Monday, December 08, 2008
THE beak of a bird is a wonderful thing. I was thinking this the other day when I was doing some waterside bird-watching.
I watched, in succession, a kingfisher, a heron and a cormorant.
These three birds look very different and are not even remotely related to each other. But they all have similar beaks: a serious looking dagger-shaped object on the front of their faces. The reason, of course, is that they all catch fish and, by a process biologists call convergent evolution, they all ended up with the best design of beak for the job.
Ospreys and fish eagles also catch fish and though I didn’t see any of them the other day, I have watched them at it in other countries in the past. But they catch fish with their feet so their beaks have remained the normal shape for a hawk or an eagle.
And though kingfishers, herons and cormorants all catch fish in their beaks they go about it in very different ways.
I’ve often watched herons doing it. They never land in the water. They wade in very quietly from the land, sometimes travelling some distance, very slowly, until they reach a spot they fancy. I was on the Aran Islands recently and I watched one wading out into the Atlantic.
Then they wait, stock still, their long necks like coiled springs, until something swims within range. I have read some accounts that say they stab their prey but I’ve never seen this and I rather doubt if it ever happens. Instead they open their beaks at the last moment and grab it across the back (a bird can only open the lower mandible of its beak, the upper is fixed). I recently examined a trout that had escaped from a heron. It had a neat scar on each flank below the dorsal fin.
They then turn the fish and swallow it head first (I think they use their tongues for this). The fish is usually still alive. They do eat other things. I once watched one catching frogs in a raised bog, miles from any open water. They will also eat young birds, which may be why they are sometimes mobbed by small birds in the same way that birds of prey are.
Kingfishers dive on their prey from a perch above the water, usually an overhanging branch, though I once saw one using the parapet of a bridge on the Royal Canal. They can penetrate up to a metre below the surface. Our species is not a very big bird and only takes fish about the size of your little finger. They thrive in places where there are shoals of minnows and also take sticklebacks and the fry of larger species. They also eat quite a lot of underwater invertebrates, such as diving beetles and freshwater shrimps.
Herons can take fish up to about 30cm, longer if it’s an eel. But they have huge difficulty turning an eel to swallow it.
Cormorants catch fish by out-swimming them underwater. This isn’t easy so they have a number of specialised adaptations to help them. One is that they can expel all the air from their feathers to make them less buoyant so they can go faster. But this has the disadvantage that they’re not waterproof. This is why when a cormorant surfaces it swims very low in the water compared to a duck, for example, and why they have to hang their wings out to dry after a fishing trip.
The three species do different things but all with the best beak for holding live fish, a task so difficult it must be like eating jelly with chopsticks.
* dick.warner@examiner.ie
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