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
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.
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