Thursday, 9 July 2015

Return of the Whiff-whaff

For the third year in a row there is a Whiff-whaff - a bird singing a mixture of Willow Warbler (WILWA) and Chiffchaff (CHIFF) song - on St Edmund’s Fen at Wicken.  This bird seems to have gone unnoticed by the bold Fen ringers so we have no record of how it started out.  I first heard on 23-6-15, quite late in the season, but it was still singing heartily, switching readily between Willow Warbler and Chiffchaff, but usually giving at least a hint of the other in each song.

The first time I heard one at Wicken was when I was conducting a survey on St Edmund’s Fen back in 2013.  I marked it down as a Chiffchaff but kept a mental tag on it so as not to double count.  Suddenly it’s Chiffchaff-ing gave way to the liquid cascade of a Willow Warbler.  I followed that bird through the summer and gradually it introduced more Willow Warbler song, but still keeping plenty of Chiffchaff in its repertoire.  I captured a Willow Warbler in its territory and fitted it with a metal ring coloured red with a permanent marker pen.  After some hours mapping out its singing circuit I managed to see the bird and its red ring while it delivered a mixed song.  By an even greater stroke of luck I also stumbled upon its nest, which Carl Barrymore confirmed was of a Willow Warbler’s construction: this bird was a Willow Warbler and furthermore had been able to attract a mate despite its odd song.  Later that year it broadened its repertoire still further with tit-like phrases unlike either Willow Warbler or Chiffchaff.  Sadly, the nest was later predated.
The first Wicken Whiff-whaff
I picked up the second bird in May 2014.  Initially it sang plenty of standard Willow Warbler plus either chiff-chaff followed by a Willow Warbler’s cascade or a typical Willow Warbler finishing with a couple of chiff-chaff notes.  But again the song tended to become more elaborate starting with a crescendo-ing wee-wee-wee and ending with some chiff-chaffs.  I also recorded some repeated see-see-see notes (followed by chiff-chaffing) that were so far from a Phylloscopus that I can’t be sure it was coming from the same bird at all.  This bird did NOT wear a ring.

This year’s bird was, by comparison, quite conventional, as you can hear from the attached recordings.  In the first the bird is singing quite Willow Warbler-like with Chiffchaff inserts from the top of a birch tree; in the second, recorded a couple of minutes later, after the bird had dived into a willow thicket (bees and flies and leave rustle giving a lot of hiss) it proceeded to sing like a Chiffchaff, often after a very brief aborted snatch of WILWA song.  I can’t be certain whether this bird had a ring, but probably not.
There have been plenty of others: I recorded another in Witcham last year and almost everyone I speak to seems to have heard one.  Ben Green has even reported on from Ely this year.  It seems that this “mixed singing” has been noted for over 100 years.  British Birds had an article about it in 1940, there are notes on the phenomenon in BWP (vol 6, p657) and Gwentbirding blog wrote about it at length in April 2011.  It also appears that it is much more common and widespread than is generally realised. Those that have been identified with certainty have almost always turned out to be Willow Warblers.  At this point, when it is clear that this in not something exotic, too many birders lose interest.  But quirky phenomena such as this are of more than passing interest: anomalies can be useful in testing hypotheses.

So what is going on here?   Some common themes that emerge from the records:
(i) the Chiffchaff-like elements of their songs are usually delivered more rapidly and at a slightly higher pitch than a Chiffchaff;
(ii) the bird often starts with a predominantly Chiffchaff-like song but elaborate with much more Willow Warbler as the season progresses.  But is this progression an artifact of the fact that the mixed singing is what draws attention to the bird in the first place?  The Willow Warbler elements of its song may become more apparent later as the bird is followed.
It is unlikely that the Wicken birds were the same individual but perhaps they were related, raising the intriguing possibility that this type of singing is genetically inherited.  (Alternatively, sons may have learnt their aberrant song from their father.)  They may be imitating Chiffchaffs, though it is hard to explain why they all do it at too rapid a tempo and too high a pitch. All Willow Warblers do deliberately vary their song, one individual hardly repeating the same version twice.  (No two Chiffchaffs sing exactly the same song either.) It does appear that these “mixed singers” also vary their song more than most.  Whether that is because they are dunces who are not sure what they should sound like or superstars with a particularly varied repertoire is hard to tell.
     Contending theories include the following:
  1. It’s neither a Chiffchaff nor a Willow Warbler
a)    Phylloscopus trochilus x collybita hybrid.
b)   Vagrant species.
[Hybrids and vagrants (P. tristis and P. ibericus) certainly occur but do not usually sound like Whiff-whaffs.]
  1. Song mimicry, mixing or switching
a)    It’s learned the wrong song.
[If imprinted on the wrong species why do birds become worse at reproducing their model song as time passes?]
b)   It incorporates song elements from another species in order to augment its own song.
[If the purpose is ornamentation why do these birds, at least initially, often deliver such a simple chiff-chaffing song?]
c)    It copies song of another species in response to competition from that species.
[If singing is about real estate rather than sex, why doesn’t the female sing too? And why does the song become more WILWA-like as the season progresses, when mating is over and inter-specific competition relatively more important.]
  1. No mimicry involved
a)    Unusual variant of normal song.
[While typical WILWA song is surprisingly varied, and often begins with a number of repeated syllables, rarely do they alternate two notes.]
b)   Primitive song produced by immature bird, perhaps one that has never heard a male Willow Warbler sing.
[Perhaps WILWA and CHIFF start with a similar primitive innate song that normally the WILWA elaborates in a process that fails or is retarded in Whiff-whaffs.]

 We need more observations to provide further clues.  I would be very interested to know if anyone else hears one, or has heard one recently.