Back in January a few of us involved in Curracag and the Outer Hebrides Biological Recording group decided to launch a phenology survey we ended up calling Signs of Spring. Phenology is the study of when things happen and for centuries people have systematically recorded when certain natural events occur.
Perhaps the most famous phenologist was Robert Marsham who lived at Stratton Strawless just north of Norwich. I first became interested in him as a result of the old country rhyme – ‘oak before ash, we’re in for a splash, but ash before oak, we’re in for a soak’. Now I’ve been interested in natural history in all its forms most of my life and I don’t ever recall seeing Ash come into leaf before Oak but I certainly remember years when there was a definite soak contrary to the tree’s prediction. Modern day records from surveys such as the Woodland Trust’s Nature’s Calendar confirm that ash leafing before oak is very unusual nowadays.
Robert Marsham was recording events such as bud burst way back in 1735 and his family continued doing it through until at least 1925. Their early records are available from a paper read by Robert Marsham to the Royal Society in April 1789. From those records it appears that ash preceded oak in about a quarter of the years. In that context the old saying makes more sense. Ash did come into leaf earlier that oak fairly frequently.
What’s changed since the 1700s is that springs have got warmer and earlier following increased carbon dioxide levels in the atmosphere, and subsequent global heating, starting really with the industrial revolution. For every degree (C) of warming oak comes into leaf about 7 days earlier. Bud burst in that species is triggered very strongly by rising temperatures. In Ash bud burst is related more to changes in day length. For every degree (C) of warming Ash comes into leaf just 2 days earlier. So as springs have got warmer the oak has pulled away getting into leaf much earlier than ash does. It’s only in very cold late springs such as 2013 that ash comes before oak.
The idea that natural events are affected by climate change led the UK government to develop a phenological “Spring Index” as one of its UK Biodiversity Indicators. Their index is based on four events; the first flowering of horse-chestnut and hawthorn, the first record of a swallow and first recorded flight of orange-tip butterflies. The dates recorded by their index suggest that since 1999 springs have been on average six days earlier than they were at the beginning of the 20th century.
Using the UK Spring Index species would probably show that spring never arrives in the Outer Hebrides. There are very few places at which you could record the four events. It’s probably only the grounds of Lews Castle and at Northbay on Barra that all four species occur together. That’s why we came up with our own version. Our “Signs of Spring” was based on nine events. First sightings of three insects (White-tailed Bumblebee, Green-veined White butterfly and Large Red-damselfly), three birds (Wheatear, Swallow and Corncrake – hearing that one counted) and three plants (Lesser Celandine, Lady’s Smock and Yellow Flag Iris).
When we launched our survey in February we didn’t really expect the year to turn out the way it did. Despite that we managed to get enough responses to let us have a look at how spring happened this year. We deliberately tried to include events that cover the whole progress of spring. There were things such as the first flowering of Lesser Celandine, the first emergence of queen White-tailed Bumblebees and the arrival of the first Wheatears that we expected to happen early. The first records of all of these came in in late March with average dates of 2nd April, 3rd April and 6th April respectively. These are the sorts of things that all of us can look forward to as the first sign that spring is really just around the corner.
A little later Swallows had been spotted, the first on 15th April and then a little later they were joined by Corncrakes (19th April). The return of both of these species seemed a little late in comparison to other years but were they really late? All too often we rely on a fallible memory. Once I’ve spotted the first Wheatear I eagerly expect it to be followed by the first twittering Swallows and the interminable rasp of Corncrakes. Looking back through my notebooks my first Swallow was nearly three weeks earlier than in the previous couple of years and Corncrake was about ten days earlier. This is why recording things properly really helps.
The average date of first flowering of Lady’s Smock (30th April) was exactly the same as the average date of sighting of the first Green-veined White. I found that surprisingly reassuring. The main food plant for caterpillars of the Green-veined White is Lady’s Smock. The fact that both events occurred at the same time made me think that despite all the horrors in the world, in our little corner, nature was still well in synchrony. One of the expected effects of global heating is that these synchronicities may start to break down. Corncrakes arriving before there’s enough cover for them, Cuckoos come back to find that the Meadow Pipits have already got past the egg stage and it’s too late for them to lay, plants flowering before there are sufficient pollinators on the wing.
By the time we get through to the later events, the first Large Red-damselfly and the first Yellow Flag Iris I find that in all the excitement of other spring stirrings I can’t actually remember whether these are early or late. They do though mark in my mind, though, the final flourish of spring and indicate that we are well on the way to high summer and are only three weeks from the longest day.
What we don’t know yet is how typical spring 2020 is in comparison to other years? We have laid down a marker, though, for future comparisons. Listening to the weather forecast on national tv or radio suggested that we had a warm early spring. But things are different here on the edge. Edinburgh or Inverness may bask in warm spring sunshine whilst we are faced with bitter NE winds. Sometimes it’s the other way around. Collecting our own long -term records will help us in future years put our spring into our own context.
There are some long-term records that do help give us a clue though. The met station at Stornoway has been collecting data for nearly 150 years. Looking at the average spring (March-May) temperatures over that period there seems to be a clear long-term trend towards warmer springs. The average maximum is about 1C and the average minimum temperature nearly 2C warmer now. That might now seem much but remember that oaks come into leaf about seven days earlier for every one degree rise in temperature. The oaks in the grounds of Lews Castle are perhaps coming into leaf about a fortnight earlier than they were when they were first planted. That’s a big change.
As to whether 2020 was an early or late spring, the met date doesn’t really help. The average maximum spring temperature is about half a degree above the long-term trend whilst the average minimum is about the same amount below the trend. Perhaps the natural world provides a better more integrated measure of spring. I think we can say with some certainty that it wasn’t as cold and as late a spring as the dreadful one in 2013 and probably not as warm and early as the one in the following year, 2014, but looking at how it changes in the future is something that this survey may help to show.
The other thing we can do better is to look at how things change as we go from Barra to the Butt of Lewis. How representative of the whole island chain are data from Stornoway Airport? We can’t answer that just yet but we get a hint from the two most complete sets of date. One from Bragar on Lewis, where the average “Signs of Spring” date was 2nd May whilst at Eochar on South Uist it was the 21st April. I suspect it would probably be slightly earlier still on Barra but let’s find out properly next year. Thank you to all who sent in records in 2020. We’ll be doing it again in 2021 perhaps with better publicity and some streamlining of how we get data in and of how we keep you up-to-date with the progress of spring.