3.2 Choosing study areas
Where will you carry out your study? For a small population of a rare, threatened
species there may be no choice, but most studies of breeding birds are conducted
in particular areas that cover only a small part of the species’ total or regional
range. Often the study area is chosen by the researcher because it is easy to get to,
because the owner of the land is friendly, because it has many of the birds that you
wish to study or because they are easy to observe there. For many applications,
this is not a good way to choose a study area. If you want to make estimates of
breeding parameters that are typical of the population as a whole, then it would
be better to have several study areas chosen in such a way that they are represen-
tative. These might be a sample of sites chosen at random or at least from repre-
sentative habitats or regions. If your study concerns the causes and correlates of
rarity or population decline, then it may be appropriate to select study areas with
a range of population densities or recent population trends. Comparisons among
these areas may then help to identify factors that influence population status as
well as breeding performance.
3.3 Measuring the success of individual breeding attempts
3.3.1Finding and selecting nests
Studies of the success of individual breeding attempts usually involve, as the first
step, locating nests that are being used by birds. In some species with durable
nests it may be possible to find them at the end of the breeding season and assess
whether they have been used and whether they have fledged young from signs.
However, pilot studies would be required to confirm that the probability of
finding a nest was not related to its outcome and that the signs gave a reliable
indication of use and outcome.
The ideal stage at which to find nests is before egg-laying has occurred. If some
nests fail, and if the probability of failure is correlated with attributes of the envir-
onment or parents, then nests found at later stages will be a nonrandom sample
because those that fail early will have been selected out. There may even be some
pairs or females that make a single breeding attempt and then leave the study area
if it fails. To overcome this problem it is not necessary for all nests to be found
pre-laying, but it is desirable to have as high a proportion in the sample as possible.
Some species have nests that are easy to find and access, but if finding or
accessing nests is difficult, then there is a risk that you will be studying an unrep-
resentative sample which may have a different success rate and differences in
58 |Breeding biology