The warden's hut at the Offa's Dyke end of Bala Lake, an unassuming venue in which to be inducted into the art of detection in the underworld. But my subjects were not the stuff of James Bond movies - instead the everyday country life in Snowdonia. This was a Mammal Detective workshop!

When I spotted the first otter in my life, a friend said I should report it and I did. This obviously showed a wildlife interest that should be encouraged and shortly afterwards I received an invite to this workshop.

There's a lot of wildlife out there in Snowdonia, but most of us only see the tip of the iceberg. We have a pretty good idea of the birds because they show off in front of us during the day, but mammal life is mainly nocturnal, going about its business when we're fast asleep. We can only guess or imagine what's out there, unless we know what we're looking for.

But guesswork is not good enough if we are to make informed planning decisions, so the goal is to create a mammal map of Snowdonia. The national park has been divided up into segments that volunteers are taking responsibility for surveying over the next couple of years. The good news is that the volunteers are being trained to spot the differences and search out the vital clues that evade most of us.

Otter or Mink? Spraints or scats?
Otters are staging a comeback in all parts of Wales after years of persecution and the overuse of pesticides. They are magnificent and friendly creatures - unless you're below them in the food chain. Have you noticed the diminishing population of ducks on Llyn Mair (above Plas Tan y Bwlch)? Night time is danger time when predators are on the prowl, so ducks take to the safety of the water, well away from the fox, but well within the clutches of hungry otters. Otters have long whiskers which sense vibrations in the water, enabling them to track their prey in the dark.

They obviously don't rely exclusively on this sense as it's the white ducks that go first. Some years ago they came across the Koi Carp at Portmeirion and within a short space of time all the bright-coloured ones were gone, but the dark ones are still swimming today.

Detecting mammals is more about searching for the evidence than observing the animal itself. You need to be lucky to actually see it. Much of the workshop focused on the evidence or clues as to what is around. Otter spraints (poo) are the main clues - small piles of black tarry poo with a very strong smell (jasmine tea? Bombay duck?) and generally lots of fish bones or scales. The spraints are deposited at prominent (to otters) sites, usually on large boulders or on the base of tree trunks and in places that mark the boundaries of an otter territory. If there's nothing obvious to do it on, the otter will build a small sandcastle and do it on that.

The dog otter generally has two or three females that he breeds with and will jealously guard his patch against all comers. Vacant territories are in strong demand and if the dog otter falls victim to a road accident, his place will be taken by another dog otter whose first response will be to kill off all the existing cubs (just like lions do). They will also see off food competitors and are displacing neighbouring mink.

The mink is much smaller, blacker, and with minimal whiskers. If the ground is suitably muddy for an imprint, the tracks of an otter are about double the size of a mink and with slight webbing between the toes. But now I've been initiated I know to compare the otter spraints with the mink scats (another poo word). The scats are black and shiny, full of fur, long and twisted and stink.