The birds
I admit that I haven't been cutting edge sharp on the finer details of tristis identification. Tristis, the Siberian subspecies of the Common Chiffchaff (Phylloscopus collybita) [gransanger]. The common chiffchaffs are indeed that in our region: common, whereas tristis is only visiting in few numbers, and mainly during the autumn migration.

I've mostly hold in memory that tristis has a quite different, more chicken-like call, iiie, compared to the standard huit-call of the common birds. These two calls are distinctively different in the field, and in our region the main confusion regarding the birds saying huit refers to Willow Warbler [løvsanger], although willow tends to give it as two syllable call more like, huu-it. However, without an analysis of the sound, that can be difficult to perceive in the field. To the untrained ear, I'm sure many tiny calls from various song birds can sound like a tristis.

More than focusing on the difference in calls, I've also noted, when studying the subject, that there exists a general warning-opinion about common birds being able to give calls similar to the ones given by tristis. I've come to interpret this as: "Be aware that the common birds can have many different calls, why it has to sound exactly like a tristis before it can be called a tristis". However, this provokes my logical sense! Most birders and professionals who dig into this matter are based in Europe and naturally have extensive experience with the individuals from here. And that has provided them with the knowledge that home-ground birds speak in many ways, while their very limited experience with tristis on its natural grounds make them conclude that these birds always call with just one type of call. That is perhaps a logical step in the learning process to reason like this, but it is not very convincing in my mind. Tristis too very likely has a variety of calls that differs from the most common one.

And this assumption was probably what in the beginning made me put a great effort into convincing myself and others that the three tristis-candidates that I found in one day on April 15 were exactly that!

In Oct 2022 I had my to date first and only calling tristis on Kalvebod Fælled. I detected it instantly by the unusual chicken-call. All was well, and I was happy with a good find.

And then, on April 15, this year, in exactly the same spot, I had a new tristis. My first since the 2022 bird. I was positively sure, when I heard it: The rather sharp iiit call that I perceived as being similar to the 2022 bird. I only got a few photos of the bird, which soon retreated to the birch tops, when I chased it. My recorder was in winter dead-mode, why I couldn't make recordings. Anyway, the sound was spot on, and the few photos were definitely not against such a bird, why I happily moved on with my new discovery (bird 1). But then the unlikely happened, that a kilometer further ahead, I heard another bird (bird 2). Exactly the same sound. Insistingly calling, same rather sharp call, so different from the calls all the other phylloscopus warblers close by were giving - willows and chaffies. Photos again that looked good with a bird in an overall greenless look. And the call so different from huit. And the only sound the bird made, as had it been with the first bird. No deviation, and no signs of a normal chiffchaff just adding in an odd call on occasion. It sounded like the real deal, and I put number two in the book, and left that bird too. And from there I continued and finally reached the southern end of the area, near Hejresøen, where I heard a new bird (bird 3) calling spontaneously from the thicket! Spot on, the same experience and sound. Incredible. What an influx that was! I got photos of that bird as well

Photos, by the way, are not that definitive when dealing with the phylloscopus identification. Color tones are reproduced so differently on different devices that it in many cases makes it impossible to know what the objective original was looking like. But still it is nice to have the photos as a secondary support when considering identification on this sophisticated level. And in my case, the photos all showed birds that looked grayish brown without the yellow-green touch that is expected on collybita birds.

Perhaps most birders would have stopped for a short while, and given it an extra thought, as to how realistic it sounded that three rare birds of the same species should show up at the same locality on the same day. And maybe I did too. But the fact was, that during these days several other birds were being reported from the Copenhagen area, and backed up by my 2022 experience (in memory) I had no reason to be in doubt.

But then what about the warning that common birds could be calling like a tristis, didn't I take that into consideration? Well, I guess I did. Because I had never gotten the impression that such deviating birds would be deviating on a consistent basis. I imagined that they would be normal birds giving an odd call now and then. Therefore, while all my three birds were calling consistently with the chicken call and rather eagerly, actually, I had no reason to suspect other than something truly unusual. So I entered them into dofbasen.dk in good faith.

Next day I and all others, who did not provide sound documentation, had their observations flagged. And I could see that several persons had removed their observations. And a birder contacted me to listen to his own observations of other birds, and as such, all of a sudden, the boat was no longer in smooth sailing, and I had to re-consider. I needed better proof than the photos, so I charged my recorder and got out there the next day, hoping the birds would still be around. And bird 2 and 3 were in place.

I got really good recordings and was looking forward to the confirmation. But to my own surprise the spectrograms revealed a shape not fully consistent with what a normal tristis should look like. In the field I could only hear the sound as a straight linear sound without any ascend or descend. But the shape was continuously, on all calls, an asynchronous inverted u, a strong rising and a weak descend, on both birds. Ok then. So we were talking about birds that deviated. I tried to find similar looking spectrograms on ebird and xeno-canto, but couldn't find anything conclusive. And I tried to dig up articles on the subject, and found information that gave me some confidence in the birds still being tristis. But one article was from 2013 (birdingfrontier) why I needed to find more up-to-date information on the subject.

