When counting seabirds, care must be taken or much damage can be done. For example, if parent birds are disturbed in cold, wet or windy conditions eggs and young birds can become chilled and may not survive. When the parents leave, nests are vulnerable to attack from gulls and other predators which come in looking for food.
There is an example of human disturbance on the Lamb in 1969 which resulted in no young cormorants being fledged there that year.
Counting seabirds accurately takes some practice – especially from a moving boat – and serving an “apprenticeship” when a new, would-be counter matches his counts with those of a more experienced counter, is necessary.
Unlike, say, a blackbird’s nest which can be examined to see whether there are eggs or
chicks or, if a bird is sitting in the nest it can reasonably be inferred that a breeding
attempt is going on, seabirds present a different kind of challenge. It may be difficult
or impossible to see whether a bird has an egg and some adults may be on ledges but not
breeding at all. Methods of counting have been determined which give the most reliable
results with which to compare one colony, or one year, with another.
There are three main methods of counting seabirds:
Counting individual birds:
We find a convenient, safe, point where we have a good view of the area to be counted. We then split the colony into distinct sections, using natural features or convenient large gaps between clusters of nests. A count is then made of all nesting birds in that section. Once we agree on a figure, we move on to the next section and count it. Finally, we add up all the figures to get a total for each species.
It makes it easier when two people count together, as one will sometimes spot a bird that the other has missed. There are often arguments as to whether part-built nests are judged substantial enough to be counted!
For most species we count the number of nests, or nest sites - whether the adults are
present or not. However, when we count guillemot we count the number of birds present
on the breeding ledges. These counting units have been found to give the most reliable
picture of breeding bird populations.
Counting part of the colony, and then estimating the total:
The breeding habits of some species make them very difficult to count. Puffins, for example, nest underground in burrows. The density of the burrows may vary across the colony and not all burrows will be occupied.
To count these colonies certain areas will be chosen and each burrow will be examined. Those that are thought to be occupied will be counted. From these sample areas, an estimate of numbers of occupied burrows can be made for the whole colony. Obviously, a lot of thought needs to be put into selecting the areas of the colony to be counted so that they are representative of the whole colony. All factors which affect the breeding density, such as type of terrain, soil type and vegetation must be taken into account.
Counting from photographs:
Some species are very difficult to count in the field, simply because there is no suitable vantage point, and moving through the colony is not practical. If clear photographs can be taken from a distance, these can then be examined later and birds counted using either of the two methods described above.
The Gannets on the Bass Rock are counted this way approximately once every ten years. Aerial photographs are taken and from these an estimate of the number of breeding pairs is made.