He understands the disinclination to get outdoors (indeed, one of his teenage memories is “sighing when Mum said, ‘take the dog for a walk’”). His children originally found it embarrassing when he read The Lost Words aloud to them – “the equivalent of dad-dancing”, he laughs – although they soon fell under its spell. “My daughter went on this conservation camp for the first time this summer, and she’d been sighing about going, but rang me on the first night in West Wales and said, ‘Dad – you know all that nature stuff you’ve been talking about my whole childhood – I used to think that was baloney but now I get it.’ And I thought, wow, it took a day! That’s all it took – without parents and without screens there. I loved that moment.”

Does he limit his children’s screen/social media time? “I think I’m probably worse at it now than they are!” Six months ago he joined Twitter and has amassed a huge following – his Word of the Day tweets take his project of “rewilding the language” and prizing its biodiversity into cyberspace – something he finds an “utter joy” and a “daily gladdening”.

Macfarlane wrote poetry in his teens and is an admirer of Hopkins, Hughes and Heaney. There are also echoes of Edward Lear and Edward Thomas in some splendid spell-poems. “I wanted to catch the eeriness and strangeness as well as the comedy and magic and beauty of nature. And with each ‘spell’ I tried to capture what Hopkins calls the ‘this-ness’, the quidditas, of each creature and what makes it astonishing to be close to.” As we walk, he delights in naming the creatures we see, soaring in the sky or scuttling on the earth. We forage for food and devour delicious blackberries and apples.

He has also written the script to a documentary, Mountain, about our love for high places, which has its UK premiere at the London Film Festival. It’s narrated by Willem Dafoe, and Macfarlane is still a bit star-struck. “When Dafoe came on board I was like, that’s my dream, I can die happy now with my words spoken by Willem Dafoe. It’s an experimental film – 90 minutes long but the script is only 820 words. We wanted to create a dreamscape, almost like a film poem.”

Does he have hope for the future? “The bigger picture is dismal. Plastic pollution, climate change, extreme weather events…” Contemplating the seriousness of planetary-scale problems, though, can throw people into paralysis – and paralysis, he says, is truly hopeless. “Small acts of care are crucial – grass-roots charities, individuals, books, words, are doing magic work – so to say there’s no point is an abandonment of everything. Hope is a greater agent for change than despair.”

The Lost Words is published by Hamish Hamilton on Thursday 5 October. Mountain premieres at the London Film Festival on Oct 9