requires a high level of experience and care and effort to visit nets frequently.
In pure bird survey terms, the return is poor in relation to the effort required, but
there might, of course, be other reasons for catching birds and the bird in the
hand may be needed for reliable identification.
1.4 Standardizing the effort by time and space
The most obvious source of bias in multispecies surveys is the variation of effort
put into different sampling units. This can be standardized in several ways. The
simplest is time. Species lists or counts are accumulated for a fixed period and when
this has elapsed a new one is started. The hour is a commonly used and practical
unit. It is a simple quantum of effort and many can be completed in any single
field session. It is common to collect data from dawn to the point in the morning
when bird activity subsides, and there may be another good period in the late
afternoon. The day is another commonly used unit. If several observers are
involved and perhaps other work is being done, it is convenient to be able to tally
a day’s records every evening. The length of a day is often less explicit. Daylength
varies, as does the duration and intensity of the quiet patch in the middle. Also,
the longer the recording period, the less likely it is that observers are working at
full effort to cover more ground and find more species.
Smaller time units can be used. This approach makes particular sense if
the search area is also constrained. The second Australian atlas used 20 min within
2 ha, which is equivalent to a circle of radius about 80 m. This makes each list
rather like a point count of all the birds within detection range from a fixed point.
The critical difference is that you walk around to try to flush or otherwise locate
birds that you would not necessarily find by standing still for 20 min. Hewish
and Loyn (1989) found that this method particularly appealed to observers.
It overcomes a frustration of point counts of knowing, from what you see while
walking, that there are birds out there that you are missing while keeping still for
the actual count. Two hectares is a practical minimum area to use and is roughly
equivalent to a point with a search of the area around it. Twenty minutes is a
plausible time to search this much ground in an average habitat. At a slow
walking pace you might cover some 500 m in this time and thus get to within
20 or 30 m of everywhere in the imagined plot.
If area is to be constrained, then shorter time periods go with smaller areas.
An advantage of this approach is to generate results with finer spatial resolution,
if that is important for the study aim. Also, more lists can be collected in a given
time, enabling frequencies to be measured with more precision. The disadvantage
is that more time is needed for subsequent data handling and writing.
6 |Bird diversity survey methods