vegetation or sky is recorded. Thus, if 37 out of 100 points have vegetation, then
the cover is 37%. This method is unbiased but requires a considerable number of
sample points for a reasonable estimate of cover. Using a spherical densiometer
involves looking at the reflection of the canopy in a curved mirror and recording
the number of points on a grid that are covered by vegetation. It has the advant-
age that a number of data points can be obtained from one location thus speed-
ing up data collection but as many of the points are not vertical they are thus
more likely to hit some vegetation and thus overestimate cover.
Recent developments in airborne remote sensing make the rapid measurement
of vegetation architecture possible over large areas. Airborne laser scanning (also
called LIDAR) uses laser reflections to make a detailed map of the height profile
of woodland and scrub which provides valuable habitat measures for woodland
birds (Hinsley et al. 2002). The method can also be used to estimate the height
of agricultural crops less than a meter high (Davenport et al. 2000). More subtle
attributes of woodland vegetation architecture can also be estimated using this
technique (Lefsky et al. 2002).
Summary statistics can be used to describe the diversity of the vertical distribu-
tion of foliage. The Shannon–Weiner information statistic, which is frequently used
as a measure of species diversity, can be used to estimate foliage height diversity.
11.5 Quantifying habitat selection
Habitat selection can be studied in several ways depending on the type of data
available. Commonly used approaches include the following.
11.5.1 Comparing the relative abundance of birds or records
of tracked birds in each of several habitats with the relative
areas of the habitats available
A key feature of this type of analysis is that every bird record can be attributed to
a habitat. If this is possible then the selection or preference shown can be assessed.
Underlying the concept of habitat selection is the idea that the number of birds
or records per unit area, that is, density, varies among the habitats present within
a particular region in a way that reflects the birds’ preferences for using some
habitats over others. The number of birds in each habitat reflects habitat utiliza-
tion. However, differences in utilization do not imply differences in selection or
preference because a rare preferred habitat may not be utilized as much as an
abundant, less preferred habitat. Hence, measures of habitat selection have to
take into account how much of each kind of habitat is available to the birds.
Similar principles apply when analyzing the number of records obtained in
260 |Habitat assessment