The sewage outfall stream from Ecton Sewage Farm into the River Nene near Cogenhoe has long been a regular magnet for Chiffchaffs during the winter months although it, along with the adjacent extensive Phragmites reedbed, receives surprisingly little attention from local birders. In winter the temperature of the treated water from the processing works is a few degrees higher than the surrounding environment and, as a result, provides a microclimate favourable to various insects and other invertebrates, which act as a ready source of food for insectivorous species like Chiffchaffs.
I visited the site in early January and was delighted to find at least three Bearded Tits and four or five Chiffchaffs in the general area. When Bob Bullock went there on 25th, however, the number of Chiffchaffs had increased to at least a dozen, all concentrated along the banks of the outfall stream, probably as a result of the recent cold snap. On 26th Bob visited the site again and found three ‘non-conformist’ Chiffchaffs among them – individuals which, on plumage, were clearly not of the nominate race collybita and which looked good for having an origin from much further east.
Armed with a phoneload of Chiffchaff songs and calls, plus a Bluetooth-enabled, thumb-sized remote speaker, I visited the site the following day, 27th, where I met up with Bob and together we eventually had prolonged views of two ‘grey’ individuals along the 150 metre length of stream immediately before its discharge into the river.
On plumage alone these two birds, almost identical in appearance and, differing markedly from the accompanying ten or so ‘regular’ Chiffchaffs, looked good for the Siberian race tristis based on identification criteria recently published here by Martin Garner.
Racial identification of Chiffchaffs has endured a chequered history in recent years. Initially it was thought to be relatively easy: if a grey-brown Chiffchaff sporting supercilia and secondary covert wing bars of varying degrees of prominence greater than a nominate race Chiffchaff called with a distinctive “peep” note, sounding like a ‘lost chick’, then it was tristis.
Then came the abietinus problem. This race, from Scandinavia/Western Russia, was believed to be greyer, prone to exhibiting wing bars, and was deemed a potential source of confusion with tristis. It was therefore believed that the only ‘good’ tristis were brown, buff or shades thereof. Then there was the tristis/abietinus hybrid zone with the potential to produce birds unassignable to race and which could wander to Britain … With the publication of MG’s work on tristis identification it has come full circle. In it he outlines a study undertaken on trapped birds in The Netherlands. The bottom line result was that, in a small sample unit, all individuals identified as abietinus in the hand were actually tristis on subsequent DNA analysis! MG postulates that almost all abietinus probably migrate southeast in autumn and that this race is likely to be very rare in the UK. So tristis is back on the menu for British birders as being relatively straightforward to identify – especially if you hear the call.
Which brings me to an initial point of worry: neither of the Siberian Chiffchaffs was heard to call. I played recordings of calls and songs of all three races to them and, aside from one tristis and a couple of nominate collybita breaking cover just once to see what was going on when the regular Chiffchaff song was played, there was no reaction. None.
By good fortune MG was visiting the Bedfordshire Bird Club two days after the observation so, armed with numerous photos, Bob and I met up with him briefly to discuss the ID of the Ecton birds. Upon seeing Bob’s photos his view was we could confidently identify our birds as tristis – without even hearing the call!
So, moving forward, firstly, we should be on the lookout for more of these birds, especially this winter as there seems to be more about nationally than usual and, secondly all the past records of abietinus in Northants can surely be removed as it appears they are not even readily identifiable in the hand – let alone in the field! These include individuals which have occurred at Ecton in past winters as well as others elsewhere.
For the record, there have been a handful of sight records of presumed Siberian Chiffchaffs in Northants and one of a bird trapped at Stanford Reservoir on 6th December 2008, which was confirmed as tristis by DNA analysis of feather samples. Field identification, though seemingly a lot clearer now, is still probably a ‘work in progress’.
I would like to thank Martin Garner for his input on the ID of the above birds and Bob Bullock for his excellent series of photographs.
* Nils Van Duivendijk Advanced Bird ID Guide – The Western Palearctic