above the ground using pulleys or telescopic poles. Because some dense habitats,
such as scrub and reed bed, can be successional, care needs to be taken to ensure
that population trends reflect real changes in bird numbers rather than local
habitat change around the nets. Although the Constant Effort Sites scheme uses
mist nets, any accepted capture technique (Chapter 4) can be used, providing
that effort is standardized (same number of traps, places, time periods, etc).
As a general survey or monitoring tool, catch per unit effort has some limi-
tations, such as requiring specialist equipment and training and thus being
expensive to maintain.
2.3.13 Tape playback
Some species of bird are particularly difficult to see or hear. Examples of such
species are those that have skulking behavior, live in dense habitats, are nocturnal
or crepuscular or nest down burrows. The probability of detecting these species
can sometimes be increased by the use of tape playback, in which the taped call
or song of a bird is played, and a response listened for. Recordings of the calls and
songs of many species are now commercially available, and can be copied to tape.
Ideally, use a tape loop, so that a short length of call can be repeated continuously
for as long as is required. The call can be broadcast from a simple hand-held
loudspeaker but care is needed to keep disturbance to a minimum and not to
affect the bird’s natural behavior.
The results from census work involving tape playback need careful interpreta-
tion. If the aim is simply to determine whether a given species is present in an
area, then tape playback may simply increase the chance of finding it. If, however,
the aim is to estimate population size or to produce a population index, then
more care is needed. To generate a reliable population index, the probability of
birds responding to the tape needs to be held as constant as possible. This can be
helped, for example, by standardizing the manner in which the tape is played
(same volume, recording, playback length, time of day, season, etc), and ensur-
ing that the tape is not played to any one individual too frequently, causing it to
habituate and respond less frequently. Tape playback has been used widely for
monitoring populations of marsh birds, owls and raptors (Gibbons et al. 1996;
Newton et al. 2002; Lor and Malecki 2002).
Estimating absolute population size from tape playback is more complex, as the
probability of the average bird in the population responding to playback needs to
be known. Frequently, detailed additional work will be required to determine
response probabilities. Such work has been undertaken on owls and nocturnal
burrow-nesting seabirds. For example, Brooke (1978) has shown that responses
to playback of their call were obtained only from half of all occupied Manx
50 |Bird census and survey techniques