How to ID Caspian Gull, and what are the structural differences at rest? (1 Viewer)

[leifvester]

Hey,

I'm wondering what are the best ID features to look out for that might be a good candidate for Caspian Gull when the bird is sitting or in water, i.e. I cannot see the legs, nor primary feathers.
Just so I don't end up overly focusing on a Herring gull - not that they aren't a nice bird!

For example see the attached image. I got very over-excited when I saw this bird, I really thought it was a Caspian initially tbh. It was only when it stood that and showed its wings that I clearly saw it to be a Herring, as discussed here: What is this strange gull? Ireland, Co.Down

Would you have hesitated on this bird, or are there obvious features that would rule out Caspian quite quickly here? At the time I was told the head and bill shape weren't right, but I don't understand in what way, or how much variance there is.

Furthermore, here is another Caspian gull at rest in water: https://ebird.org/checklist/S79650671

To be very clear, I'm not disputing the ID and it looks like a Caspian to me, but at the same time I'm personally struggling to spot the differences.
The gonys spot is definitely smaller in the checklist gull. Is there any other key difference that stands out that I'm missing and can look out for?

Cheers!

 

[Evan Atkinson]

Hey,

I'm wondering what are the best ID features to look out for that might be a good candidate for Caspian Gull when the bird is sitting or in water, i.e. I cannot see the legs, nor primary feathers.
Just so I don't end up overly focusing on a Herring gull - not that they aren't a nice bird!

For example see the attached image. I got very over-excited when I saw this bird, I really thought it was a Caspian initially tbh. It was only when it stood that and showed its wings that I clearly saw it to be a Herring, as discussed here: What is this strange gull? Ireland, Co.Down

Would you have hesitated on this bird, or are there obvious features that would rule out Caspian quite quickly here? At the time I was told the head and bill shape weren't right, but I don't understand in what way, or how much variance there is.

Furthermore, here is another Caspian gull at rest in water: https://ebird.org/checklist/S79650671

To be very clear, I'm not disputing the ID and it looks like a Caspian to me, but at the same time I'm personally struggling to spot the differences.
The gonys spot is definitely smaller in the checklist gull. Is there any other key difference that stands out that I'm missing and can look out for?

Cheers!

Hi there, I am still learning most of these large gulls but I'd like to think I am getting quite good at them. If I saw this bird in the field I would hesitate to put an ID on it till I could see the wing formula. Caspian's have a very large mirror on p10 that is quite distinguishable from herring. I'd also expect the eye to be more dark and the bill to be longer. I overall just don't get much of a casp vibe here but I can understand why you might have thought this was a casp. In the first link you put you can see that the p10 mirror is quite small, it should be quite a bit bigger in an adult caspian. At this time of year you get a lot of clean headed herring gulls and some can have dark eyes like I learnt today on the Thames. I have also found that the best time of year for looking for adult Caspian's is late november-early december, as not many herring gulls will have the clean white heads of a Caspian at that time of year. That doesn't mean that you can't get herrings at that time of year with clean heads though, so be cautious. Hope this helps!

Evan

 

[leifvester]

Hi there, I am still learning most of these large gulls but I'd like think I am getting quite good at them. If I saw this bird in the field I would hesitate to put an ID on it till I could see the wing formula. Caspian's have a very large mirror on p10 that is quite distinguishable from herring. I'd also expect the eye to be more dark and the bill to be longer. I overall just don't get much of a casp vibe here but I can understand why you might have thought this was a casp. In the first link you put you can see that the p10 mirror is quite small, it should be quite a bit bigger in an adult caspian. At this time of year you get a lot of clean headed herring gulls and some can have dark eyes like I learnt today on the Thames. I have also found that the best time of year for looking for adult Caspian's is late november-early december, as not many herring gulls will have the clean white heads of a Caspian at that time of year. That doesn't mean that you can't get herrings at that time of year with clean heads though, so be cautious. Hope this helps!

