On August 8, Valerie Hurst was looking out her window when she saw a squirrel. In picturesque Stratford-upon-Avon, the appearance of a squirrel near an oak tree is not anything out of the ordinary. But for Valerie, the question was whether the bushy-tailed manifestation heralded the arrival of acorns in the old oak tree.

“As soon as you see the first squirrel, you know the acorns are just about ready,” Ms Hurst told The National.

Whether in her back yard or along the way on her morning walk, for almost 20 years Ms Hurst has contributed her sightings to Nature’s Calendar, a project of the UK’s Woodland Trust.

She is one of thousands of volunteers across the UK whose observations of flora and fauna are helping to shape scientists’ understanding of the interplay between climate and species — a citizen-driven contribution to the field of phenology.

We’re hoping it encourages stewardship for their local environment Lorienne Whittle, Citizen Science Officer at Nature's Calendar

Phenology, as defined by the UN’s Intergovernmental Panel on Climate Change, is “the timing of seasonal activities of animals and plants … perhaps the simplest process in which to track changes in the ecology of species in response to climate change”.

The information gathered by Nature’s Calendar volunteers is perhaps the most comprehensive data set available to scientists in the UK, with about 3 million records. The project will soon celebrate its 20th anniversary.

“Using historical data, we have records dating back all the way to 1736 — very useful for those wanting to study change over time,” Lorienne Whittle, Citizen Science Officer at Nature’s Calendar, told The National.

Anett Kiss is an undergraduate studying for a bachelor's degree in ecology at Edinburgh University. She is looking at what increased temperatures portend for frog spawning.

Frogs are ectotherms, which means they rely largely on external heat sources and cannot regulate their own temperature. For this reason, Ms Kiss says they are more affected by environmental variables like temperature.

Building on previous work by her supervisor and collaborators, she is drawing on data collected by Nature’s Calendar volunteers.

“We’re looking across the UK, and we have almost 100,000 observations spanning about 30 years, from 1983 to now,” Ms Kiss told The National. “There is a general trend: frogs are shifting their breeding earlier in response to an earlier spring, as temperatures become warmer earlier.

“We're using more complex models and more complex statistical analysis to try to explain how frogs might be responding to temperature changes, and whether they can flexibly decide when to spawn, or whether they are required to evolve and adapt to changes,” she says.

In the pond ecosystem, this phenological shift can have a ripple effect.

“Looking at newt emergence — they don’t hibernate in winter, they hide in burrows — we’re looking at how temperature might be affecting when newts emerge,” Ms Kiss explains.

“Newts also breed in the pond. They would usually encounter frogspawn [when they emerge], and newts tend to be predators of tadpoles. Looking at how synchronous or asynchronous the [frog and newt] breeding times are, there might be higher predation.”

This, in turn, could affect the number of tadpoles that survive to become frogs — thus affecting overall frog populations.

After all, Ms Kiss notes, “Organisms are almost never found in isolation.”

Nature’s Calendar is hoping to expand its volunteer base in the UK, and encourage a new generation of citizen scientists to submit observations online.

“I would say to someone signing up to try and make this a part of their lives — like a new hobby that gets you outside and in nature. In the process we’re hoping it encourages stewardship for their local environment,” says Ms Whittle.

With many species and events, Ms Whittle says, it is simply a matter of being more attuned to one’s surroundings: “The first butterflies in spring, redwings and fieldfares which begin arriving about now to overwinter in the UK – many people are [noticing] this already, but by watching for certain events and reporting what they see on Nature’s Calendar, they are contributing to a really important dataset which is increasingly used by scientists and policy makers to study climate change.”

For Ms Hurst, basic scientific observation has become hard-wired into her dawn walks.

“I’ve always been interested in the natural world and conservation of nature,” she says. “It brings a little bit of purpose to a walk … When I’m walking, I observe these things — I mean, you trip over the remnants of the acorns!”

Whether it is noting the arrival and departure dates of migratory birds, pondering the seemingly mutually exclusive presence of swallows or starlings, hearing the song of a robin in the hedgerow or recording the cycles of the ash tree, Ms Hurst says it is a rewarding hobby.

“It takes you through a path that you wouldn’t really expect. They’re all interconnected, but perhaps we don’t really realise it.”