We didn’t just go to the New Forest on Sunday to look at crossbills, fantastic and charismatic as they are. The main reason for the trip was the visit to Blackwater Arboretum: a locally renowned roosting site for…. Hawfinch Coccothraustes coccothraustes. The site is so renowned that, at this time of year, a large number of people turn up in the later afternoon to watch the birds come in to roost. A wooden carving that appears to depict a hawfinch (confusingly, the accompanying plaque tells you that the carving is named ‘Fire crest’ [sic]) can be found at the site, as you see from the adjacent image.
The Hawfinch is a remarkable and unmistakeable finch. Robust, stocky and short-tailed, it sports bizarre curled tips to its inner primaries and a characteristically massive bill. This is used to crack open the kernels of plums, cherries, maples and other pits and stones and the bird is able to literally crack these rock-hard objects in half. Reaching 18 cm in total length and weighing c. 54 g, it is one of the largest finches of the European field guide region. Some books say that it is the region’s largest finch, but it is matched or exceeded by the White-winged grosbeak Mycerobas carnipes (known from Iran eastwards to China), Caucasian great rosefinch Carpodacus rubicilla (the most westerly subspecies of which inhabits the central and eastern parts of the Caucasus), and Pine grosbeak Pinicola enucleator (the nominate subspecies of which occurs in Scandinavia). All of these are pretty obscure and exotic finches, or they are if you live in Britain at least.
We saw our first hawfinch some time around 15:30, and I’m pleased to say that the very first individual that turned up was first spotted by a member of our natural history group. It sat right at the top of a bare conifer for some time over ten minutes, in an unusual display of lethargy uncharacteristic for a small passerine. Within an hour between five and ten birds had appeared and, after most people had left, those of us remaining were treated to a view of 16 birds in the same tree. Like most small birds, they don’t of course sit out in the open at night, but deliberately tuck themselves in among dense foliage. Several conifers in the arboretum seem to be favoured for this purpose.
The Hawfinch is unique (or is it? Read on): it’s usually imagined to be closely related to grosbeaks but is the only member of its genus, Coccothraustes. Actually, this wasn’t the case for much of the twentieth century, as until fairly recently various Asian and North American grosbeaks now included in the genera Mycerobas and Hesperiphona were regarded as additional species of the hawfinch genus. There are also several hawfinch subspecies – most of which differ from the nominate Eurasian form in being paler, darker, or slightly smaller – and it’s always possible that (as is the case with so many ‘subspecies’ these days) at least some of these will be regarded by some workers as worthy of specific status. I’d speculate that this is most likely for C. c. buvryi of northern Africa, a hawfinch notable in having a more gracile bill than the other forms. There are also a few fossil hawfinch species, with C. balcanicus and C. simeonovi both named from the late Pliocene of Bulgaria (Boev 1998). So the hawfinch may not be so unique after all. I’ve actually spent some considerable time trying to find a tetrapod that is truly unique: without any close living or extinct relatives. So far no luck. Let me know if you think of one [adjacent hawfinch image from here].
As with the crossbills I discussed in the previous article, hawfinches are part of the finch clade Carduelinae, the group that also includes grosbeaks, bullfinches, rosefinches, goldfinches, redpolls and siskins. Phylogenetic studies support the view that the hawfinch is ‘unique': Arnaiz-Villena et al. (2001) found it to represent an isolated lineage most closely related to bullfinches (Pyrrhula) and grosbeaks (Mycerobus). If this is correct then hawfinches are relatively basal within Carduelinae and have a ghost lineage going back to the late Miocene, but there are few phylogenetic studies of carduelines and further study is needed before we can be really confident about this.
Given its massive bill and (assumed) impressive bite, there has been quite some discussion in the literature as to what these birds are capable of in biomechanical terms. The hawfinch skull is massive, deep-jawed, equipped with a particularly long and well-reinforced symphysis in the mandible, and with hypertrophied musculature. Both Sims (1955) and Bock (1966) discussed hawfinch bite strength, the former in a study of hawfinch cranial morphology and the latter in a paper on bill form and function. Van der Meij (2004) tabulated the amount of jaw musculature a hawfinch has (an incredible 1454 mg: most other carduelines have between 120 and 600 mg), and while she recorded bite forces for carduelines and other finches she didn’t manage to get this data for hawfinches.
Cracking open a cherry stone apparently requires a force of 27-57 kg: Witmer & Rose (1991) stated that this was about equivalent to 470-700 N while van der Meij & Bout (2004) discussed the force produced by the hawfinch bill as being 310 N. Both Sims and Bock go into a bit of detail as to what this actually means for how the finch’s bill is loaded during biting, and where the compression and so on occurs. I’m rubbish on biomechanics so won’t attempt to discuss it, but I’m sure the data is very impressive if you can understand it properly [adjacent hawfinch pic from here].
Anyway, I’d never seen a single hawfinch before, so to be suddenly saturated with a mass sighting of 16 birds at once, at quite close range, was a great experience. They looked fantastic. They weren’t the only finches there: we also watched siskins and bramblings too, and there were also goldcrests and firecrests in the area. Later that day Phil Budd and I went off to look for a Black-throated diver presently haunting the New Forest, but I’d better stop there otherwise I’ll end up writing yet more about birds.
So in a deliberate effort to avoid birds, I’ll be getting back soon to vampire bats. Will (aged 5) encouraged me to do some internet research on Godzilla recently, so you can look forward to the soon-to-appear ‘The science of Godzilla’ post (really). There is also some stuff close to completion on Piltdown, at last.
Refs – –
Arnaiz-Villena A, GuillÃ©n J, Ruiz-del-Valle V, Lowy E, Zamora J, Varela P, Stefani D, & Allende LM (2001). Phylogeography of crossbills, bullfinches, grosbeaks, and rosefinches. Cellular and molecular life sciences : CMLS, 58 (8), 1159-66 PMID: 11529508
Bock, W. J. 1966. An approach to the functional analysis of bill shape. The Auk 83, 10-51.
Boev, Z. 1998. Late Pliocene hawfinches (Coccothraustes Brisson, 1760) (Aves: Fringillidae) from Bulgaria. Historia naturalis bulgarica 9, 87-99.
Sims, R. W. 1955. The morphology of the head of the Hawfinch (Coccothraustes coccothraustes). Bulletin of the British Museum (Natural History) 2, 369-393.
van der Meij, M. A. 2004. A tough nut to crack: adaptations to seed cracking in finches. Unpublished thesis, Leiden University.
– . & Bout, R. G. 2004. Scaling of jaw muscle size and maximal bite force in finches. The Journal of Experimental Biology 207, 2745-2753.
Witmer, L. M. & Rose, K. D. 1991. Biomechanics of the jaw apparatus of the gigantic Eocene bird Diatryma: implications for diet and mode of life. Paleobiology 17, 95-120.