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

The final, and arguably most important, aspect of the new scheme was the formal sampling
strategy. Our trials, and similar experience with national surveys of single-species (Shrubb & Lack
1991, Donald & Evans 1995), have demonstrated the feasibility of a random sampling design in the
UK. The advantages of this are considerable as it provides the only mechanism for producing what
is demonstably unbiased and thus representative information (Bibby et al. 1992). A desk-based
study assessed a number of different strategies for the selection of survey squares (Gregory &
Baillie 1994, 2004). These simulations showed that simple random sampling compared very well
with any of the more complicated strategies involving landscape type. It was concluded that a
random selection, stratified by potential observer density, should be used for the new survey.
BREEDING BIRD SURVEY
Against this background, an annual survey of terrestrial breeding birds was introduced in the
UK in 1994 (http://www.bto.org/survey/bbs.htm). The strategic aims of the BBS are:
1) to be based on a formal randomised sampling design,
2) to improve the geographical representation of monitoring,
3) to improve the habitat representation of monitoring, and
4) to increase species coverage of monitoring (as a consequence of the other points).
The BBS thus aims to provide precise information on year-to-year and longer term changes in
population levels for a broad spectrum of common species across a range of regions and habitats.
The parallel recording of land use and habitat change will facilitate a better understanding of the
factors responsible for population changes, and this will be particularly important for populations in
decline. In a wider context, the BBS aims to promote a greater understanding of the population
biology of British birds through a partnership between large numbers of voluteer and a small
number of professional ornithologists.
Selection of survey squares
Survey squares were selected as a random sample, stratified by potential observer density,
from within regions across the UK (Gregory & Baillie 1994). The 128 BTO regions were reduced
to 83 regions by the amalgamation of the smallest ones. Sampling from fewer, larger regions avoids
the problem of extremely small administrative regions. The number of squares to be surveyed in
each region was calculated by multiplying observer desity by a constant and setting a minimum
level of coverage. We anticipate that professional will be necessary to ensure minimum coverage in
remote areas.
At a local level squares are allocated to volunteers through a network of voluntary regional
organisers. Each organiser receives a list of target squares for their region with the instruction that
squares should be allocated in strict order from the top downwards. This is essential in maintaining
the random design of the survey. The same squares are surveyed year after year and new volunteers
are found if the original volunteer drops out. The way in which observer changes are handled in the
analysis will be addressed once sufficient data have been gathered. Regional organisers play a vital
role in coordinating fieldwork and overseeing the return of data. In limited circumstances, they can
report squares as being uncoverable, for example, because a landowner refuses access permission or
the square is complelely inaccessible. In such cases, the square can effectively be ignored, although
organisers submit information on land use and habitat type to check for any bias introduced by their
exclusion.
- 104 -