Evan

Hey Evan,

I appreciate the response! I definitely agree that seeing the primaries is the best way to be (almost) sure. The conundrum is that if a bird is sitting, swimming or standing, it could be an hour before it takes flight! So getting a sense of the bird without the wing tips would be handy!

I found this cool blog which shows that even if the bird is standing, you may be able to see the underside of p10, if it preens, which should reveal a lot of white: Species Gallery - Caspian Gull

I also just made a head collage, which does help quite a bit with comparison, particularly in the bill. In winter the green tinge should be more common in Caspian, however even in the bottom right summer bill, you can really appreciate the length difference as you mentioned.
Other notable points include:

• the distinctive gape,
• hooked upper mandible which sharply curls below the lower mandible at the tip,
• a thin lower mandible

In the Herring gull the bill appears more compact and the tip doesn't curl below the bottom mandible. There does also appear to be a subtle head shape difference, but this might be because in the pictures of the Caspian gull, they are standing. Either way I can't put the difference in head shape into words, so I might not worry too much about that feature.

Also as I learned recently, a Caspian gull winter plumage doesn't end until April at the earliest, whereas Herring can be in summer plumage as early as late December (as was the case here) so at this time of year there should be a shawl on the neck of a Caspian and the black mark on the bill seems fairly common as well.

I do think going forward I would be able to confidently ID this bird as a Herring gull without the need for primaries.

 

[Evan Atkinson]

Hey Evan,

I appreciate the response! I definitely agree that seeing the primaries is the best way to be (almost) sure. The conundrum is that if a bird is sitting, swimming or standing, it could be an hour before it takes flight! So getting a sense of the bird without the wing tips would be handy!

I found this cool blog which shows that even if the bird is standing, you may be able to see the underside of p10, if it preens, which should reveal a lot of white: Species Gallery - Caspian Gull

I also just made a head collage, which does help quite a bit with comparison, particularly in the bill. In winter the green tinge should be more common in Caspian, however even in the bottom right summer bill, you can really appreciate the length difference as you mentioned.
Other notable points include:

• the distinctive gape,
• hooked upper mandible which sharply curls below the lower mandible at the tip,
• a thin lower mandible

In the Herring gull the bill appears more compact and the tip doesn't curl below the bottom mandible. There does also appear to be a subtle head shape difference, but this might be because in the pictures of the Caspian gull, they are standing. Either way I can't put the difference in head shape into words, so I might not worry too much about that feature.

Also as I learned recently, a Caspian gull winter plumage doesn't end until April at the earliest, whereas Herring can be in summer plumage as early as late December (as was the case here) so at this time of year there should be a shawl on the neck of a Caspian and the black mark on the bill seems fairly common as well.

I do think going forward I would be able to confidently ID this bird as a Herring gull without the need for primaries.

Hi, yes the difficulty with birds sat on the water is that they can be quite deceiving. I think if you do have a casp on the water it will be obvious, however they can vary a fair amount. I've always thought to myself that adults are the easiest to identify but I now believe that 1st and 2nd winters are much more easy after learning on the Thames today. Also, I have never really considered this green tinge you speak of as being a contributing factor for ID'ing casps, its never something I have noticed and I don't really think it matters as much as the structure of the bill itself. It's very difficult to confidently ID casps in the summer as they aren't really on our shores at that time of year but I know that they can appear as early as August and even July; see here -> Summer Caspian Gulls .

Evan

 

[leifvester]

I think if you do have a casp on the water it will be obvious, however they can vary a fair amount.

Could you elaborate some more on this point please?

Also, I have never really considered this green tinge you speak of as being a contributing factor for ID'ing casps, its never something I have noticed and I don't really think it matters as much as the structure of the bill itself.

Ah this is really useful to know - thank you!

 

[Evan Atkinson]

Could you elaborate some more on this point please?


Ah this is really useful to know - thank you!

