to go into these methods, a range of approaches is available. The simplest of
these, which is known as the Lincoln index (or Petersen method) assumes one
capture and one recapture (or re-sighting) event only, and that the population is
closed. The calculations for this model are essentially those described above.
More complex models allow for multiple capture (re-sighting) events, and for
open populations. The latter types of model, generally known as Jolly-Seber
models, provide information on both population size and survival rates. Further
information on these models is given in Chapter 5.
The principle behind standard effort captureis that populations of birds can
be reliably monitored by capture methods if capture effort is kept constant over
time, and done at the same season each year. Several programs for monitoring
birds with this method exist, but perhaps the best known is the Constant
Effort Sites scheme of the British Trust for Ornithology (Peach et al. 1996,
http://www.bto.org/ringing/ringinfo/ces/index.htm),,) which is being followed by an
increasing number of European countries. The Monitoring Avian Productivity
and Survival (MAPS: http://www.birdpop.org/maps.htm)) program is a similar initiat-
ive in North America.
Catch per unit effort data can be used to:
- Monitor population trends of adult birds, based on the numbers caught.
- Estimate absolute population size using the capture-mark-recapture
methods outlined above. - Monitor changes in productivity using the ratio of juveniles to adults
caught late in the season. - Estimate adult survival rates from between-year re-traps of ringed (banded)
birds (see Chapter 5).
For the Constant Effort Sites scheme, the capture method involves placing
the same types (e.g. mesh size) and lengths of mist nets (see Chapter 4), in the
same positions, for the same length of time (about 6 h per visit) over a series of
12 visits during May to August. These methods are held constant from year to
year. All birds caught are identified, aged, and sexed, and all un-ringed birds are
ringed. While it might be tempting to vary net lengths from visit to visit, particu-
larly if the number of fieldworkers varies from visit to visit, this could influence
the catches. Simply calculating the number of birds per 10 m of net is insuffi-
cient, because doubling net lengths does not necessarily double the number of
birds caught. Similarly, catching for twice as long with half the length of nets on
some visits is not advised as capture success may vary with time of day.
Constant effort ringing is commonly used in dense habitats (scrub, reed beds,
undergrowth, etc), but it can also be used in forest canopies, with nets raised high
Field methods| 49