Estimation of the numbers of smaller nocturnal migrants is particularly
difficult, but considerable progress has been made in this field (www.birds.
cornell.edu/brp). Many smaller migrants call as they migrate, allowing at least
minimum numbers to be assessed and species to be identified. Recently developed
methods use microphones and complex computer programs to try to estimate
total numbers of calling birds passing, as well as their height and speed (Evans and
Rosenberg 2000, http://www.birds.cornell.edu/brp)..) Radar has been used to not only
detect passing flocks, but also to estimate their numbers, direction of flight, speed,
altitude, and even wing beat rate, but not their specific identity. This method
requires access to extremely sophisticated, and usually militarily sensitive equip-
ment and is generally beyond the reach of most researchers. Counts of migrants
passing in front of the moon, or passing through the beams of bright lights, are of
limited use, because only a small proportion of birds can be seen and most cannot
be identified. A further indirect method of measuring changes in numbers of
migrants, although not the absolute numbers, is ringing (banding), and a high
proportion of ringing effort is concentrated at migration stopover points (Dunn
et al. 1997). These methods are described in detail in Chapter 7.
2.3.12 Capture techniques
Because most species of bird tend to be visible and vocal, methods to survey them
generally rely on observers seeing or hearing them. Occasionally, however, this
may not be the case, as in species that live in dense undergrowth, or in the forest
canopy, which may be rarely seen or heard. Under such circumstances, one way
to census them is to catch them using mist nets. Capture techniques have been
widely used in the tropics where they can be usefully combined with other census
methods (e.g. Whitman et al. 1997). Broadly, two separate approaches can
be used; either capture-mark-recapture(also known as mark-release-recapture,
MRR) which allows estimations of population size, or catch per unit effortwhich
can be used to produce population indices.
Capture methods can be time consuming and require substantial training
to develop the skills necessary to catch, handle, and mark birds. The safety and
welfare of the birds are always of paramount importance. In many countries,
these techniques are licensed, and anyone considering using them should
apply to the relevant authority well in advance. As we have seen in the previous
chapter, mist netting is a relatively poor method for surveying birds. Further
information on methods of capture and marking are given in Chapter 4. Despite
these disadvantages, capture techniques yield much information besides popu-
lation size and trend estimation. In particular, they can provide valuable infor-
mation on demographic parameters, such as survival and breeding success, in
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