|Eurasian Wren (Troglodytes troglodytes)|
Wrens, Troglodytes troglodytes, are one of the smallest British birds, being only marginally larger than our smallest bird, the migratory goldcrest (Regulus regulus). The Eurasian wren inhabits a zone extending from Britain, across Europe and into Iran, Afghanistan and across Central Asia to Japan. Over most of this range, wrens are sedentary.
At the northern extremity of it's range it has to cope with occasional periods of extreme cold such as occurred in Europe and especially Britain in 1963 when our winter weather became Siberian for about 3 months. We are experiencing a period of cold now where the temperature is struggling to get above freezing during the daytime. The wren is one of the smallest warm-blooded animals to survive our winters and yet it doesn't hibernate like some small mammals.
Wrens have a mating ritual which entails males building several nests and then enticing a female to mate and lay eggs in it. She will inspect the nests first and will only choose a mate who has built a nest she finds suitable. Only then is the nest lined and prepared for the brood. She then broods the eggs and tends the young whilst the male goes off to entice another female into another of his nests. This leaves many nests, some of which will not be used for brooding.
So how does the small wren, which because of its small body mass, and consequently a large surface area to mass ratio - meaning it will lose heat very quickly and will need a high metabolic rate to maintain its body temperature - keep warm in these winters?
In normal winters, the old nests from spring and summer are used as roosts. Several wrens will gather together in the same nest, so decreasing their surface area to mass ratio, and so keeping warm.
They do the same thing in these occasional periods of extreme cold. The only problem is that, if too few wrens gather together, or the temperature falls too low, or the nest isn't quite as good an insulator as is needed, very many wrens freeze to death overnight and the population will crash.
As a youngster I lived near the beautiful, and poetically named, River Evenlode in North Oxfordshire, one of the tributaries of the Thames. This river had been diverted at several points when the Oxford to Worcester railway line had been built by the Great Western Railway Company to make it easier to build bridges across it. In one such place in an elm wood, the old river bank was still exposed and formed a sort of shallow over-hang hung with tree roots and wild clematis. I could always guarantee finding several wrens nests under these ledges.
In the summer of 1963, after one of the severest winters on record in the UK, when searching for these nests, I found nest after nest full of mummified bodies of wrens, probably some twenty or thirty dead wrens in all. All of them victims of the winter.
So, how does evolution account for this?
Quite easily. Wrens have evolved a survival strategy which works well enough in most years and which leaves a few survivors even in harsh winters. Even when a local population is wiped out entirely in a single year, as may well have happened in my small elm wood in 1963, sufficient survivors are there in other areas to move into the vacant territory. Wrens also produce several batches consisting of 6-8 eggs in a year so a population can bounce back quickly, especially since food will be plentiful in areas where the population has been reduced.
In this way, the environment and the habits the wren has evolved to cope with it, work well enough together to allow the species (i.e. the wren genes) to survive and to produce more wrens next year. This, of course, takes no account of the suffering and death from cold of the individual wrens. There is no survival value for the genes in evolving mechanism to prevent this altogether, only in as much as it leaves a few gene carriers to carry them into the next generation. Because it works well enough, there is minimal evolutionary pressure to evolve more complicated strategies such as hibernation or a larger body mass.
Interestingly, on the point of a larger body mass, the wrens on St Kilda, a remote North Atlantic island, are measurably larger than the mainland population. The differences are enough for this wren to be given sub-specific status, Troglodytes troglodytes hirtensis.
And of course, the northern limit of the Eurasian wren's range will be determined by a line above which there are no survivors over winter, so a dynamic has been created which will fluctuate a little each year and by a larger amount as climate changes over the long term. The northern limit will always be an area of large-scale occasional population crashes and recovery.
No intelligent, compassionate, loving creator would be this callous and indifferent to the suffering of individuals. Quite clearly, the wren, it's range, its reproductive strategy and its winter survival strategy have all been created by a dynamic and selective environment.
No mystery and no magic required. Evolution by natural selection is again the most parsimonious explanation.