Common Birds Census
The Common Birds Census was started in 1961. It is organized by the BTO and is the most satisfactory system for measuring bird populations, at present. If you wish to undertake a Common Birds Census in your area, you should get the full details from the BTO at Tring. Basically you select a study area ranging in size from approximately 20-25 hectares with woodland habitat to 80 hectares in open country such as farmland. You need to make eight or more visits lasting several hours, before the end of the summer. To have any value you must be prepared to carry this on over several seasons; continuity of record is most important. If you are working through the BTO 1:2,500 outline maps will be provided free of charge. One of these should be used to show the major plant communities which are one of the main factors influencing the availability of nesting-sites and food; a standard list of habitats is shown in the Ecology and Habitat section.
Drawing any form of vegetation map has problems, particularly for the birdwatcher, as it is more often the structure of the individual plant and, perhaps its fruit which is of fundamental importance to the bird. Very often a sketch of the profile of the habitat with some idea of the scale will give you a reasonable idea of the habitat. You can draw the basis of a vegetation map on the 1:10,000 or the 1:25,000 scale maps and then on these try to map the boundaries of the major plant communities such as grassland, heath, bracken, woodland, water and so on. You can indicate the distribution by symbols such as the initial two letters of the name. When you superimpose a map of the breeding distribution of various resident species you may be able to get some idea of the broad habitat requirement for the species. But in making your vegetation map the BTO recommend that in woodland areas you indicate whether the canopy is open or closed, and similarly describe the density of the field layers, e.g. dense, medium, sparse, giving the average heights and dominant species.
When you have completed the fieldwork you can start analyzing the results. You take a fresh map for each species and transfer the symbols to the map, substituting the letter of the visit for the original symbol, accompanied by the appropriate convention. Therefore on the Blackbird map, the symbol ‘A’ now means Blackbird in song on the first visit; the second visit will have ‘B’ and so on. When each map is complete it will show discrete groupings of letters indicating positions held by territorial males on different visits. Territory boundaries will be checked by BTO staff.
Line transect method
While the Common Birds Census is the most precise method of censuring bird populations it is also somewhat time consuming and complicated. There are two further methods of sampling, which are simpler to use, although they forfeit some precision. The first of these methods is the line transect which is and has been used by people sampling a wide range of habitats in a number of different countries. This also has the advantage over the Common Birds Census in that it can be used throughout the year, enabling you to census winter populations.
On a line transect you walk through your chosen habitat along a regular path and record all the birds you identify either by sight or sound. You do not need to map their positions, although it is better if you do. There are, however, various conventions which you should observe. Most observers seem to recommend an average speed of about 3 kilometres an hour. Also you count all birds within 25 metres of the line. Some birds are more conspicuous than others and so it is difficult to compare results between one species and another or for one species from one wood to another, except where the birds are conspicuous. Nevertheless, the system can be quite useful for comparing a number of singing passerines.
From the figures resulting from the line transect, which is in effect a sample census, you can derive a rough estimate of the total number of birds in the wood. Furthermore, if you continue to make line transects throughout the year you can detect seasonal and annual variations in the bird populations.
When I am travelling and I have to move faster than a line transect allows, whether on foot or some form of transport such as a boat, I keep a fifteen minute check-list of the birds observed. Normally I use squared paper in my notebook, listing the birds on the left-hand side and using a column for each fifteen minutes. I use a ‘tick’ to record a sighting, or a figure, if I have counted, with an additional symbol for a song, nesting, etc. This can give some indication of the distribution of birds over a large area. If you can also add notes about the types of habitats through which you are passing, the record will be that much more valuable.
This is another sampling method which lacks many of the good qualities of the Common Birds Census, but it can be used throughout the year and is good for assessing the relative abundance of birds in different habitats. Spot censuses can be made in a number of ways. In my local wood, I used ten points which were roughly the same distance from each other and which were easily identified. I waited for five minutes recording all the birds which could be seen or heard within a 25 metre radius.