might be one of several predefined habitat types, patch sizes, management
histories, or whatever is the target of study. Sample locations are then selected at
random within each stratum.
One problem with the random selection of sample locations is that the overall
distribution of effort may well look geographically patchy. While random points
are the best way to get a good overall result, they may not give the best chance of
picking up any geographic patterns of potential interest. A common way round
this is to use a systematic selection, visiting plots uniformly distributed on a
grid every kilometer or hundred kilometers or whatever scale is appropriate.
Hybrids between systematic and random are possible too. It might be sensible to
visit every 100-km square to get a good spatial spread but to pick actual sample
points within each at random.
Another difficulty with random selections can be cost and access. In large and
remote areas the time and cost in getting even a short distance away from a road
could be considerable. What is good from a statistical viewpoint can look ridicu-
lously impractical. For this reason, expedience often plays a large part in select-
ing where to go in the field. The best advice that can be given is to acknowledge
and understand what is going on. What makes for easy access for an ornitholo-
gist does the same for other people too. Species that are hunted or trapped
may be scarcer or absent in more accessible areas, which will cause those species
to be underestimated if only the accessible areas are sampled. Many other edge
effects are possible. If sampling does not penetrate very far from edges, then
forest interior species will be underestimated.
1.3 Finding the birds
Finding all the species in an area depends primarily on good bird-watching skills.
The observer needs to get to the right place at the right time and to identify every
bird species there. Fast and accurate identification is essential. Few people can
identify absolutely everything from a brief sight or sound but if more than about
10% of contacts are unidentified, you need to improve your identification skills
to collect worthwhile data. In an unfamiliar area, especially in the tropics, this
might take several days, but they are days well spent. Most detections will be by
ear, but to begin with you may need to see the bird to identify it. Help from
a local expert can be invaluable and increasingly it is possible to get sound record-
ings to practice with. Observers can work in pairs and compare notes as to what
they are recording to see that identification is consistent and everyone is up
to standard before serious data collection begins. Sounds can be recorded and
brought back for subsequent expert identification.
4 |Bird diversity survey methods