As I mentioned in an older blog post, I volunteer on the Wetland Bird Survey every month – counting the birds along a section of the river near where I live. This river is too long for one mere mortal to count so we have a small but dedicated team, and our great river is divided into nominal segments. Eleven to be precise. These segments have arisen somewhat organically, based on ease of access, volunteer locations and count efforts. The end result is that if you want to know how many birds are on the river you need to co-ordinate the counters to avoid the risk that the highly mobile birds are not double counted.
Consider a flock of 20 Canada Geese that forage all along the length of the river. Without proper planning we could count this flock up to eleven times, and therefore over-represent the real population by 1100%! This is an unacceptable error by any margin.
So we count on the same day (and as close to the same time as possible). We notify counters in neighbouring segments if a material number of birds head off from ours into their segments in the counting window. This lively email exchange sometimes includes special sightings (like Spoonbills and Seals) that bring all the counters straight back out doors again!
The WeBS method allows for some uncertainty and gives measures to minimise this where possible. Which is fine for all the count dates where Snowmageddon has not descended on the ‘English Riviera’ that is South Devon. So what do we do when access and visibility means that the counts are spread over three days?
Well, that’s where you can use a maximum daily count as apposed to mathematical sum. By comparing the counts across the segments that did manage to get out there we can pick the total seen on any given day per species and use this as the ‘least worst’ total. This is the same approach you can take with patchy or sporadic survey data when trying to compile annual summaries across the region – something else I seem to spend my time doing.
This might seem overly conservative – after all, some of these sites are many miles apart and we may be talking about less mobile species than Canada Geese. But with the levels of uncertainty that we are dealing with it is quite a proportionate response. And don’t get me started on the impact of surveyor effort or distance weighting! Maybe that’s a post for another day.
This is not the photogenic side of ecology and conservation work, nor does data management get me out of doors. But without this ‘back end’ effort, the value of the field work is not realised. It is in the analysis, at a National and County level that we can learn what is happening to our wetland birds.