How to score primary moult in gulls

During the International Gull Meeting in Spain this year, I gave a presentation about primary moult in adult European Herring Gulls. I was happy to see afterwards that there was a lot of interest in this subject from various attendees from all over Europe. Some of them also indicated that they wanted to start keeping track of primary moult in the the gulls that they observe.

When moult is scored at different locations, it is important that this is done by using the same method so that the results can be compared.

In this post I will explain how primary moult is scored and what it can tell you about a group of gulls.

Numbering primaries

When studying primary moult, each primary feather is numbered so that it can be referred to. In gulls, numbering starts on the inside of the wing.

The primary on the inside of the wing is indicated as Primary 1 (P1), followed by Primary 2 (P2), up to Primary 10 (P10) on the outside of the wing.

Scoring primaries

The primary moult score (PMS) is obtained by assigning a value to each primary.

I use the following well-known scoring system:

0 = Feather is still present (not replaced)
1 = Feather is missing or just growing
2 = Feather is a quarter grown
3 = Feather is half grown
4 = Feather is three-quarters grown
5 = Feather is full grown

I have also come across the following system (as per the BTO Guide 1983, Moult in Birds by Ginn & Melville), in practice I don’t think there is much difference with the method above:

0 = Feather is still present (not replaced)
1 = Feather is missing or just growing
2 = New feather just emerging, up to one third grown
3 = Feather is between one and two thirds grown
4 = Feather more than two thirds grown but not fully grown
5 = Feather is full grown

The values of all feathers are then added up to give a total score. A wing with 10 old feathers will have a score of zero, a wing with 10 replaced feathers will have a score of 50.

Example: the following adult Lesser Black-backed Gull has the following score:

  • P1 fully grown = 5
  • P2 almost fully grown = 4
  • P3 to P10 old = 0
  • PMS score: 5 + 4 = 9

This is displayed as ā€œ9 (5400000000)ā€.

(The method in the BTO Guide replaces the value 5 with the letter N. The above score would then be ‘N400000000’. Personally I prefer using all numbers instead of adding an extra variable such as ‘N means 5’.)


The main challenge in scoring the primaries is of course getting a good view of the wing. The best method is to take clear photographs, preferably of the gull in flight so that the wing is viewed when fully stretched. Ideally, a high enough shutter speed is used to prevent motion blur.

The other challenge is interpretation: how long is ‘a quarter grown’ or ‘half grown’. This can be subjective and even with practice, differences can occur. Here, good photographs again play an important role so that they can be re-evaluated, or evaluated by other people.

Keep in mind also that when you are able to score a bird in the hand that this will result in a much more accurate score than when having to calculate the score from a photo. Especially when it comes to the inner primaries for example you will be able to see if a feather is scored as 1 (missing/just growing) or as 2 (a quarter grown). However, the effect on the final result will be minor and when plotting many results in a graph it will hardly affect the median, if at all.

Keeping track of scores

The simplest way of keeping track of primary moult is by using a spreadsheet. I advise to at least use the following columns:

  • Bird ID
  • Day of the month (as a value)
  • Month (as a value, 1 to 12)
  • Year
  • A column for each primary: P1, P2, etc
  • PMS score
  • PMS pattern (the 10-digit summary)

Use formulas to automatically fill in cells such as the PMS score (the sum of all values from each individual primary column) or the PMS pattern (concatenate all values from each individual primary column).

You might also want to keep track of the primary score of both wings because there might be differences. Alternatively, take the highest score.

When using my Marked Birds Database you can keep track of primary moult as part of an observation.

What to do with primary scores

Keeping track of any kind of data allows you to discover trends: is what you are tracking always the same or are there differences? In case of differences: do these occur in individuals (over seasons), age groups, different populations (ranging from nearby to those abroad), or similar (gull) species? Being able to compare such data can give valuable insight into the differences between individuals and species.

On a personal level, keeping track of primary moult has given me the opportunity to document suspended primary moult in European Herring Gulls in great detail, something of which little is known. I had no idea that I would discover this simply by keeping track of a group of individual gulls.

I would be very interested to find out in what way this occurs in other (Herring) gull species around the world. To do this, primary moult needs to be recorded on a large(r) scale.

Now you know how to do this yourself.



8 thoughts on “How to score primary moult in gulls

  1. Thank you for posting this Maarten. I have seen scoring on Gull Org. so your straightforward article will help enormously – so much to learn!

  2. Is it possible to have views from the front? I am a bird carver and I need measurements of width of the mouth!

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