Tuesday, 29 June 2010

14th June 2010, Catch Me If You Can, Deeside

Today started off too windy to catch crossbills with mist nets, so it was frustrating to watch the little beggars come in and quench their thirst at drinking pools:

However, later in the afternoon the wind dropped and I managed to get the net up for an hour and caught an adult (4F) female Parrot Crossbill:

And she was in wing moult, in the process of replacing her first two primaries P1 and P2 ( and associated primary coverts already renewed):

This bird's bill structure is typical of so many on Birdguides and in publications that are labelled as "Scottish Crossbill". However, the bill depth on this specimen was 12.7mm which would make it more likey a Parrot Crossbill - it is the 'minimum' bill depth that is measured as this apparently provides the most consistent measurement within and between ringers. Some might argue it is a 'big' Scottish Crossbill. However, it also gave Fc2 on it's release, identified first aurally, then confirmed by sonogram analysis. So, what do we give prevalence to, the biometrics or the call structure ? Some ringers are happy to classify on biomterics alone and for many crossbills this is possibly safe, but there will always be those iffy ones in the overlap zone.

There has been some recent chitter chatter in the crossbill world that the calls are 'cultural' possibly implying that they are not a valid taxonomic tool or diagnostic criteria. Also, calls can be learned or change. So what ? For me, as a musician and a humanties graduate, this cultural aspect, if it exists, is all the more important and may provide some clues and insight within the overlap zone where biomteric analysis alone may struggle.

In the end 'local knowledge' formed my diagnosis that this was indeed what is currently classified as a Parrot Crossbill. At this site birds that have bill depths in the region of 11.5 plus or minus a bit give what I would classify as Scottish Crossbill calls. Birds in the region of 12.5 plus or minus some give Parrot Crossbill Calls. Thus there is a positive correlation of biometrics to call structure......for most birds ! There has been the odd 11.9 or 12.1 bill depth specimen that gives Parrot Calls where you might expect it to give Scottish Calls. This, I feel, reflects the limitation of minimum bill depth as a completely reliable measurement on its own. For example, it does not take into account the shape or structure of the bill, often a very large gonys bill mass which is so important for the Pine species in opening closed cones ( and why Parrots often have a pronounced gonys). Effectively, a crossbill can have quite a narrow minimum bill depth but still have a massive gonyal 'bulge', so this measurement may be biased, though as stated, for most it is accurate. Bill width is also important structurally, but a difficult measurement to make consistent between workers. Parrot Crossbills also have thick, chunky necks, the 'bull necked' appearance often referred to in field guide books and these neck and cheek muscles contribute to their feeding preferences.

Parrot Crossbills power their way into a closed Scots Pine cone, 'smash and grab' if you will - they use the downcurved culmen to cut into it, large bill width and depth ( the gonys increasing this) to lever 'open' and make the cone split (they do not "slice" it per se, though sometimes "peel" the scales), and the powerful neck a cheek/ jaw muscles to literally brutalize the cone, manupulating the cone the whole time with the powerful feet ( not legs as was suggested to me). A female Parrot Crossbill feeding on Scots Pine, October 2006 upper Deeside:

Notice that the tail is used for balance during the feeding process, often firmly angled or wedged on to the feeding perch. I have some more detailed stuff I am preparing as a short article on Parrot Crossbill feeding, how they work the cones etc, maybe on here or the new site.

1 comment:

Loxia Fan said...

I should possibly add a caveat to the statement that crossbill can change their calls. This is not my experience, though I have instances of call sychronization within a pair. Some other workers may experienced "call changing" and I am merely relaying this.

I do have a recording of released Common Crossbill that gave all 3 variations of a Fc1, though this would not affect its classification as a Fc1 (but does possibly conflict with EcA and B). In this case I can't prove the bird changed its call as I don't know what the original call was !