Meanwhile, I continued to follow the birds during the next days, and while doing so, I managed to find 4 other birds giving the same type of call. Two of the birds, bird 4 and 6, were looking rather greenless too, the last two, bird 5 and 7, I only heard and recorded.

All the birds except bird number 4 showed the inverted u spectrogram, whereas number 4 only shows the rising part of the u. Birds 1, 2, 3 and 6 are still in place (May 7), and during all my visits the birds have been calling spontaneously without being stressed or provoked, and they have been calling exclusively with the same call, and often eagerly, although, as the days have passed, it seems like the overall enthusiasm has dropped a bit. A few of them have also added in some of the small-chatter that can often be heard at the beginning of the chiffchaff song, but never more than just a few hinting notes. None of the birds have been interacting with any of the other birds around them, and this despite of several willow warblers and chiffchaffs being present, singing and calling. The only time I've experienced some interaction was when bird 4 one day started calling, and all of a sudden was attacked by another phylloscopus, which I think is another unusual chiffchaff. That bird retreated to where I got the photos as seen, but then it disappeared without returning. Bird number 4 continued to call afterward.

Another change in behavior that I've experienced was when bird 1 suddenly started to add in a new call that reminds of the traditional huit call, but the quality of it better resembles the one given by the willow warbler than that of the chiffchaff - meaning that it shows a flatter start to the call than the chiffchaff normally does (hockey-stick). The sharp tristis-like call, however, continued to be the main call, only adding in the new one a few times.

And a last noteworthy behavior has been that of bird 3 which one day was fluttering its wings, like a juvenile would do when begging. And another day I saw that same bird preening itself exposing what looked like a brood patch, which, if so, would indicate a female bird (but still without any interaction).

All the birds have been rather stationary within a 40 x 40 m2 area, except for bird 2 which has been moving within a rectangular space of about 20 x 100 m2. And the habit has mainly been semi open edge of young mixed birch forest, with sea-buckthorn [havtorn] and hawthorn vegetation [tjørn] mixed in.

The thoughts
So, after having investigating the subject (read the articles linked to below), I've come to accept that these birds are not tristis. This also by comparing them with the calls of the 2022 bird - it is obvious that there is a different, stronger and rising quality to the calls of the present year birds.

Instead of, I now accept them as the calls that have become commonly known as the sweee o sweeeo call in English communication. By both Dean and Lindholm called the alternative call. I call them iiiih calls to be fateful to my own interpretation of the sound.

When digging into the matter it soon becomes clear that during the last 20+ years more birds giving this type of call have showed up. And have gained focus. Not only among common birders, but among ornithological authorities as well. To my knowledge the origin of the call is still a mystery today, but different suggestions have been given:

1. ranging from juvenile calls being used into adulthood
2. over the idea that it has to do with a 'seasonal fashion of the year' several birds being inspired by a 'cool' sound that they adapt to,
3. to the birds having an eastern unknown origin and likely being birds of the eastern part of the abietinus complex.

The latter was indeed my own first assumption, when they couldn't be attributed to real tristis. The fact that the birds in Denmark showed up as a cluster around Copenhagen (assuming the other tristis candidates were of the same type as my birds) made it reasonable to expect that they had arrived from the east. There were warm spells of air moving up through eastern Europe in the days before, though not reaching Denmark. But it still could have triggered easterly wintering birds to come this way. A bird was reported from Uppsala in Sweden on April 12 that shows a somewhat similar call to mine, and this bird has been suggested to be an abietinus, which is the expected subspecies for the area. The fact that the birds were all grayish brown looking made it natural to assume birds that we normally don't see in our area.

I haven't been able to find other records from neighboring countries that deviate sound-wise, which however doesn't mean much, since it is likely that many such deviant birds would pass through unnoticed due to lack of coverage outside large urban areas and birding hotspots. And today, at least in Denmark, it is also very likely that many such birds are no longer being highlighted when reporting, since they have been deemed deviants belonging to the general collybita group (collybita/abietinus), which are the normal birds in our area; so people most likely just report them as plain chiffchaffs, without comments.

But that said, reading into the matter (Dean has a good summary in the link below) it becomes clear that there is no evidence that such alternatively calling birds should origin from a certain area, even though I would still like to defend the perception that there is an overall similarity between the traditional tristis call and the alternative call. But if such an area should exist, I admit, it would be reasonable to expect that it would have been found by now, taking the extensive travel activity among birders and researchers into account.

That the birds should be juveniles in the learning process doesn't fit well with the fact that many birds use this call consistently and at all times of the year.

And the fashion-of-the-year idea seems a bit far-fetched: Why should they do it? Calling consistently in a different manner than is expected by other members of the species is not a good strategy if you want to pass on your genes to the next generation. I know mimicry is common among songbirds, but that is normally building onto an already existing repertoire, to show superiority or gaining benefits by sounding like these other species. This call doesn't resemble anything in the area where these birds show up, why there should be nothing to gain by this adaptation. One could probably accept this idea, though, if a single bird here and there felt capriciously impulsed for such a style change, but the phenomenon is too common and widespread, why the idea is unrealistic.

