Chasing the Day: the extraordinary effect of light on birds and their behaviour.

by Keith Offord.

8^th January 2008

Keith’s photographs were wonderful and of the highest professional standard. Around them he wove a story about birds and light, migration and food, barnacle geese and pied flycatchers, and waxwings that sometimes go but not always.

He started with photographs of the pied flycatcher at each end of its migration, looking at home both in the UK and in Africa, and he posed the question ‘why?’ Why doesn’t the flycatcher stay and breed in Africa? Why does it undertake the hazardous journey across desert and sea when its insect food is plentiful there all the year round? The answer appears to lie in the way the Earth tilts in relation to the sun, and the consequent longer summer day in Britain. Not more food – but longer light hours in which to feed the young which enable an increased breeding success. Then we saw photographs from the other side of the Atlantic, of the insect-eating purple martin, which undertakes a similar length journey to the pied flycatcher, wintering in South America and returning north in the spring to breed in the longer North American days.

So does this mean that all migrating birds are ‘chasing the day’ – or rather day length? Seemingly not. Another of Keith’s examples was the barnacle goose. These beautiful geese breed in the long days in the arctic and fly south to winter in their thousands in the UK in the Solway Firth. They can feed by night in the estuary mud, so it appears that it isn’t the day length that they are following, but rather to escape the arctic ice that freezes in their food. Their long journey is still about food, but is more about temperature than light.

In search of answers to the ‘Why’ of these migrations we heard of the osprey that moves south to winter, again, not so much to do with light as the fact that fish depth is related to temperature. Like the barnacle goose, it is the availability of food that varies with the temperature rather than a simple relationship with light or day length.

To add to the confusion there is variation within some species; some greenshank winter here in the UK and some fly to Africa. This results in less competition for limited food but how did it originate? Waxwings are irregular in their migration, sometimes they go and sometimes they stay, seemingly affected by weather and availability of food rather than directly by light or day length. And then there are the nocturnal birds such as the nightjar, with a long-distance migration to and from Africa that is poorly understood. However, the dusk, which is their most active time for feeding, is longer in the UK summer than it is in Africa and perhaps these migrations evolved driven by an increased feeding rate for the young, similar to the pied flycatcher.

So we ended with no simple answers; in fact, as Keith said “we end with more questions”. But we enjoyed the journey and the photos were magnificent.

Penny Avant