Scientific name: Phylloscopus inornatus
UK numbers: 8 wintering birds/320 passage
'Tswe-eep!’ For the keener birders among us, it’s one of the calls of autumn.
The prospect of seeing a Yellow-browed Warbler turns an ordinary birding day into a special one. The eyes suddenly focus on every leaf of the coastal scrub, hoping to catch a glimpse of this half-Chiffchaff, half-Goldcrest autumn sprite.
Birders love Yellow-browed Warblers because few birds so completely capture the magic of autumn migration. Breeding no nearer than western Russia, virtually unknown in spring and always arriving when lots of other birds are moving too, the
Yellow-browed is the icing on the migratory cake. Where Yellow-browed Warblers are, other great birds will be nearby. The Yellow-browed Warbler is a fixture in many a happy memory; of bushes humming with hungry birds, berries red and leaves reddening, the camaraderie of fellow delighted birders, the last autumn sunshine and the first storms.
And it is something more than this.
This 10cm of stripes, flitting, hovering and leaf-searching, also embodies the marvel of migratory flight, its wonders and its mysteries. The Yellow-browed has a story to tell which is almost unique. It tells of the dearth of our real knowledge about many species and shows how birds are often so much more remarkable than we think they are.
The story of this bird’s capture of British birding hearts begins back 50 years ago. No birdwatcher these days could conceive now of just how rare the Yellow-browed Warbler used to be. It was recorded only a handful of times before the year 1958, but the following years saw a remarkable and unexpected increase.
Once a rarity
By the 1970s, about 70 individuals appeared each year, and by the early 2000s it was more than 400 a year. Appearance records kept on tumbling, with more than 800 in 2003 and an incredible 2,000 in 2017, beating 2016, which was itself a record year. Nowadays, a single Scottish island can host more individuals than used to appear in the entire year in the whole of Britain. And yet these are birds that should be on their way south to winter in south-east Asia, in places such as Thailand and Singapore.
Although there is no doubt that some Yellow-browed Warblers were missed in the past, these numbers add up to a remarkable change in this bird’s patterns of appearance (and something similar has happened, on a smaller scale, to another Asian rarity, the Pallas’s Warbler). What is going on? Why are they almost routine now, when they were once major rarities?
You can imagine that there has been much learned discussion and speculation about this – the only thing that certain types of birders love more than seeing rarities is talking about rarities. However, trying to get to the root of a tiny waif’s migratory tendencies is not easy. Some colourful arguments have been flung to and fro. One of the first suggestions as to why these warblers were finding their way to Western Europe, when they should be travelling south-east, was that the change was the result of a vast collective mistake. In other words, thousands and thousands of Yellow-browed Warblers were becoming disoriented and flying in the wrong direction. It so happens that the direction into northern Europe is a neat reverse of the great circle route into south-east Asia, so the theory was that it wouldn’t take too much misreading of the birds’ migratory instruments, such as their day-clock, to cause the error.
It was even suggested that individuals breeding near mountains with unusual magnetic properties might cause the shift.
The idea of mass misdirection is not as unlikely as it sounds. Part of what we all love about birding – the chance of seeing something unusual – in part depends on just this. Rarities don’t intend to come here; if they did, they wouldn’t be rarities. Birds make mistakes all the time in all aspects of life; they eat the wrong food, they don’t spot the Sparrowhawk coming, and they take the wrong turning.
Nice and plump
However, the sheer volume of the Yellow-browed flood has convinced most people that these birds know what they are doing. After all, they are often in excellent condition and nice and plump when they arrive here, and they aren’t furrowed-browed warblers, looking lost and confused. There is no evidence to suggest that they are all young, inexperienced birds, either.
Instead, it could all be down to Zwischenzug. This splendid German term describes unexpected counterintuitive movements made by birds prior to their main southward migration. It occurs in, among other species, Starlings, which may fly from Switzerland as much as 300 miles to the north-west to summer in Germany.
The Yellow-browed Warblers travel more than 1,200 miles, making it something of an ultra- Zwischenzug, but there are several arguments in its favour. Several birds trapped in Europe and then placed in Emlen Tunnels to register their intended direction of further migration, firmly oriented east. Secondly, the phenomenon is increasing, suggesting that birds are surviving the winter, breeding and passing on the migratory quirk to their offspring. And thirdly, very few are finding their way to north-west Africa, or indeed southern Europe, suggesting that many are either perishing or reorienting.
Why is the biggest question
Why are they coming in the first place? This isn’t known, but perhaps the habit of flying westwards is an exploratory flight, taking advantage of the relatively benign and productive autumn of north-west Europe? Without interviewing the birds, it might be impossible to find out.
There is no doubt that the Yellow-browed Warbler’s recent migratory shift is remarkable, but in recent years scientists have unearthed some other remarkable aspects of this Siberian sprite’s life. We have to remember that, on its breeding grounds it is a very different creature from the one we see here. Take its typical problems: it has to avoid the cruel intentions of Oriental Cuckoos, which commonly parasitise it, and it has to avoid the depredations of Siberian Chipmunks, which are the sworn enemy of its eggs and young. This makes us realise how far away from us it breeds.
But the coolest new fact to emerge about this bird is that it shares some aspects of its breeding behaviour with Black Grouse and Ruffs – operating a pairing system similar to a lek. Out in the taiga, the first arriving males quickly acquire a territory in the forest. Later arrivals don’t spread out, but instead set up their own territories, often very small ones of a few tens of metres, right next to a previously arrived male.
These aggregations of singing birds attract females, which themselves gravitate to the males with the central, larger territories, which are presumably those of the highest quality. The central males frequently mate with more than
one female and appear to get the lion’s share of copulations, because their position on the lek demonstrates their overall fitness. It is thought that more peripheral males sacrifice setting up a proper territory in the hope of intercepting a receptive female on her way to the central territory!
Imagine that – a small warbler operating the same system as a Black Grouse, one recorded for only a handful of birds. That is enough to raise your eyebrows, whatever colour they might be.
Where to see them
Look for this rare bird in coastal trees and shrubs. They arrive on the east coast initially, but do make it into south-west England. In the Northern Isles these can be easily the commonest warblers during October!