Since there has been an awareness about the subject for many years now, it is reasonable to accept that the periodic emergence of these birds is real, and not just an expression of coincidental coverage by the European birding community.

And so I was wondering:

If geographic origin (genetic) is out of the question, the cause must be found in the environment, and the idea struck me, that it is known among humans, that the suppression of certain hormone levels in the male body affects the vocal expression (eunuchism). What if the same is the case with these birds? The phenomenon seems to have emerged during the later decades, and taking into consideration how we continuously keep polluting the environment with toxic chemical substances that accumulate throughout the ecosystems, it is not hard to imagine that birds are to be affected negatively, and some more than others, due to the natural variation of the physical conditions that the individuals constitute.

However, one problem with this theory is the unpredictable emergence of the phenomenon. It appears that the number of alternative birds show up with years interval, why it shouldn't be caused by a constant chemical effect. But then again, what if unusual boosts of pesticide use, or other sources of contamination, in the wintering areas intoxicate the birds, through their prey items or water consumption, leading to a periodic or permanent hormonal disturbance affecting the vocal expression? That these birds then seem to disappear could be explained by a sooner death than expected among the individuals in general.

According to the Lindholm article, linked to below, it is thought that the alternative call is part of the normally occurring juvenile sound repertoire, why the idea of the birds calling like this, is, that they are merely birds that, for unknown reasons, have continued to use that call - idea 1 listed above. But what if we accept that idea, that it is indeed an innate juvenile call, that under normal circumstances would have been suppressed as the bird grows up and aquires the learned sound repertoire that is normally used by adults. and then a hormonal disturbance, caused by outer factors, would trigger a 're-accessibility' of the juvenile calls adding them into the reachable cerebral pool of sounds, or perhaps replace it entirely? It's an idea.

What I don't like about it, though, is that all my birds have had grayish-brown looks without the normal appearance that we expect to see in our area. And for that, I would still like to think there is a geographical component to this phenomenon. Unless: Could it be that both behavior and physical appearance can be affected by chemical intoxication?

Another thing that provokes doubt about this idea is that it would have been reasonable to expect that the phenomenon would show up in other similar species as well. That should then be species wintering in the same areas. But only few songbirds winter in the Mediterranean region, as the chiffchaffs do, why it might be difficult to detect such a similarity. I can only think of the Eurasian Blackcap (Sylvia atricapilla), but I'm not aware of any unusual appearance among the blackcaps. Eurasian Starlings (Sturnus vulgaris) also winter in southern Europe, but would they share feeding habitats with the chiffchaffs? However, being ignorant about avian physiology I wouldn't know if responsiveness to environmental changes is similar in all song birds why the phylloscopus warblers could respond uniquely...


When I started to investigate the phenomenon I hoped to find a solid and straight answer, but have failed. The question is still open. I like the environmental explanation but would prefer the real answer to be a pool of birds from an unknown geographic region because both looks and sounds are quite distinctive. Not the looks as such, since both abietinus and tristis can look similar, but the combination of the two. A good researcher would keep on investigating, but I have to leave it with this, adding my information as a small contribution to the pursuit of an answer. Blood samples, however, is where I would start if I were to continue. But how to obtain them from enough birds that only show up occasionally without prediction is of course the big question. Unless you decide to kill them for later examination, but that is of course not a modern world consideration worth giving even the slightest hint of thought!





Good links to read:

A. R. Dean - 'Alternative' calls of Common Chiffchaff
http://deanar.org.uk/general/articles/chiffchaffcalls.htm

Antero Lindholm - Occurrence of the alternative call of Common Chiffchaffin Finland and Estonia:
https://caluta.fi/Caluta/Caluta6.pdf

M. Hellström - Gransångare av rasen tristis
https://cdn.birdlife.se/wp-content/uploads/2019/03/Tristis_Waern__Hellström_2011.pdf


Links to my birds (all are marked as abietinus? to be able to find them later):
Bird 1 (April 15 to May 7):
https://jesperbayjacobsen.com/page_start.php?file=bview&search=samebird&speciesid=6624&id=176

Bird 2 (April 15 to May 7):
https://jesperbayjacobsen.com/page_start.php?file=bview&search=samebird&speciesid=6624&id=173

Bird 3 (April 14 to May 14):
https://jesperbayjacobsen.com/page_start.php?file=bview&search=samebird&speciesid=6624&id=174

Bird 4 (April 18 to April 24):
https://jesperbayjacobsen.com/page_start.php?file=bview&search=samebird&speciesid=6624&id=175

Bird 5 (April 19 to May 2):
https://jesperbayjacobsen.com/page_start.php?file=bview&search=samebird&speciesid=6624&id=182

Bird 6 (April 21 to May 2):
https://jesperbayjacobsen.com/page_start.php?file=bview&search=samebird&speciesid=6624&id=180

Bird 7 (April 24):
https://jesperbayjacobsen.com/page_start.php?file=bview&search=singlephoto&id=5833