This week we completed our yearly ringing session in the gull colony of IJmuiden, the Netherlands by fitting the last of some 50 juvenile Lesser Black-backed Gulls Larus fuscus graellsii (Dutch intergrade) and Herring Gulls Larus argentatus argenteus with a colour ring.
I was asked recently to show how we identify nearly-fledged juveniles of these 2 species in the field.
This can indeed be tricky because at this age both species look very similar and it comes down to checking specific plumage characteristics.
And while some individuals are textbook examples of how a juvenile Lesser Black-backed Gull and a Herring Gull should look, others show overlapping features.
On the other hand, once you know what to look out for (and with the benefit of being able to handle them), they can be easily separated.
Which species: Lesser Black-backed or Herring Gulls? The one with the colour-ring is a Herring Gull…
The Golden Rule
Before we start, a reminder that determining the species or age of any gull is done by verifying multiple characteristics, never by one feature alone.
In general, juvenile Lesser Black-backeds have a much darker appearance than Herring Gulls and are more chocolately-brown coloured compared to the just dark-brown color of a Herring Gull.
The absence of a blocked pattern on the outer greater coverts in Lesser Black-backed also adds to the impression of a darker gull, especially when seen in rest. More about that pattern later.
Features that are similar in both species and therefore cannot be used are the patterning of the head and body, the color of the legs and the shape and color of the bill (although Herring Gulls tend to have a paler bill base where the bill of Lesser Black-backed tends to be fully black).
In the next photo, compare the color of the Lesser Black-backed Gull (top) with that of the Herring Gull (bottom). Click for a larger view.
About the tail pattern
When fully grown, the tail pattern can also be used to differentiate between the two species (with less patterning present in Lesser Black-backed), but because the plumage of the gulls that we handled was not fully developed and the tail feathers not long enough, we could not use this characteristic.
Our main method of telling Lesser Black-backed’s apart from Herring Gulls is by looking at the primary pattern.
Whereas that of a Lesser Black-backed has an overal dark appearance, that of a Herring Gull shows a distinct light panel.
This is first of all because the outer web of the first 4 primaries is lighter, leaving only the tip of the outer web dark.
In the following image I’ve indicated this area in red:
The effect is further emphasised by the deep black color of the primaries in Lesser Black-backed, where those of a Herring Gull are at most a dark brown.
Note that the inner webs of the inner primaries of Lesser Black-backeds are also light but because they are mostly obscured by the overlapping adjacent primaries, the overall dark impression is created.
Only when viewing fully spread primaries can you sometimes get the impression of a light inner panel in Lesser Black-backed, but it is never as obvious as in Herring Gull. So when in doubt, you’re looking at a Lesser Black-backed…
Greater covert pattern
When not being able to get a view of the primary pattern (for example when the gull is in rest), a feature to look out for is the pattern on the outer greater coverts.
Where the inner greater coverts of both species show a similarly blocked pattern, a Herring Gull will typically show this on all greater coverts.
As a result, the wing of a Lesser Black-backed appears much darker than that of a Herring Gull.
In the following image I’ve marked the greater coverts in red. Notice that those of the Lesser Black-backed have a uniformly colored center whereas those of the Herring Gull have a distinct pattern.
The differences can best be viewed when comparing both species side-by-side.
All images were taken in the gull colony of IJmuiden in the first week of July 2016. I hope you find them useful.
Lesser Black-backed Gulls on the left, Herring Gulls on the right. Click for a larger view.