Sewage plant birding.

11th February 2014
Brecon sewage treatment plant, as with other similar plants around the country, continue to provide a very good environment for birds, especially in winter. Insects are attracted to the small life forms around the decomposing organic matter, these insects in turn provide food for the birds. In addition, when organic matter like sewage decomposes the resultant exothermic reaction gives off heat which turns the settlement clarifiers at the plant into large radiators providing a warm place for birds to roost in severe weather.
A local birder has made this area his regular 'Patch' and it has turned up some very nice birds over the last few years. The first, a few years ago was a Citrine Wagtail, again feeding near the sewage outfall from the plant which discharges into the river Usk some 100yards away. The same food chain event occuring, the discharge water is warmer than the ambient temperature of the river and this warmth attracted insects and this in turn attracted this rare Wagtail.
Also there is a regular roost of Pied Wagtails at this plant, sometimes up to three figures can be found there.
In addition there is a hedgerow adjacent to the plant and on closer examination a huge amount of flies and insects can be found therein. This winter this hedgerow has provided food for a large number of birds. Yesterday I saw up to fifteen Wrens, ten Goldcrests and at least twenty Long Tailed Tits. However, that was not the reason for my visit, it was because the aforementioned birder has now found what is thought to be a Siberian Chiffchaff. These birds are hard to accurately identify and their call is often needed in addition to the sighting.
There is at least one other wintering Chiffchaff in this hedgerow and a good comparison can be made, always a bonus. On arrival I could immediately see the tonal differences between the birds, the presumed Siberian bird being much lighter and buffier brown, making the now obvious Collybita Chiffchaff, (the nominate race), look almost olive green by comparison.
To summarise;
Collybita is the species of Chiffchaff we see every spring in the UK, also there are some other newly declared species which are in Spain and the Canary islands. In addition there is the Tristis race and the Abietinus race of Collybita. Abietinus is from Scandinavia and Tristis is from Siberia. This bird looks a strong candidate for a Tristis, it has all the classic characteristics as follows;
· Absence of olive in the crown and mantle.
· Absence of yellow away from the underwing.
· Presence of a grey-brown or pale brown hue in the upperparts.
· Presence of warm buff in the supercilium and ear-coverts.
· Presence of buff at the breast-sides/flanks.
· Very black-looking bill and legs.

In addition a bird has been calling like a Tristis in the hedgerow area, a monosyllabic high pitched 'TEE'.
This would complete the ID of the bird in question.
Below is a comparison of Collybita ( Nominate) and the presumed Tristis, I welcome any comments or corrections to the ID.
Also please see UK Birds, Siberian Chiffchaff.

Presumed 'Tristis' notice the ID criteria mentioned above.

Nominate Collybita ( UK Spring/Summer ) Much more olive green etc etc.