24 September 2015

Exceptional Siskin Surge

The Siskin movement this autumn has been exceptional, for both the early start and vast numbers involved, as indicated by BirdTrack. With the peak movement and arrival of continental birds still to come, numbers in the UK are expected to increase considerably.

BirdTrack Siskin reporting rate

Jeff Kew from Thetford, Norfolk writes:

This has been an exceptional September for Siskins in the UK, with large numbers being reported across southern Britain, due to a major irruption from Northern Britain.

Last autumn Siskins were fairly scare in Southern England with a few being reported on feeders in February and March, the usual peak time to see and ring this species. Reports from Scotland indicate a bumper cone crop last summer and consequently most Siskins stayed at home for the winter, this Autumn with dwindling food supplies the picture is very different.

In the Norfolk / Suffolk Brecks area we have not seen anything like the numbers coming to feeding stations in September since 2005.

In our Thetford garden we have already ringed 611 Siskins this month (in just 4 ringing sessions), with an amazing 368 caught on the 18 September.

One of the hundreds of Siskins recorded taken by Jeff Kew

We have had birds ranging from exceptionally light (10.4g) to exceptionally heavy (17.0g) - which presumably indicates a mix of recently arrived birds and birds with serious intent to travel much further south. The majority are juveniles, which appear to have done less post juvenile moult than our local breeders.

Post juvenile moult strategies are interesting with the amount of post juvenile moult being variable and probably linked to both fledging date and food supply. We believe unmoulted juveniles (3JJ) can reach us from Scotland.

Thinking back to what we saw in 2005 we had high numbers through September, followed by two recoveries in Spain in Navarra and on Mallorca - which could give an indication of where some birds of this years birds may end up this winter.

Greg Conway from Thetford Forest Ringing Group writes:

The extent of post juvenile moult is highly variable, with some typically retaining one, or more, old greater coverts along with juvenile tertials and tail feathers, whereas others replace all coverts, tertials and tail (see below).  However, there are also many in between that replace different numbers of tertials and tail, often symmetrically but not always centrally!

Juvenile Siskin with replaced greater coverts, tail and tertials - Greg Conway

Juvenile Siskin with replaced greater coverts and two inner tertials - Greg Conway
Juvenile Siskin that has replaced outer pair and 2 central pairs of tail feathers contrasting with juvenile feathers (pointed and browner) - Greg Conway

To better understand the mix of different migration and post juvenile moult strategies occurring this autumn, and add to long-term data collected from Thetford Forest, ​any ringers catching Siskins are encouraged to record the following biometrics (in addition to age/sex, wing & weight):

All birds if possible!
1) Number of unmoulted greater coverts (including those with zero)
2) Fat score (0-8 scale)​

And if time allows!
3) Tertial moult (record in IPMR moult card)
4) Tail moult (record in IPMR moult card)
5) Pectoral muscle score (0-3 scale)

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