When I was seven, which truth be told was 50 years ago, I won the annual photography competition of the 12th Cambridge cub scout brigade with a series of photographs, all black and white of course and taken with my little box Brownie camera. The only thing digital about cameras in those days was the fact that you used your finger to press the button to take the photograph! My winning series of photographs charted the story of two swans laying their eggs in a nest at the back of St John’s College in Cam-bridge. They showed the hatching of seven lovely cygnets and their growth into young independent swans.
So, when I joined the college as a student a little more than 10 years later, it was great to see the same swans still raising their young there. And now, half a century on they are still there – or presumably now their grand-cygnets if one can use that term. While a virus may cause havoc for us humans, some things in nature never change, as these swans show. Well, I say that, but a quick trawl through Google Scholar shows papers demonstrating an annual mortality rate of about 40 percent for young swans, so while all may look the same, clearly individuals come and go.
Last week I was almost dive-bombed by three over-enthusiastic swallows just back from their winter jaunt to Africa while I was walking down the lane next to our house. “Summer has arrived!” we say without really thinking what an arduous journey these birds have had. The RSPB web-site tells me that most British swallows spend their winter in South Africa, travelling through western France, across the Pyrenees, down the eastern side of the Iberian Peninsula and then on to Morocco. Then, unbelievable to me at least, they fly right across the Sahara. These birds migrate at low altitudes finding food on the way. They cover 200 miles a day, flying at 15 to 20 miles per hour. “How do they know where to go?” one might ask.
For me as an ophthalmologist it seems remarkable that birds use their eyes in ways we could never imagine. Birds and insects manage to see the polarisation of light quite as much as its luminance and colour. And more incredible than that they manage to detect magnetic fields using similar photoreceptors to those that see light. It seems that they use polarisation vectors of sunlight and magnetic fields to work out which way is south. But how does a bird return to exactly the same nest year after year? These are, as far as I can see, unanswered, maybe even unanswerable, questions just in the same way one marvels at how a salmon returns from its sea travels to spawn in the same place it was produced.
And how do birds manage to travel quite so far? Migration is hazardous with many birds dying of starvation and exhaustion along the way. And we think that we are having a hard time these days! Worse still, climate change is having a profound effect on bird migration.
A quick sweep online shows a 300-page volume on Effects of Climate Change on Birds which I must admit I haven’t read from cover to cover but does indicate that global warming is changing the timing of birds’ migration. If a species arrives at a time which does not correspond to the availability of its food source, this causes a problem. Indeed, these population declines in migrating species were some of the first effects of climate change reported in the litera-ture over 10 years ago. Perhaps if we understood more of the knife edge that so much of nature lives on, we would be more concerned about the effects we are having on it. Maybe the crisis we as a human population have found our-selves in currently will make us realise a bit more what we are doing to our fellow inhabitants of this fragile planet.
