Ornitho.ch, 13 december
"Up to now no invasion of this subspecies had ever been mentioned in Switzerland.
The proper identification of this subspecies is still a difficult matter as the
head color of the local A. c. europaeus subspecies is subject to strong
variability. Therefore, only the birds that could be positively identified as
belonging to the A. c. caudatus subspecies may be announced under "A. c.
caudatus". For this reason a homologation form had been requested for every
observation of A. c. caudatus individuals. Nevertheless, due to the huge amount
of data collected this winter the Swiss Rarities Committee has decided to suspend
the homologation excepted for isolated individuals or for single white-headed
birds in a group of europaeus individuals. ...
In order to better document this invasion, it might be interesting to know
whether the mixing of the two subspecies increases with the time or not,
therefore a precise count of the individuals of both subspecies in mixed groups
as well as any sighting of local europaeus birds should be sent. Photos are of
course most welcome."
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http://www.ornitho.ch/index.php?m_id=21--
edit: De Zwitsters hanteren voor de gelegenheid dus een ander beoordelingskriterium dan Waarneming.nl. Alleen nog solitaire
witkoppen of een enkele witkop in een groep gewone.