Brilliant Bird Beaks
Last year, I wrote an article about five birds with brilliant but bizarre beaks – from the wrybill, which has a bill that bends to the side, to the impressive shoebill, which has a massive, hook-tipped monster of a beak that is perfect for catching and holding onto large, slippery fish. But picking only five beaks to include in the list proved difficult, and there were several birds that I initially considered that were eventually cut from the final selection.
Here, finally, is the promised (albeit much-delayed) follow-up article, showcasing such avian oddities as the sword-billed hummingbird, the ʻakiapōlāʻau, and the extinct huia, all of which prove just how versatile and varied bird beaks can be.
1. Toco Toucan
Disproportionately large and usually very colourful, the spectacular beaks of the toucans are surely the most distinguishing feature of this bird family. One species, the toco toucan, has the biggest beak of any bird relative to body size, providing 30–50% of its overall body surface area. At first, this oversized, banana-shaped bill seems to be a hindrance to the toucan, especially in the air, for the bird flies with weak, unsteady wingbeats. Even at rest, when the toucan is perched on a branch, the bill looks awkward, making its owner look slightly unbalanced.
But despite its size and cumbersome appearance, the beak is not as heavy as you might think. It is largely hollow, supported internally by a framework of thin, crisscrossing struts, making the overall structure quite light, fragile, and easily broken. But what is the purpose of such a large, striking beak?
This is a mystery that has puzzled scientists for a long time. Charles Darwin hypothesised that it might have something to do with sexual selection, but the beaks of both sexes are big, bulky and brightly coloured, so this seems unlikely to be the case. Some people have theorised that the bill enables the bird to obtain hard-to-reach fruit and nuts that are on the ends of thin branches that wouldn’t support the toucan’s body. Others have suggested that it allows them to plunder nests and reach into holes to snatch eggs and small chicks. Perhaps the bright colours of the beak make toucans look more frightening, allowing them to intimidate other frugivorous species with which they compete for food.
But recent research has shed more light on the toucan’s beak, suggesting that it also acts as a radiator. When an infrared camera is used, you can see that the toucan’s bill glows brightly – it is very hot. This means that it probably acts as a heat-exchange system, rather like the ears of an elephant. The bill is packed with a network of blood vessels and is not insulated, which results in rapid heat loss. When this blood returns to the toucan’s body, the entire animal is cooled down. The bigger the beak, the better it is at this job, which explains its disproportionate size.
In fact, the toucan’s bill is so efficient at radiating heat that, for a brief period, it may release up to four times more warmth than the bird’s body actually produces, making it the largest known ‘thermal window’ in the natural world. At night, when the air can get quite cold, the toucan often tucks its bill under its wings – it’s now trying to stop excess heat from escaping.
The skimmer looks like a large black and white tern, except that the lower mandible of its beak is noticeably longer than the upper. To catch fish, it flies over a still body of water such as a river, lake or shallow lagoon, so low that its wingtips almost touch the surface with each beat. Then it drops that elongated lower mandible and trails it through the water, cutting a furrow across the surface. It has a blade-like tip, which allows it to slice through the water with minimal resistance. If the lower mandible comes into contact with a fish as the bird skims over the water, the upper part of the bill snaps shut in a reflex mechanism, catching whatever triggered it.
A skimmer may not be successful as it flies over its chosen body of water, but once it reaches the end it simply turns around and heads back over the course it has just covered. By now, fish may have come closer to the surface to investigate the ripples caused by the skimmer’s previous run, so this second traverse is often more fruitful than the first.
When a skimmer is newly hatched, both parts of its beak are of similar length. But the act of skimming induces a lot of wear and tear on the lower mandible – the tip may sometimes even break if it hits something hard such as a submerged rock. Therefore, although both mandibles of a skimmer’s beak grow continuously throughout its life, the lower one grows much faster, creating an imbalance in length.
3. Sword-billed Hummingbird
The passionflower Passiflora mixta from South America has such a long trumpet (or corolla tube) that you might think nothing could possibly collect nectar or pollen from it. But the sword-billed hummingbird can. It has a longer bill in relation to its body than any other species of bird. In fact, at 9–11 cm in length, this bill is even longer than the hummingbird’s body, making it look almost like a hypodermic needle with some feathers and wings attached to one end.
A bill of this length, however, presents a few problems. The bird cannot use it to groom itself, for example, so it must instead use its small feet to do so, preening with one while balanced precariously on the other. And since the bird might easily overbalance, it perches with its unwieldy bill held upwards at a steep angle to maintain stability.
