Counting Cliff-Nesting Birds
The counting of seabirds on nesting cliffs in large colonies can be very difficult and, for the fool-hardy, dangerous. It might be quite simple to count the number of individual birds on some cliffs, but that figure does not tell you the numbers that are really nesting there, for you cannot always see the eggs or young birds. Clare Lloyd, who has counted Razorbills and Guillemots for some years in the past, has pointed out some of the hazards which could lead to errors in the results. For example, there can be considerable variations in the number of auks visiting the colonies at different times of the day. The numbers are least variable just before the young hatch, particularly in June and she recommends that five to ten counts should be undertaken at this time, as near mid-day as possible, when the error is reduced to between 5% and 17% for Razorbills and 4% to 8% for Guillemots.
It is not always possible to see from one point the whole of a very extensive tern or gull colony on flat ground. In these colonies it is possible to count a sample area and estimate the total from this. However, these colonies should not be disturbed unless it is for an important scientific reason and then only by an ornithologist who knows what he is doing.
Counting birds on cliffs poses yet more problems. Unless it is a very small colony you will probably not be able to see all the birds at once. You, therefore, have to move your position to get a new angle on the colony and have the added difficulty of determining whether you are looking at the edge of the section you have just counted, or, indeed, whether you have missed some. Counting from a boat is not easy since, more often than not, it moves up and down! A photograph of a cliff from a boat or land and then a later count can help too, but, if you have a mixed colony, identification is virtually impossible.
When counting such colonies, cameras that produce an instant print might well be invaluable in providing a basis for a sketch on which you can mark the ledges you have been counting. Using your print or sketch you would be able to number the ledges and mark in the count for each ledge.