11.4.2Species composition of vegetation
Detailed recording of all plant species present in quadrats can provide useful
measurements of vegetation as habitat for birds (Rotenberry 1985). One method
is to remove all the plants from each of a series of quadrats, sort them by species,
dry them, and measure their above-ground dry weight. However, although
having this level of detailed information for large numbers of quadrats can yield
useful insights into the ecology of the focal bird species, collecting it can be so
time consuming that it would only be available for a miniscule sample of the area
to be evaluated. Furthermore, although sometimes certain plant species are
critically important for nesting or providing fruit, it is usually difficult to relate the
bird abundance to the abundances of a long list of species. Furthermore, in prac-
tice the habitat structure is usually more important than species composition.
Instead, bird habitat studies usually involve more rapid measures of the cover of
dominant species and broad taxonomic groups or morphospecies (e.g. grasses) or
plant species or higher taxa that are known to provide the focal birds with impor-
tant resources that are specific to them, such as palatable leaves, seeds, or nest sites.
To gather data on vegetation composition, quadrats may be placed on a regular
grid or in random locations within the area to be assessed. Estimates of percentage
cover can be sufficiently accurate, especially if carried out by a single individual.
For species feeding on the ground it is often useful to measure bare ground.
The standard method is using quadrats (often 0.50.5 m) but larger quadrats
(11, 22, or 55 m) have the advantage of reducing the local variation.
Even more rapid, but rougher, assessments can be made using a sighting tube.
A researcher looking down into a 50-mm length of 30-mm diameter plastic pipe
fixed vertically to a holder on a belt around the waist height sees a circular area of
about 10 cm diameter on the ground. The vegetation cover of this circle can be
rapidly assigned to a category and the appropriate box ticked on a recording form.
It is important to score at least 10 (and preferably about 30) circles at each
sampling place, but this approach often yields more accurate measurements per
unit fieldwork time than making a detailed assessment of a single quadrat. The
researcher can walk rapidly between sampling points on a regular grid or transect
with the sampling places being located by pacing or use of a GPS.
11.4.3Vegetation architecture
The height, structure, and density of vegetation often affects birds by providing
perches or cover and by limiting the bird’s field of view and ability to run or fly to
capture prey. The height of ground layer vegetation is a useful measure, but the
maximum height is often unsatisfactory, because a single flowering grass stalk
258 |Habitat assessment