Saturday, 24 July 2010

Catching Pine Crossbills, Upper Deeside, 21st June 2010

Crossbills are notoriously difficult to catch in order to ring as you cannot bait sites like you can for other finches - they are specialised to feed primarily on various pine cones and there are usually millions of them about so no joy there ! Usually crossbills are caught when they come in or out of drinking pools. I believe Dutch ringers use 'artificially' created drinking sites and also use caged decoy birds to lure crossbills in which sounds very efficient ( and interesting). Here in the Scotland we only tend to use sites that the crossbills themselves visit naturally on their own accord and we don't use decoys ( though an endorsement to allow this can be applied for). Many man hours can be spent finding and reconnoitring these sites which is why I, and others, are quite guarded about where we are operating and catching birds. Last year one of my pools was compromised twice by photographers (the worst kind of disturbance) so don't take offence if I don't disclose sites, it is nothing personal !

Crossbill's drink obviously to sustain themselves, however, there is also a more 'cultural' purpose to the process. The birds will often sit in a perching tree near to or directly above the pool and the dominant male will sit right on the crown of the tree, sometimes singing or preening. There is a 'pecking' order to how the bird are organised within the tree, with more dominant birds asserting their authority, and often these birds will drink first. One often finds that these birds may actually drink twice - I have observed this several times thanks to colour ringing individuals. However, sometimes extra keen juveniles bail in first and these are often caught more easily. I refer to such pools as 'cultural' pools that is they are long term historical drinking sites where social behaviour and interaction can be performed. This is opposed to sites where they drink 'ad hoc' or opportunistically eg. near to where they are feeding and are thirsty at a particular moment, a puddle below a tree for example. All of this makes them exceedingly difficult to catch in any sort of numbers.

Monday 21st June had near perfect conditions: warm. overcast, and very importantly, little wind. Whilst setting up at dawn several birds came in to one of the pools I was hoping to catch. I backed off and let them down to drink - they could be trapped later when they came back ! I set up two nets and settled down very nearby so that the nets could be monitored constantly.Only an hour had passed and I caught some birds, a family of Parrot Crossbills and a Siskin (other birds drink at crossbill pools). The Siskin was processed first and released quickly.

The male Parrot Crossbill was an adult and was in post-breeding moult ( inc.wing moult):

Parrot Crossbill

Again, the bill structure to me is clearly Parrot, but for many birders this would be classified as Scottish. The minimum bill depth was in excess of 13mm so not Scottish !

The female hadn't started moulting her primaries but some tail feathers were being moulted. She also had a brood patch score (BP) of 4 (meanind she had finished brooding/breeding). The bill structure was Parrot:

Parrot Crossbill

The juvenile had a rather 'clean' look to its bill structure, almost Scottish in appearance but still too deep biometrically to be anything but Parrot Crossbill:

Parrot Crossbill juvenile

These birds were colour-ringed and processed quickly and released into the same tree whilst sound recordings were obtained of their flight calls. The colour ring sequence is unique to each bird and allows the birds to be 'recaptured' (by observation) without actually physically catching it again. Much useful information can be gathered from these observations of colour ringed birds and so far I have very important data on call association, feeding ecology, breeding behaviour as well as site fidelity and movements. The rings have no impact on the activities of the crossbill and some that were ringed in 2006 are still being observed at the same location.

I only had a chance to have a quick drink myself and another bird was caught. This bird, an adult male Scottish Crossbill, already had a metal ring on its right leg so was a retrap. Consulting my notebook it was ringed by myself and my trainer on 3rd September 2006 at the exact same pool:

Scottish Crossbill

This bird was also in post breeding wing moult with several primaries in the process of being replaced.  The minimum bill depth, believe it or not, is over a milimetre less than the Parrots that were caught, even though the shape is similar. This bird was processed and released very quickly, with the flight call recorded as he flew.

There was a lull of a few hours in the early afternoon. This so often happens, the birds just dossing about, preening, singing. Some interesting by-catch kept me on my toes with several Siskins and my first ever juvenile Stonechat. The latter was very interesting as there are usually several pairs with territories at this site but this year there have been none, so the bird I caught was most likey a roving juv from further up the Glen.

By late afternoon the action started again with two juvenile Parrot Crossbills caught in the other net I had set. These were probably caught as I have described above, that is piling in eagerly before the dominant adults ( who then sussed out the situation !). However, though they don't perhaps look as good, catching juveniles is actually better in many ways because we can age them exactly, that is as a first year or Euring 3J. We can also assume that these birds are on their natal site or certainly not far from it, so any movements can be valuable.

The first juvenile I processed had a stonking bill:

Parrot Crossbill Juvenile

From the profile you can really see that this bird, a typical Parrot Crossbill, is 'bill heavy'. Notice the very pronounced gonyeal bulge in the lower mandible.

The next bird, possibly a sibling, also had a pronounced Parrot type bill bill and was slightly smaller overall:

Parrot Crossbill Juvenile

The bird is being held in the 'reverse' ringers grip in order that the tip of the lower mandible can be photographed in the profile. Crossbills can be either left or right handed, or billed as it were, just like humans. I class the cross as being to the right or left based on the direction of the lower mandible tip, in this case to the left.

All in all a very sucessful day with a total of 5 Parrot Crossbills, one retrap Scottish Crossbill, 5 Siskins, 1 Chaffinch and a Stonechat caught naturally without bait. The retrap Scottish for me was the highlight in several ways. Hopefully the in hand bill profiles will be of help to birders in determining ( or not !) the Crossbill species and the text giving an insight in to the intricacy of crossbill study


Dougie Preston said...

Sounds like a good day to me mate!, I suppose you'll have a lot of people very jealous indeed. Though with all the field work involved I reckon it's probably well earned too...

Loxia Fan said...

It was a good day Dougie, 6 Pine Crossbills per 'man day' is good by anyone's standards.

It's not all 'glam' though - just back from a trapping session today, and nil return. 4.30am start,a long drive there and back, midges, ticks, sunburn ! Loadsa xbills around but just weren't drinking where I had my nets today. Did get some great recordings of in situ birds which I will post a few of later.