Tetrads

I have already described the research I am undertaking on the survival of Marsh Tits. I am also participating in another nationwide ornithological survey organised by the British Trust for Ornithology (BTO) in which all the birds that occur in Britain in summer and winter are mapped out. The scheme is called The Bird Atlas 2007-11.

The basis of the mapping is the British Ordnance Survey grid system which divides the country into a system of 10 km x 10 km grid squares, each square containing a hundred 1 km x 1 km squares. These little ones are aggregated in groups of four to generate tetrads (there are thus 25 tetrads in a 10 km x 10 km grid square).

This is the 2 km x 2 km tetrad that I live in. You can see the intersection of the grid lines dividing the tetrad into four 1 km x 1 km squares. The whole tetrad is therefore 4 square km.

In order to maximise efficiency and to prevent wasteful duplication, each participant applies for (or is assigned) a number of tetrads in their area to survey. I have been assigned 22. Each tetrad must be visited a minimum of four times, twice in winter and twice in summer (the breeding season). For consistency and comparability, each visit is timed as exactly two hours. Not all species are likely to be found in a particular visit, but a tetrad can be subsequently visited as many times as desired to "mop-up" any species that were missed during the timed visit.

It is fortunate that in Britain we have a network of public footpaths and it is invariably possible to gain access to a sufficient area of the tetrad.

I have pinned the 22 tetrads that I have been assigned onto a cork board so that I can keep track of my progress. Each square (tetrad) will need to gain four "ticks", two in winter, two in summer, before the work is complete. Each tick represents a two-hour timed visit. There is therefore a minimum of 88 hours work on this board!
We have four years to complete the work but I plan to take much less time than that!

To conduct the survey, a circular walking rout is planned in the tetrad, and all birds seen are recorded, together with any breeding activity (such as a singing male, evidence of nest building and so on). The work is rewarding because it can get you to places in your area you might not have otherwise bothered to visit, and certainly reveals bird species living in an area that may not have been previously suspected.

This week, I was fortunate to have my friend Drinking Ken up from Leicester to stay with me and he was keen to help me carry out surveys in a couple of tetrads.

Drinking Ken helps to plot out a route for a timed-tetrad survey using the network of public footpaths