The upside to this ludicrously long bill is that it can extract nectar from flowers with long corollas, such as Passiflora mixta, that other birds, even other hummingbirds, are unable to access. This way, the sword-billed hummingbird avoids competition and finds its own niche in a very crowded world.
4. Hawaiian Honeycreepers
The Hawaiian Islands in the Pacific Ocean are so remote that only a relatively small number of plants and animals have managed to settle there naturally. Few birds other than those that are adapted to spending months flying above the sea were able to colonise them. But those that did make it to this lush, fertile land found a paradise, with few competitors and plenty of ecological niches to fill. Most of the birds that now live in Hawaii are found nowhere else in the world.
Perhaps this archipelago’s most famous avian residents are its honeycreepers, of which there are many different species. All of these birds are sufficiently similar to make it reasonably certain that they are all descended from the same ancestral species, probably a small sparrow- or finch-like bird that got blown to these isolated islands a few million years ago. Finding its forests largely empty, this bird, over time, developed into several different species to occupy the vacant niches and exploit the many sources of available food. This is known as ‘adaptive radiation’. Today, the honeycreepers have changed so much that it is extremely difficult to determine exactly what that ancestral bird was.
Perhaps more so than any other group of bird, the Hawaiian honeycreepers show vividly just how dramatically beaks can be moulded by evolutionary pressure, and change to suit the diet of their owner. The palila, which lives largely on seeds and has a short, thick, powerful beak needed to crack and open them, is probably similar to the ancestral bird from which all the other types eventually radiated from and is quite finch-like in appearance. The ‘amakihi has a very similar body but a more slender bill suited for picking up small insects; it could be said to be the honeycreeper equivalent of a warbler. The ʻiʻiwi, a crimson-coloured bird, has a particularly long, curved bill that allows it to probe deep into trumpet-shaped flowers, which, along with its ability to hover, means it is comparable to a hummingbird.
Perhaps most remarkable of all, however, is the ʻakiapōlāʻau (pronounced ah-kee-ah-POH-LAH-OW). This bird has a dual-purpose beak. The lower mandible is small, straight and chisel-like to chip away bark to find insects, while the upper mandible is elongated into a curved probe with which to winkle them out. The ʻakiapōlāʻau is able to dextrously use the two halves of its beak for these different purposes. It’s about as close as a bill gets to a Swiss Army penknife.
The crow-sized huia was the largest and strangest member of a family of birds called New Zealand wattlebirds. It was a weak flier and prefered walking along branches through the canopy or on the forest floor. It also possessed the most pronounced sexual dimorphism in bill shape of any bird species in the world. The male had a fairly ordinary-looking, short, stout bill, which he used to chisel insects from wood, while the female had a long, decurved, curlew-like beak, used for probing in deep cracks and crevices. So different were these beaks that the two sexes were initially described as belonging to different species.
Sadly, not much is known about the biology or behaviour of the huia; it was little studied before it was driven to extinction. As such, we do not really know why the huia developed such sexually dimorphic bills. Perhaps the divergence arose because of a lack of competitors in the male and females’ respective foraging niches in the North Island forest ecosystems where it lived. It has been suggested that because the female was the main provider of food for its young, this sex evolved the longer bill to obtain the protein-rich invertebrate diet required for the chicks. Perhaps the extreme differences in bill shape and sizes meant that the two sexes exploited the habitat in different ways. Maybe females foraged in all-female groups and males in all-male groups. We simply don’t know.
The peculiarity of the huia astounded 19th-century naturalists and there was considerable demand for specimens. Even by this time, the huia was already in decline due to hunting by the Māori. They used the females’ bill in jewellery, while the feathers of the species were highly prized and worn as adornments and symbols of status. Predation by invasive mammals, such as rats, cats and mustelids, along with avian diseases introduced to New Zealand by non-native birds, also contributed to the huia’s decline.
Then things got even worse for the huia. In 1901, a Māori guide took a huia feather from her hair and placed it in the hatband of the visiting Duke of York (later King George V of the United Kingdom) as a token of respect. Many people in England and New Zealand wanted to emulate this royal fashion and wear huia feathers in their hats. This caused the value and popularity of such feathers to soar, hastening the bird’s demise.
The last official confirmed huia sighting was made on 28 December 1907, when three birds were spotted in the forests of the Tararua Ranges. Unconfirmed but convincing reports suggest that true extinction for this species came several years later, in the 1920s or beyond, but now there is no doubt that this amazing bird and its unique beak have gone from the world forever.