When I have seen and ID'd adult Caspian gulls on the water, they have always been quite in your face so to speak. Usually the combination of a very dark eye, long bill and more of a pear shaped head. Below is a picture of one that I saw at Dungeness in SE Kent that was initially picked up on the water. You can see that this individual is rather dinky looking amongst the black backs so it's probably a female. This is quite a classic looking individual as well with the long bill, small and dark beady eye and the head structure. This is all things that I would be looking for in a typical adult Casp. As you have experienced, you can occasionally get herring gulls with either long bills or apparent dark eyes which is why I like to rely on the wing formula to confirm an ID.


They are definitely a learning curb though, I still mix up herrings with them and will probably continute to do so haha.

Evan

 

[leifvester]

I think I'm fairly in tune now with what to expect with Caspian gull.

I've been browsing photos of Caspian gulls in water on eBird, and I did come across this one: https://ebird.org/checklist/S98966315
Not to be a wet blanket for the sake of it, but this is probably a Herring gull? Or can there be this much overlap in the structure of the head and bill? (That would be somewhat of a spanner in the works!)

I'm assuming without primaries it may be best to leave that one unidentified?

 

[Evan Atkinson]

I think I'm fairly in tune now with what to expect with Caspian gull.

I've been browsing photos of Caspian gulls in water on eBird, and I did come across this one: https://ebird.org/checklist/S98966315
Not to be a wet blanket for the sake of it, but this is probably a Herring gull? Or can there be this much overlap in the structure of the head and bill? (That would be somewhat of a spanner in the works!)

I'm assuming without primaries it may be best to leave that one unidentified?

You can get Herring Gulls on the water that might stand out as possibly being a Caspian (i.e. longer than usual bill, dark eye) which is why seeing them in flight is so important. At this time of year the above point really is noticeable and it's easy to be caught out by an adult herring gull. The image suggests herring gull.

 

[Evan Atkinson]

Just as an example, I had this gull on the water at abberton reservoir a few weeks ago which I initially thought was a caspian. But better pics suggested a herring with a dark eye.

Evan

 

[leifvester]

Just as an example, I had this gull on the water at abberton reservoir a few weeks ago which I initially thought was a caspian. But better pics suggested a herring with a dark eye. View attachment 1625384
View attachment 1625385
Evan

Yep I would certainly give that one a double take, but the points relating to the bill hold for this one too, even at this distance.

A couple of additional softer points I've found which may not be overly reliable, but the Caspian gull may appear to have a flatter back running into the primaries (not angled up as much at the back of the bird) and a little more black on the primaries as can be seen in this blog: More Caspian Gulls

 

[Evan Atkinson]

Yep I would certainly give that one a double take, but the points relating to the bill hold for this one too, even at this distance.

A couple of additional softer points I've found which may not be overly reliable, but the Caspian gull may appear to have a flatter back running into the primaries (not angled up as much at the back of the bird) and a little more black on the primaries as can be seen in this blog: More Caspian Gulls

Exactly. This was before I went to a site where there were lots of adult casp's so I do know what to look for now because before I mainly just looked at any herring with a dark eye and immediately assumed Casp. Seeing how they range really does help with ID'ing them though, have seen one individual who's mantle was almost YLG in terms of darkness and it was a brute of a bird. Will send pics if you want.

Evan

 

[leifvester]

Exactly. This was before I went to a site where there were lots of adult casp's so I do know what to look for now because before I mainly just looked at any herring with a dark eye and immediately assumed Casp. Seeing how they range really does help with ID'ing them though, have seen one individual who's mantle was almost YLG in terms of darkness and it was a brute of a bird. Will send pics if you want.

Evan

Yes please of it's not too much hassle!

