recorded (for cryptic and shy species this may not be true), and that birds do not
move in response to the observer prior to detection.
2.3.8 Colonial birds
Around 15% of bird species nest in colonies, either on cliffs, in trees, on the
ground, in caves or in burrows. In some ways, this makes them easy to count,
since birds are concentrated in generally conspicuous aggregations. However,
counting birds in colonies also poses problems:
- Numbers may be huge, making counting difficult; it may be necessary to
sample parts of the colony (using strategies described above) and extrapolate. - Breeding may not be synchronous. At any time, part of the population
might be elsewhere, and the birds present on the second visit might not
necessarily be those present on the first; individual marking of birds may be
necessary. - There may be large numbers of non-breeders or “helpers” present, or birds
might be absent from the colony for long periods; it may be better to count
nests rather than individuals. - Old nests might appear to be active; it might be advisable to count
apparently active or occupied nests only. - Colony attendance might vary greatly during the day and over the year; it
may be necessary to make a number of counts at different times.
A critical step is to decide what it is that you want to count. Is it the total number
of birds present, the number of breeding pairs, the number of apparently active
nests, or the number of occupied burrows? This decision will help to determine
the count method used.
Counts of large colonies often involve breaking the colony down into smaller
units for ease of counting. In the case of cliff colonies, photographs can be used
to divide the cliff into counting units, or even to count the birds directly.
Cliff colonies should always be counted from opposite the colony rather than
from above when nests are more easily missed. Aerial photography has been used
to estimate numbers of large colonial birds, such as Gannets Morus bassanus.
Tree-nesting colonies can be counted in a similar fashion, with nests in either all
trees being counted or just a sample of trees. Large colonies of ground-nesting
birds can be subdivided into smaller counting units by using a grid system
marked out with string. The counters can then visit all, or a random stratified or
regular sample of grid squares. Alternatively, densities of nests can be estimated
usingdistance sampling(see above) and extrapolated for total colony area.
Burrow-nesting seabirds are particularly difficult to count, many of them return
44 |Bird census and survey techniques