Jonathan's Mistletoe Diary

February 12, 2012

More blackcaps

Filed under: Uncategorized — Jonathan Briggs @ 6:58 pm

After yesterday’s female Blackcap-eating-mistletoe pictures I thought I’d try and get a male today.  We’ve got one that appears every now and then, but he frequents the waste mistletoe pile further away from the window, and is more difficult to photograph.

[Of course there may be several others nearby, of both sexes, but they only ever appear one at a time – rarely two, and then one chases the other away.]

But he did come closer a few times (possibly several, but I can’t spend all day crouching in the window!) and so here are a few pics, just to show what a handsome little brute he is.

And just look at the way he starts to squish those berries, separating the seed very quickly – much more manly (or is it just male impatience?) than yesterday’s female!  (though it takes a bit more than that to actually stop it all sticking together).

These overwintering Blackcaps are, almost certainly, part of the new migration of a particular genetic sub-race of Blackcaps from Germany. They breed there and, traditionally, migrate to Spain in the winter.   As do our own breeding Blackcaps.  But in the last few decades some of the German summering birds have changed their winter migration habits, and come to Britain instead.

Numbers were few at first, just a few dozen 30 years ago – but now they come over in their thousands.  Why?  No-one’s quite sure, though some experts suggest that the easy pickings from British bird tables are partly to blame.

Some people are beginning to suggest some of our own summer breeders are now not bothering to migrate, staying put to mingle with the German visitors each winter.  No evidence for that yet, but there was a brief discussion about it recently in the comments on one of Paul Evans’ contributions to the Guardian’s Country Diary.

There are several side-stories to this.  The most interesting is the evidence that these British-wintering birds, despite spending their summers in Germany mixed up with Spanish-wintering birds, are a distinct genetic group.  They mix with the Spanish-winterers all summer in Germany – but don’t breed with them. They keep themselves to themselves and breed within their wintering groups.  So much so that there are now some morphological differences between them – evolution in action!

Another side issue relates to mistletoe.  Blackcaps are very efficient vectors (spreaders) of mistletoe, taking one berry at a time and wiping each seed off onto a tree branch.  But in Britain they are relatively new winter visitors – we’ve not experienced this level of efficiency for mistletoe spreading before.

Other British wintering birds that tolerate the white sticky berries of mistletoe (most birds don’t like them) are much less efficient.  Mistle Thrushes, for example, swallow the whole berry, seed and all, excreting the seeds en masse, with most failing to touch a branch.

So, what are the implications of a rapidly increasing winter population of ultra-efficient  (German) mistletoe vectors?    No-one knows – but there is coincidental evidence of mistletoe increasing*, in the same time period the Blackcaps have increased.

.

Intriguing stuff, with no definitive answers yet.  In the meantime, as promised yesterday here’s a (very brief) video of that female Blackcap taking a mistletoe berry:

(*If you’ve got thoughts or observations on UK mistletoe populations, Blackcaps or not, have a look at the Mistletoe League Project, collecting info on mistletoe management and abundance)

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