Tuesday, 9 January 2018

Meisen Wood - Gold Reflections

Where are they?
Are they coming?
Why are there so few of them?
There numbers are so low!
What’s happened to them?

These thoughts, pertaining to Goldcrests, pre-occupied much of our ringing last autumn.  The Goldcrest is the species we have most ringed in Meisen Wood with an annual average of 556 birds.  This average would have been substantially higher if in 2015, and 2016, we had not committed the ringer’s heinous crime of running out of rings.

In partial mitigation, we would plead that on realising that this was going to happen we telephoned the Heligoland Offices in Wilhelmshaven and placed an emergency order for additional rings.  And the lady at Heligoland HQ, who is responsible for ring distribution, efficiently dispatched them immediately for overnight delivery; as evidenced by the packages’ postmarks.  Lamentably Deutsch Post were thoroughly inefficient taking nine, and ten days, respectively to deliver the rings; by which time the Goldcrest passage had finished.

Thus in anticipation of the 2017 Goldcrest migration I ordered an additional 1200 Vrings (the equivalent to the BTO’s AA rings).  Part of the anticipatory excitement was the thought of collecting more measurements to statistically strengthen the data set for our own studies on this beautiful, diminutive bird.  Additionally, we had conducted a good deal of preliminary work to gather data to test a couple of hypotheses we have about Goldcrest passage along the Teutonburg Hills.

Oh my; the best laid plans!

Our final figure was a 114 Goldcrests caught and processed; this is an 80% decrease in numbers.  This disturbing numerical decline is compounded by an examination of the age profile of the captured birds.  In previous years the adult to juvenile ratios were in the order of 1:5; last autumn the ratio was 1.5:1.  For such a short lived bird this, if repeated across its European range, will mean recruitment into next years breeding population will be severely compromised and so inhibit population recovery.

So yes, this data maybe depressing but it is invaluable! The data indicates that a population event has occurred; this is part of the fundamental value of standardised and consistent bird ringing.

A fascinating aside is that an interesting migration pattern between UK Goldcrests and German Goldcrests exists.  Many UK Goldcrests in the autumn migrate to the near continent: Belgium, France, Germany and the Netherlands.  Simultaneously, going in the opposite direction, are Goldcrests from the near continent’s breeding population.  Then in spring these movements are reversed; an intriguing cross-over.

The Goldcrests passing through Germany naturally include some of the German breeding population but many more originate from Fenno – Scandinavia; for instance we have controlled Goldcrests from the Russian Baltic enclave around Kaliningrad (a straight line distance of 925km).  Baltic State bird observatories ringed low numbers of Goldcrests last autumn but, unfortunately, year on year comparative figures are currently unavailable. Though Falsterbo, the Swedish Bird Observatory, in its preliminary analysis of its autumn 2017 ringing results report a 58% decline in Goldcrest numbers.

So back to the original questions and thoughts – why such a decline?  Several possibilities exist.  Firstly the Goldcrests could have had a disastrous breeding season in Fenno-Scandinavia.  Such poor productivity could be a consequence of poor weather; inadequate food availability; or high predation rates.  Secondly Goldcrests are typically double brooded (an amazing energetic cost for such a tiny bird), perhaps local conditions, this year, limited brood size and breeding attempts. Thirdly, did poor weather effect their migration?  Alternatively, does the Goldcrest population periodically fluctuate, like that of many small mammals, with regular peaks and troughs?  And this was a trough year.  An intriguing part of biological studies, to which ringing contributes, is that answers are rarely the result of a single factor. 

With over a thousand V rings we are definitely prepared to monitor next autumn’s Goldcrest passage. Thus we are looking forward to ringing more Goldcrests, and increasing our data set to gain further insights into Goldcrest migration through this area of Germany

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