Female Common Crossbill
Common Crossbills are usually fairly recognizable in the hand being much smaller bodied than Parrots and a bit smaller than Scottish. The key measrement however is bill depth. This one was quite big with a minimum bill depth of 10.6 mm but still well within range for Common Crossbill.
Common Crossbills can sometimes be difficult to age precisely due to an extensive breeding season that can last most of a calendar year (depending on cone/seed availability) and also due to the fact that some suspend their moult (if irrupting and/or breeding in their 2cy) whilst other juvenile 1/2 cy's can have extensive or erratic post juvenile moult - though the former will still often show 3 generations (or ages) of feather and can be aged quite easily. Another factor is that crossbill 'years' don't necessarily conform to our calender years eg. January to December, and as such the age codes ringers use may not conform like they do for other bird species. The dead crossbills that Dougie Preston sent me from the 2009 invasion had examples of arrested or suspended post juvenile moult - these will feature in a later post.
The female had adult type wing feathers that were quite worn, no old (or juvenile) greater coverts so was aged as a Euring Age 4 or a 4F. She had also replaced two primary coverts (indicated below, and much darker than the older feathers):
Moulting Primaries and Primary Coverts
One of the associated primaries P1, slightly darker than the older feathers, was nearly fully grown and the other was just coming in so this bird appeared to be active wing moult. Another useful ageing criteria was provided by Jenni and Winkler's fantastic Moult book which states that Common Crossbills never replace PC1 and PC2 (primary coverts 1 and 2) or even PC3 as part of their post juvenile moult so the age of a Euring 4 ( born before the present calendar year is safe). This female also had a brood patch score of 4 meaning she must have bred some time over the Summer and would explain the commencement of wing moult with the first two inner primaries.
The male's bill depth was 0.1mm bigger - positive assortative mating ? ! This bird was also in wing moult with two new primaries and associated primary coverts. Quite a big billed curvirostra, though overall bill is 'rectangular', culmen is not steeply downcurved and the tip of the lower mandible is at a shallow upward incline:
Male Common Crossbill
Notice how the bill appears big relative to the size of the head (which was small) ? After releasing the two birds one after the other, and recording their flight calls (both Common type Fc's) I noticed a colour ringed crossbill feeding in a Scots Pine tree right by the ringing station. I got the scope out and it took some time to get the positions of all 3 colours and the metal but I eventually confirmed it as Parrot Crossbill that was ringed 15/10/06 at the same site. This bird was on its own and I took the opportunity to collect and measure some of the cones it was foraging on. The first one it dropped was this cone:
Parrot Foraged Cone
The above cone resembles the shredded appearance of a Scots Pine cone that has been worked by a Scottish or Common Crossbill , yet the bird that foraged it was defintely a Parrot on bill depth and also flight call. So what gives ? Well, clearly like all things crossbill there is no definitive, and inconsitency and variation prevails ! I guess it depends on the structure of the individual cone as the bird then dropped a cone that was more typical of a Parrot foraged closed Scots Pine Cone:
Typical Split Parrot Foraged Cone
I have observed Parrot Crossbills only partly prising open certain cones many times so my conclusion is that crossbills cannot be identified with 100% certainty by examining the condition of the cones that they depredate, though for most birds this may be a safe assumption and is generally useful. Parrot Crossbills 'split' the cones by making an entrance to the scale with the culmen tip then 'working' that hole, by inserting the mandible then manipulating it through up to 90 degrees which then splits the scales - which is why Parrots are 'bull-necked' and have extremely well developed neck and cheek muscles. They will also sometimes literally 'peel' the scales. It is a bit of a 'domino effect' - once they have got one scale open the others are much easier. They will work a cone like this one for anything up to one and half two minutes at this time of year.
Hopefully when the wet and windy weather finally subsides I can get out and catch some more ! A more thorough review of Parrot feeding will appear in the future as I have scads of pictures of birds feeding, including some colour ringed Parrots feeding on Larch ( which apparently they don't feed on according to some authors !).
© Lindsay Cargill 2010