A few mornings before the cold swept down, I stepped out into mild air and dense sea fog. Webbed in bare branches, a tracery fading through sepia to grey, the silence was full of chiming birdsong, each from a separate GPS among the twigs.
Invisibly they sang and invisibly were heard: song thrush, chaffinch, robin, shadowy blackbirds. Pieces of shadow were chasing each other about: you’d have thought it was spring. I recalled a mistle thrush, perched alone on the sycamore in mid-December, singing its heart out and jerking and fanning its tail. It was the 11th – I wrote it down – and far too soon for that sort of thing.
Birds are used to weather chopping and changing. They can still die in hailstorms, or be windswept into oblivion, and icy winters can kill the ones too tiny to hold enough warmth. But some of each species survive, to renew their populations within a few years.
The current pace of change makes new problems for the interconnections of life. Some birds, for example, time the birth of their young to match a peak supply of caterpillars feeding on young oak leaves. As warmer and earlier springs encourage earlier budburst, a mismatch of fledglings and peak caterpillars can leave young birds hungry. Natural selection for new links and timings may take longer than many species can “afford”.
When birds, animals and plants make homes in quite new habitats, however, their adaptations can be striking. More than half the world’s people now live in concrete settlements. They have created a new world for nature as well – warmer, denser, noisier, more light-filled and polluted than the natural ecosystems beyond the suburbs. In changes to cope with it does urbanisation produce evolution?
A study just published online by the University of Washington looked at 1,600 instances of global change in how plant and animal species look, develop and behave. It found that urbanisation is producing genetic, inheritable changes that "are crucial to ecosystem health and success". (Global urban signatures of phenotypic change, Proceedings of the National Academy of Sciences.)
The probability of this has long been theorised, but this is the first judgment from global evidence. A nice example is what's happened to a yellow-flowered wasteland plant Crepis sancta, or hawksbeard, of which Ireland has some close and common relatives. Its seeds are tufted and spread by wind, like those of dandelions.
Researchers in France found that seeds of the plants growing in urban areas have become larger than those on plants in the countryside. They suggest that, by natural selection, heavier seeds were more likely to survive – they are more likely to drop down onto nearby soil, where they can germinate, than on concrete or tarmac.
A common and accommodating species for study is the blackbird, of which Ireland has a great many, their urban eccentricities frequently noted. But do they sing not only in dead of night but in different voices from those in rural settings?
A study in Spain recorded the songs of 27 blackbirds along a gradient of increasing anthropogenic noise. The birds did, indeed, change their vocal frequencies along the road, the better to be heard.
(online at http://dx.doi.org/10.1016/j.landurbplan.2010.08.013).
Another study, comparing blackbird song in the city centre of Vienna and in the Vienna Woods, found that forest birds sang at lower frequencies, with longer intervals between less shrill songs. (http://dx.doi.org/10.1016/j.anbehav.2009.06.016).
Why urban blackbirds sometimes sing at night has been explored in experiments. Exposure to light affects their levels of melatonin and perception of length of day, and thus their biological rhythms. One study, using light loggers carried on individual birds, showed that, in March, when reproductive growth occurs, urban blackbirds reached maturity 19 days earlier than rural birds. Light pollution, affecting perception of day length, could be the cause. (Does light pollution alter day-length? Royal Society B.)
How much of all this has become genetic, heritable and part of evolution? Historically, blackbirds were birds of the forest and their urbanisation was first recorded in Germany in 1820. In a deforested Ireland they spread island-wide and the Dublin ornithologist John Watters knew them well in "the vicinity of gardens, shrubberies, low brushwood, and indeed any locality affording good cover and secure shelter".
Do they find the modern city stressful? American researchers hand-raised urban and forest-living European blackbirds and found that city-born blackbirds have a lower corticosterone stress response than their woodland cousins. The reduced stress response is , they suggest, “presumably necessary for all animals that thrive in ecosystems exposed to frequent anthropogenic disturbances”.
Including, presumably, humans. (Stress and the city: urbanization and its effects on the stress physiology in European blackbirds, Ecology, August 2006).
Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks