage with dark primaries and a dark tail band which gradually turn into gray wing feathers and white tail feathers after each moult cycle.
The same is true for Black-headed Gulls. The first-generation wing feathers contain much black on the primaries, primary coverts and alula.
From May onwards in the second calendar year, a Black-headed Gull acquires its second-generation primaries. When the full moult is complete, the plumage is identical to that of an adult.
Descriptions of the adult Black-headed Gull plumage in bird guides and other literature mention the presence of a black mark on P9 only briefly if at all and I have not been able to find detailed descriptions of their presence and variation.
I therefore wanted to satisfy my curiosity by holding my own investigation.
Apart from trying to find some kind of indication of the percentage of individuals with a black mark versus those without one, I was curious to find out if the presence of a mark is:
- Age related
- Sex related
- Genetically determined
(If you want to know the answers to these questions straight away, scroll down to the summary.)
In order to be able to find out the answers to these questions, I needed a representative group of gulls with a known background.
I found this in a Black-headed Gull colony in Zoetermeer, The Netherlands, where gulls are fitted with a color ring as part of a project which investigates dispersal and longevity in Black-headed Gulls.
The conspicuous color rings allowed me to photograph individuals in flight and identify the individuals based on the ring code. Photographs were available for the years 2010, 2011 and 2012.
Question 1: black mark vs no black mark
The individuals that were scored were those that had acquired an adult plumage at the time of observation (second cycle plumage and older). This resulted in the following totals:
- Total number of individuals: 190.
- Individuals with a black mark: 46 (24.2%)
- Individuals without a black mark: 28 (14.7%)
- Unknown: 116 (61.1%)
It is immediately apparent that with only one-third of the individuals scored, not enough data is available to make any hard conclusions. Hopefully I will be able to score more individuals in the coming seasons so that a more precise conclusion can be made.
I was still surprised to see though that in this initial sample the individuals with a black mark outscored those without a mark.
Question 2: is the mark age related?
In gulls, black markings on the feathers of the wing or tail are usually an indication of immaturity.
In large gulls especially, it is not uncommon for a 5th calendar-year individual in full adult plumage to still show sub-adult plumage characteristics (that means: dark markings in the wing and tail) whereas such markings in the following moult cycle are not present for that individual.
So can the black mark on P9 in Black-headed Gulls be used to determine the age of an individual and if so, what age would that be?
If we treat this phenomenon the same as in large gulls, it would be sensible to expect to find such markings in 2nd-calendar year individuals or possibly 3rd-calendar year individuals only and not in older individuals.
My data disproves this immediately. From the 46 individuals scored with a black mark on P9, 33 of them (71.7%) were aged as 4th-calendar year or older:
- After 2nd calendar year: 1
- 3rd calendar year: 7
- After 3rd calendar year: 5
- 4th calendar year: 1
- After 4th calendar year: 6
- 5th calendar year: 1
- After 5th calendar year: 18
- After 6th calendar year: 7
This immediately shows that black markings on P9 are very common among adult Black-headed Gulls.
Incidentally, this 24th-calendar year Black-headed Gull (caught individually and not part of the sample from the described colony) also shows black markings on P9:
A related question that could be asked is whether the size of the black mark changes over time for an individual? Does the size and position stay the same or can changes be detected?
This is a difficult one to answer because it requires detailed photos of the same individual in different moult cycles, taken from the same angle. More photos over more years are needed to come up with a conclusive answer.
One interesting exception though is White E5RH, ringed in June 2010 as a female, aged as after 3rd calendar year. Photos show a clear change in the size of the black mark in P9 on the right wing compared over different moult cycles:
- June 2010: large mark clearly present
- April 2011: only a faint mark visible (photos not conclusive)
- April 2012: small mark clearly present
Question 3: is the mark sex related?
So do black marks occur more in females, males or can no distinction be made?
From the 46 individuals, the difference in sex was as follows:
- Female: 17 (36,9 %)
- Male: 20 (43,4%)
- Unknown: 9 (19,5%)
So even though slightly more males are scored than females, there does not seem to be a clear difference in sex.
Question 4: is the mark genetically determined?
What about offspring: do they inherit the same markings as one of their parents? If neither parent has a black mark, would their offspring also not show a mark? Or if one or both parents show markings, would their offspring also show markings?
Of the 46 individuals scored with a black mark on P9, offspring for 3 individuals are known (a total of 4 individuals fitted with a color ring). Out of these 4 individuals, 3 are 3rd-calendar year in 2012 while the other is a 2nd-calendar year in 2012. I have not been able to score any of these individuals yet.
Of the 28 individuals scored without a black mark on P9, offspring for 2 individuals are known (a total of 3 individuals fitted with a color ring). Out of these 3 individuals, one is a 2nd-calendar year and the other 2 are 3rd-calendar year individuals in 2012. Only one of these has been scored: White E5RT (offspring of female White E5RA), does also not have any black marks on P9.
Of course, much more data needs to be gathered to come to any kind of conclusion.
To summarize, here are the answers to the questions I asked myself:
Question 1: Are there more individuals without a black mark than with a mark?
Answer: not enough individuals from the sample have been scored, but out of those that have been scored, a large majority does indeed have a black mark.
Question 2: Is the mark age related?
Answer: no, it is not. Black marks on P9 can be seen on a wide range of ages.
Question 3: Is the mark sex related?
Answer: from the small sample of individuals there does not seem to be a significant difference between marks present on females compared to males.
Questions 4: Is the mark genetically determined?
Not enough data is available to answer this question yet.
I went into this project thinking that black marks on P9 were not common but even though the figures are inconclusive, they show that they are far more common than I expected. Based on observations of ringed individuals (including those from other colonies than the one used here), I already knew that they are not specific to young adults but it is nice to see it in hard figures now.
My aim is to continue collecting data based on individuals from this colony and hopefully this will shed more light on all of these questions.
Gulls of Europe, Asia and North America Klaus Malling Olsen and Hans Larsson
The influence of age and sex on wing-tip pattern in adult Black-headed Gulls Larus ridibundus Domique Allaine and Jean-Dominique Lebreton, Ibis Volume 132, Issue 4, pages 560–567, October 1990.
Handbuch der Vögel Mitteleuropas. Band 8 (1.Teil) Urs N. Glutz von Blotzheim