Running like clockwork?

Why can't we expect all wildlife to respond to our changing climate in the same way? As Mark Boyd explains, it may depend on how species tell the time.

Are we nearly there yet?” How often have you been asked that question? There are several obvious responses: a straight “yes” or “no”; a measure of time – “just 15 more minutes”; a measure of distance – “only 10 miles to go”; a measure of hope – “the sat nav reckons we’ll be there in...” or expectation – “Well, from here it usually takes us...” The outcome still depends on the traffic, the weather, road works and a host of other factors.

All of these answers have parallels in nature. And the way different species answer that question is not only fascinating, but also lies at the heart of a key issue around our changing climate. In our warming world, the basis for some of the answers is shifting. It also helps to explaining why years with freak weather can be so disruptive for wildlife. 

Back to our journey. If everything runs smoothly, our 10 miles could take the 15 minutes that the satnav predicts and that it usually does and could be either nearly there or not depending on your point of view. "Nearly there” may be estimated by time, distance, external clues, past experience or crossing some threshold of closeness.

An extreme close-up of the centre of a dandelion

Dandelion (Taraxacum officinale) close-up of seedhead, Badbury Rings, Wimborne, Dorset, England, UK - Guy Edwardes/2020VISION

Natural measures

For wildlife needing to coordinate its activities for the best outcome, these same measures could be calendar time (always hatch 100 days after the shortest day to catch the insects); day length (don’t emerge until the days are longer than the nights to avoid most of the frosts); average temperature (a few warm days and it’s time to germinate); and time above a threshold temperature (chill first and then emerge, whatever the weather). 

Some species combine these measures. The date of butterfly emergence, for example, may vary from year to year. But that variation is much less if instead of measuring it by days on a calendar, we measure it in “day-degrees” above a threshold temperature. That is to say, below a certain temperature, nothing happens. Development is, almost literally, frozen. Above that threshold, the speed that a butterfly matures within its pupa is a combination of the temperature and how long that temperature is maintained. Some species, such as northern brown argus, may be better able to grow in a long, cooler development phase than a short, warm one. For others, like the common blue, the components of the day-degrees may matter less. These may thrive in a wider range of climates.

Without controlled laboratory experiments, where the impact of each factor can be assessed, we are often left with just the outcome of these timings rather than understanding their causes. Swallows, for example, arrive earlier each spring now, so that you may see them before sand martins arrive. That would have been very unlikely 30 years ago. Are these two related birds measuring time differently? 

Blue tit feeding chick

Blue tits may need to find 1,000 caterpillars a day. Image by Gillian Day

Seasonal fare

We know, too, that blue tits nest slightly earlier each spring. Blue tits respond to changing day length, which doesn’t vary with climate, and to the prevailing temperature. Blue tits try to time their breeding so that their nestlings can be fed when there are most caterpillars around. The emergence of those caterpillars, though, depends more on temperature than daylength, so in a warmer climate the tits and their food supply could get out of sync. 

The same issue seems to be happening for golden plovers nesting on moorlands. If their chicks are looking for crane flies long after the main hatch, there is a danger that fewer golden plovers will grace our winter landscapes.

These timing measures have been honed over many generations by plants and animals. Those that got it right and passed their measures on were just slightly more likely to raise more young than others. But this takes time. For each family whose timing is spot on, there may be one that just missed the boat, so the synchronicity is hard won over many years. To expect species not only to tweak their measure with the seasons, but also to tweak it in relation to their food supply that may be responding to a totally different cue within the sort of timescale that scientists predict the climate to change is unlikely. We have no idea how some of these interactions between species will pan out, so everything changes for nature, and for the scientists of the future.

Infographic showing overlap of caterpillars and blue tit and great tit chicks over the decades

Shifting breeding 

Insectivorous birds such as blue and great tits time their breeding to hit peak caterpillar abundance. The tits judge their timing by daylength and early spring temperature. Caterpillar emergence depends more on later spring temperatures. As the climate changes, what used to happen occasionally is now happening more regularly, and the overlap is reducing, perhaps leaving tits short of food.

How can we help?

First, we can document timings in nature by taking part in field surveys. This is called phenology. You can get involved in our surveys here 

You can also make lifestyle choices, to help reduce carbon emissions, the most obvious cause of climate change. Find a whole host of tips on our website here. 

Finally, you can have confidence in your Wildlife Trust - and support our work by becoming a member. Our Living Landscape schemes are designed to help wildlife to survive the stresses that climate change brings. The larger the scale we can work in, the better that will be for wildlife of any species under any new conditions. “Are we nearly there yet?” Perhaps not, but at least we are starting to understand more about our journey.


This article was originally published in the spring 2018 issue of our members' magazine Local Wildlife.

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Waxwing (Bombycilla garrulus) feeding on Cotoneaster berries in supermarket car park. Whitstable, Kent - Terry Whittaker/2020VISION