Birdwatching: Counting the Birds
Whilst I enjoy the sheer variety of birds and particularly those new or exciting ones that I find for myself, I enjoy even more being a ‘curious naturalist’ and asking questions and finding answers about the activities of birds.
In order to study many of the different aspects of the lives of birds we have to count them. Counting can tell us how many birds make use of the whole or part of a habitat for such activities as feeding, nesting or roosting. Regular counts can show us if there are changes in numbers and, if this is the case, it may become important to discover the reasons. They could be seasonal, with some individuals migrating over long distances, some only a few kilometres, and other species from harsher climates coming to take their place. Counting by birdwatchers has in recent years shown that often changes have resulted from habitat destruction and environmental hazards, such as organo-chlorine pesticides, oil pollution and even, though it was thousands of kilometres away, the effect which a disastrous drought in the Sahel Region had on the European bird populations which wintered there. The 1976 summer drought in Britain affected our own garden birds according to Mr and Mrs P. Willson who were undertaking a garden bird survey for the BTO.
Recently the Natural Environmental Research Council reviewed current and past work concerned with the monitoring of populations and communities of living organisms in marine, terrestial and freshwater habitats, and one aspect which was very clear, was that the part played by amateur naturalists in many disciplines was most valuable. This NERC report entitled Biological Surveillance, defines the work as attempting to detect temporal and spatial changes and providing a capability for predicting future changes.
Counting birds is by no means easy. In some places, for instance, a small lake, you can see all the birds quite clearly but in a wood of the same size this is quite impossible. Birds and their habitats are so variable that different methods of counting are required for each of them. To make things more difficult there is no agreement as to the ideal method of counting. It is also difficult to be certain that one person’s figures are comparable to another’s, even when working in the same area.
Counts are either direct or based on some form of estimation. In a direct count you must be able to see each individual bird. This method is suitable for lakes, estuaries, seabird cliffs, open fields and small flocks of birds in flight or at sea. If you cannot see all the birds at once, for instance, in a woodland habitat or when counting a very large and fast-flying flock, you should count one or more samples and calculate the total from those figures.
A direct count should produce a more exact figure as, with a small lake, you can see all the birds at once. If the lake is large, however, you can run into problems as a substantial portion of a flock might move to a part of the lake, as yet, uncounted.
Recently, I have been counting birds, using the direct method, on Pakistan lakes. One of these lakes had a surface area of just over 18 square kilometres and, with a telescope from high ground, it was possible to count all the ducks, herons and other long-legged water birds from two positions. However, in order to identify and count the small waders, I had to visit many more places where previous experience had taught me that the birds tended to feed or nest. Another of the lakes had a surface area of 185 square kilometres. Here I had to drive some 120 kilometres around the lake, stopping on headlands and other vantage points from which I could scan large areas of water with a telescope. In addition to the problem that some of the birds might have moved substantial distances (which I don’t think they often did), there was the problem that at 3 kilometres, particularly with some refraction in the air over the water, the identification of individual birds was not easy. Even when they were closer than this, the flocks were often so dense that it was impossible to enumerate them and I had to rely on estimates. Before all these counts I had made a preliminary reconnaissance of the lakes, mapping, as far as possible, the vegetation and learning the preferred feeding, loafing and sleeping areas of various species.
Closer to home, regular counts of lakes can be very useful. The results may be of value to the wildfowl counts which are organized in this country by the Wildfowl Trust, as well as by local county bird clubs. It is also worth counting any small area which can be seen easily, such as a pasture, field or estuary, especially if you visit it regularly. Over the years the results could show interesting changes.
The second method of arriving at a total number when counting birds is by estimating. This is done when direct counting is difficult or impossible due to the numbers of birds or the habitat. The operation almost invariably falls into two parts: firstly, a direct count of one or more samples is made and, secondly, a division of the flock is carried out using the size of the sample as the basic unit. For example, first count part of the flock, say ten birds, and then estimate how many groups of ten there are in the flock. The flock might be so large that the estimate will be in thousands. In order to account for any error an estimate of the largest and smallest size that the flock might be, should be given. Plenty of practice is needed and a useful aid to precision can be obtained if you photograph the flock and count the birds at leisure later and then compare this figure with your original estimate. Another problem for the unwary is that some birdwatchers tend to over-estimate the numbers of large birds and under-estimate the number of small birds. Whenever possible, it is a good idea to compare your counts with other people’s. Whenever I see a group of birds I count them. Some of the results I will send to the county bird recorder.
Exactly the same principle of estimating is used when you are counting the number of birds in any habitat. Several methods may be used. In the breeding season you can attempt to locate all the nests in a part of a habitat but this is very time consuming and more often it is better to locate the singing males on the supposition that they are holding territory and are probably nesting. To obtain an estimate for the whole of a habitat, which should have approximately the same ecological structure, you census one or more sample areas and arrive at an average for the different species within the area. Then a calculation of the total size of the population is made by multiplying the population of sample area by the number of times that the total area is divisible by this sample area. For instance, the sample area may cover 8 hectares and the total area may be 80 hectares. The total number of pairs of Blue Tits in the sample area may be 9, therefore there will be approximately 80/8 x 9 = 90 pairs. This system is commonly used for estimating game populations in the USA and some other countries.
Sometimes it is impractical or extremely difficult to count or even sample the number of birds. Therefore, bearing in mind that it is always best to give some idea of numbers, an indication of the maximum and minimum ranges can be given using a symbol for the order of size.