By Dominic Couzens
You can imagine the scene. A Hawfinch perches upright in a tall forest tree, looking out over the canopy, well out of sight of any human being. As with every bird, everywhere, it is hungry, and here among the twigs and branches of its favourite woodland giant, it knows there is plenty of food.
After a brief glance down, it manoeuvres its massive bill, opens the mandibles and plucks what it needs – a juicy caterpillar. Hang on, is that a mistake? A Hawfinch is eating a caterpillar? Is this the same Hawfinch that is famous for little more than having an outsize bill, capable of exerting enough force to crack a cherry kernel, or even an olive stone? Yes, this is the same species, alright. But this is the surprising Hawfinch, the real bird, which has a quiet lifestyle that very few people know about.
In the summer, oddly, it almost entirely forsakes its winter diet of nuts and fruits and feeds instead on soft-bodied invertebrates. The famous bill, its prized asset, is rendered less useful than usual and might even be a hindrance at such a time. But in the summer forest canopy, insects roll off the trees and times are easy, so the specialist becomes a temporary member of the ranks of ordinary insectivores, competing with Blue Tits and flycatchers.
Young Hawfinches, in their first few weeks of life, must wonder what their bill is actually for. Of course, the Hawfinch’s mandibles do come into their own in the depths
of autumn and winter, when seeds are everywhere.
With so many species competing for the nutrition they bring, it pays to have an dvantage, and the Hawfinch bill is simply more powerful than that of any other finch, allowing it access to larger and harder seeds and stones. And while the sheer size of the bill is what every birdwatcher can see, easily, it is the internal structure that is the marvel.
Finches are defined by their bills, and the family as a whole is highly adapted for de-husking of seeds. Just inside the rim of the outer (cutting) edge of a finch’s upper mandible is a sharp bony ridge which juts out below the palate, but not as low as the outer edge – the arrangement is like the two ridges of a molar tooth.
Hawfinch: packed with muscle
Using its tongue, a finch can lodge a seed in the gap between the cutting edge and the inner ridge. Meanwhile, the cutting edge of the lower mandible articulates into this gap, meaning that the sharp point of the lower mandible pierces the husk of any seed that is lodged there. A quick roll of the seed with the tongue peels off the pierced husk.
Hawfinches have this arrangement, but in addition their bills have a pair of bony, serrated protuberances at the back of both upper and lower jaw, either side of the mid-line of the bill. Once clenched on a seed, all the strength of the muscles closing the bill is concentrated on these four knobs jutting on to the husk.
It is this concentration of force that causes the seed casing, however hard it is, to rupture. And what force it is. Scientists have measured the crushing force of a Hawfinch bill and found it to exceed 50kg, which is a weight equivalent to more than twice the baggage allowance an airline might allow you. Think of those heavy bags at the airport and you will appreciate the extraordinary might of a bird that itself weighs only 0.05kg.
No wonder this finch has such a large, bulging head, packed with all that muscle. It is a bird that could, if circumstances allowed, probably crush your finger. Perhaps surprisingly, the bill isn’t the only part of a Hawfinch that is physically unusual. If anything, the flight feathers are even odder, indeed unique.
Too much bird bling?
The four inner primary feathers are a completely different shape to the conventional outer primaries that make up the wingtip. They are shorter than they ‘should’ be, truncated in length and curved at the ends. The secondaries are also slightly curved and, when the bird is perched, the wing looks somewhat damaged. It is hard not to think that this bling must impair the birds’ flight. Hawfinches are top-heavy and awkward in flight as it is! But what is the function of these ornaments? It is widely assumed that they play a role in sexual selection, since the modifications are more strongly pronounced in males than in females and, more tellingly still, are most fully developed in older, presumably socially dominant males. They are shown off in the subtle displays of Hawfinches
in winter flocks, the scene for pairing up. Males ruffle their plumage and droop and spread the wings, and these wing manoeuvres are not seen in other members of the finch family.
Hawfinches also have an unusual ‘bill-touching’ display, in which the prospective pair lean over to touch the tip of each other’s bill, almost falling over while doing so. Another of the many curiosities surrounding this magnificent bird is the fact that it is so unobtrusively quiet. In the spring the woodlands resound with Chaffinch song, gardens fizzle with Greenfinch song and hedgerows chatter with the efforts of Linnets. But the woodland canopy remains quiet.
Hawfinches do have a song, but it is soft, halting and unmusical, barely worth the effort – and besides, it seems that its function in pairing up and holding a territory is minimal.
You hardly ever hear it. The call note, a loud, slightly spitting ‘pix’, is also easily missed, so much so that this bird, shy and quiet for most of the time, is famously difficult to find.
A peculiar quirk of Hawfinch breeding behaviour is that there are two quite different types of ‘housing’. Some pairs nest entirely alone, hundreds of metres away from anybody else, seemingly alone in their ‘country estates’.
Others, however, the more successful birds, nest together in what be described as desirable cul-de-sacs, with a reasonable gap between nests (a few tens of metres), but in definite clusters. The reason for this distinction in breeding density isn’t known. As mentioned at the start, young Hawfinches in the nest are fed mainly on soft-bodied invertebrates, together with some seed paste crushed in the parents’ bills. This is fed bill-to-bill by both sexes.
When the young leave the nest, they may stay with their parents for much longer than most young finches do, because the unusual bill takes time to develop. It is some months before the first-year birds can handle tough seeds – indeed, they might not be proficient until deep into the winter. Thus, if you see a flock of Hawfinches this winter, you might actually be seeing one or more family parties, because in addition to the young sticking around for a while, members of a pair frequently overwinter together.
In common with most finches, they are highly sociable, although they form only modestly sized flocks. Last autumn was a bumper year for Hawfinches in Britain, with much large numbers than usual flocking to our shores.
It is a great time of year to see this charismatic and colourful bird, a species which can be very surprising in many aspects of its lifestyle.
Scientific name: Coccothraustes coccothraustes
UK numbers: 500-1,000 breeding pairs, 10-15,000 wintering birds
Habitat: Deciduous and mixed woods
Diet: Buds, seeds and shoots