 

[Evan Atkinson]

All I have of it are 4 awful digiscopes of it, but for context in the top centre there's a northern (or argentatus) herring gull and it is darker than that. I don't have any open wing shots of it which would have been really interesting to see. It's the left handmost adult on the mud looking to the right. It stood out significantly in the scope and I really thought it was a yellow legged gull at first. Beautiful looking bird though

Evan

 

[leifvester]

All I have of it are 4 awful digiscopes of it, but for context in the top centre there's a northern (or argentatus) herring gull and it is darker than that. I don't have any open wing shots of it which would have been really interesting to see. It's the left handmost adult on the mud looking to the right. It stood out significantly in the scope and I really thought it was a yellow legged gull at first. Beautiful looking bird thoughView attachment 1625500

Evan

That is a lovely gull. Very sleek and clean. I can see the sloped back extending through the primaries which also have more black than white.

I can see the distinctive head shape clearly in that photo too, with the face narrowing into the bill (the pear shape). The eye is also quite far up and forward in the head, compared with the Herring on the right.

I have read that the mantle colour can be the same as Argenteus, but don't know how frequent that is. Certainly stands out here.
Nice one!

I'll not pretend I can see the Argentatus though!

 

[Evan Atkinson]

That is a lovely gull. Very sleek and clean. I can see the sloped back extending through the primaries which also have more black than white.

I can see the distinctive head shape clearly in that photo too, with the face narrowing into the bill (the pear shape). The eye is also quite far up and forward in the head, compared with the Herring on the right.

I have read that the mantle colour can be the same as Argenteus, but don't know how frequent that is. Certainly stands out here.
Nice one!

I'll not pretend I can see the Argentatus though!

Yes, it was a belter of a bird. I notice that I chose the one picture without the argentatus in the top centre so will post that now.

Evan

 

[leifvester]

Yes, it was a belter of a bird. I notice that I chose the one picture without the argentatus in the top centre so will post that now. View attachment 1625502

Evan

Wow that does stand out! Didn't think the difference would be that striking. Looks like a brute of a Herring gull

 

[pdwinter]

I'm not sure I can contribute much as I have seen Caspian gull on only 45 occasions in 25 years. Only 6 of those have been adults - the majority being 1st winter birds. Many of these have been seen in the 300-500m range, often in failing light and usually where the camera just won't get a good picture.
I have put up a few images of the most recent adult I found and managed to pick out in early December 2019 and get past the Hampshire Recorder (it seemed to be a species only accepted with photographic evidence in this county at that time although that may now be different with a change of county recorder). Fortunately a friend with a Nikon P950 was in the hide at the same time and these are his images.
The two things obvious to me were the (textbook) bill - long, parallel-sided, dull (not very good at picking out subtleties like greyish or greenish!) yellow with a dark subterminal mark and red on the lower mandible - and the long wings with a fair bit of white at the end (not particularly visible in these distant photos!).

 

[leifvester]

I'm not sure I can contribute much as I have seen Caspian gull on only 45 occasions in 25 years. Only 6 of those have been adults - the majority being 1st winter birds. Many of these have been seen in the 300-500m range, often in failing light and usually where the camera just won't get a good picture.
I have put up a few images of the most recent adult I found and managed to pick out in early December 2019 and get past the Hampshire Recorder (it seemed to be a species only accepted with photographic evidence in this county at that time although that may now be different with a change of county recorder). Fortunately a friend with a Nikon P950 was in the hide at the same time and these are his images.
The two things obvious to me were the (textbook) bill - long, parallel-sided, dull (not very good at picking out subtleties like greyish or greenish!) yellow with a dark subterminal mark and red on the lower mandible - and the long wings with a fair bit of white at the end (not particularly visible in these distant photos!).

Awesome! Yes the "parallel-sided" characteristic seems like a fairly consistent one. The bill is quite clear in these photos too.

Thanks Paul!

 

[leifvester]

Just thought I'd share another difficult Herring from today.

Longish tibia, longish bill, dark eyes. But in flight showed too much black on wing tips, even for sub-adult. The bill probably not long enough as well.