When to breed

In Arran, May 2009 was cooler and wetter than May 2008.  The mean temperature in May 2009 was 10 degrees centigrade, three degrees cooler than May 2008.  The rainfall for May 2009 was over 80mm, nearly three times more than May 2008.  How do breeding birds react to the weather?   A simple question, but one which has intrigued people for centuries.

Leonard Jenyns was a nineteenth century eminent zoologist and botanist, who was invited to join the “Beagle” as naturalist but declined, suggesting in his place the young Charles Darwin.  The rest, as they say, is history.  Jenyns was a curate who lived near Cambridge. Starting in1820, for twelve years he recorded everything he could about the seasonal patterns of plants and animals including the timing of birds breeding seasons.  He noted that the average date on which chaffinches started to lay was 28 April, but across the different years of his study, it varied

by almost two months between 17 March and 14 May.  Other species showed similar variation.  This was the first quantification of birds’ breeding seasons and his conclusion was that breeding occurred later in cool springs than in mild springs.

Weather does influence when birds breed.

A century later, David Lack a leading twentieth century ornithologist who became Professorial Fellow of Trinity College, Oxford, working with his entomologist colleagues in Oxford, clearly demonstrated that great tits, which feed their chicks on woodland caterpillars, laid their eggs so that the time when they were feeding their chicks coincided exactly with the time when the caterpillars were most plentiful.  How do the great tits know when to start nest building, copulating, laying, and incubating, in order to have young in the nest for the feast?  Lack suggested that it was the length of the day.  The great tits that start breeding at a day length that results in having young at the time when food is plentiful will leave many descendants and the genes for breeding at the “right” time will be passed on to the next generation. Those great tits that start early or late will leave few if any descendants.

Along with weather, the availability of food influences when birds breed and the trigger in many cases to start breeding seems to be the length of day.