Refugia survey is all well and good and I would certainly use them as a tool for this project also but as you said your dogs were bitten in March and April.. this is when adder are slow after hibernation and very approachable. (for the rest of the season they would be off like rockets, hence why dogs are often bitten in the spring as this is when they are most likely to encounter the snakes). If you can start your project from mid February onwards you can take full advantage of this fact and include visual survey.
The reason I'm saying this is I know for a fact that it can take up to a year for adder to use refugia, I've seen this more than once. For adder use tin, you would be wasting your time with roofing felt unless they were extremely large and also get the refugia down early say the end of January to allow them to 'establish'.
Using visual in the spring will quickly give an idea of population size. It will also allow you to identify hibernacula, which are a vital feature of adder ecology. For example if during your project a communal hibernacula was destroyed it would skew any results as the entire population might perish!
A relative population estimate can also use a mixture of refugia and visual survey so make the most of both methods. The Froglife methodolgy for estimating relative population size is straightforward.
You can use both refugia and visual survey. The maximum density of refugia should not exceed 10 per hectare (this is a vast number of refugia at most sites and few people would use such a high density in practice)
All you do is check your refugia and carefully look for snakes. The best count you get for a single day in any given survey period is your relative population estimate. The only stipulation is that the count must be by a single observer.
Once you have your number the population is estimated as follows
<5 Low Population
5-10 Good Population
>10 Exceptional population
As you will be recording areas over time you will be able to monitor if populations increase of decrease as a result of the management work.
There is a little book in the Shire Natural History Series by Peter Stafford simply called 'The Adder' ISBN 0 85263 879 5. It costs about ú2.50. I would recommend it to anyone setting out to work with adder, it gives a very good description of visual survey techniques and when and how to find them. There is also a fairly good background of 'facts' though some might be disputed these days.
Just to emphasis the point about visual, if at one site last year I had relied on refugia only to estimate the population after 12 visits the population would have stood at 1 neonate adder.
Visual survey revealed that the population was actually 'good'.
Oh just one more point, relative population estimates are based only on adult animals, you can't count juveniles and sub-adults.
The difference with relative population estimate is you don't have to capture all the snakes and mark them or at least photo ID them, so it's much easier and a lot less stressful for both you and the snakes!
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