monitoring breeding birds in the united kingdom - European Bird ...

COMMON BIRD CENSUS
The Common Bird Census (CBC) has been the main monitoring tool for common birds in the
UK over the last 30 years. The CBC is based on territory mapping and is one of the longest running
schemes of its kind in Europe, having been established in the early 1960s (Marchant et al. 1990). It
has proved highly valuable in advancing our knowledge and understanding of population
fluctuations among British birds (e.g. O'Connor & Shrubb 1986, Marchant et al. 1990, Baillie 1990,
Greenwood & Baillie 1991, Peach et al. 1991, Fuller et al. 1995, Greenwood et al. 1995). There is
general consensus that territory mapping provides a reasonable estimate of most territorial
populations (Dawson 1981, Verner 1985, Verner & Ritter 1988, Bibby et al. 1992) and the aims
and methods of the CBC have been reviewed on a periodic basis (O'Connor & Fuller 1984,
Marchant et al. 1990, Baillie & Marchant 1992).
Despite the considerable achievement of the CBC, the scheme has a number of limitations and
the BTO has been exploring alternative methods for population monitoring for some time (Baillie &
Marchant 1992, Gregory et al. 1994). The limitations of the CBC are as follows:
1. Survey sites are not chosen at random. In consequence, there is the potential for species,
geographical and habitat bias. In fact, the CBC has been restricted to farmland and
woodland, and other habitats have been excluded. The distribution of survey plots mirrors
the distribution of the human population and is baised towards the south and east of the
UK.
2. Fieldwork, data compilation and mapping analysis are extremely time consuming (Gregory
et al. 1994). Fieldwork involves nine or ten visits to a census plot each breeding season and
many hours in data collation. Note that in Britain territory analysis is carried out by BTO
staff rather than by volunteers, as is the case in other countries (van Dijk 1992). The time
required for territory mapping fieldwork and analysis is roughly seven times that of an
equivalent technique, such as point counts (Gregory et al. 1994).
Taken together, these factors have limited the expansion of CBC coverage in the UK and to
some extent limited the use of the data, although much can be done at the analytical stage to
alleviate problems (Greenwood et al. 1995). Over a number of years the BTO has tested a number
of new approaches to population monitoring in a UK setting (Gregory & Baillie 2004). The aim is
to develop a monitoring scheme based on a formal sampling strategy for volunteer birdwatchers
which uses simple and efficient methods and thus allows large numbers of people to participate
over a wide geographical area.
ALTERNATIVES TO TERRITORY MAPPING
Line and point-count transects
The obvious alternatives to territory mapping are line or point count transects, both of which
are relatively efficient and have been used widely in population monitoring in Europe and North
America (Bibby et al. 1992). A number of field trials have tested the suitability of these methods
(Gregory et al. 1994, Gregory & Baillie 2004). They showed the comparability of monitoring
results derived from different counting methods, the efficiency of the methods, and the
practicability of using randomly chosen survey sites in the UK. A line transect approach was
recommended because it was preferred by volunteers and was slightly more precise. Furthermore,
line transects can be applied effetively in a wider range of habitats than point counts (Bibby
et al. 1992).
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