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All the survey forms were designed so that the data could be readily computerised. For example, coded information on species names, county codes, weather conditions, and habitat type allow detailed information to be summarised and input efficiently. Habitat information is recorded on the first reconnaissance visit to the square. The appropriate habitat codes are chosen from an established hierarchical system (Crick 1992), which is common to a range of BTO schemes. Observers record the primary and secondary habitat for each transect section with up to four levels of detail (Fig. 3). The primary habitat is the dominant habitat type within a particular transect section. In the example given, transect 1 comprises tilled land with a hedgerow without trees, an active farmyard, and the dominant crop being an autumn cereal. There is no secondary habitat within the section and so this space is left blank. Note that the ideal transect lies within 20 m of the actual route and the habitats do not differ (Fig. 2). Transect 2 is a similar area of farmland which contains a small woodland. The first habitat codes are the same as those above. The second codes are for a young coniferous plantation with low human disturbance, a moderate shrub layer, and sparse field layer. Note that the ideal route is 50 m from the actual survey route and the ideal goes through the young plantation. Figure 3. An example of a BBS habitat recording form (see text for details). Count visits are timed so that the first is in the early part of the breeding season (April to mid- May) and the second in the late part (mid-May to the end of June). It is recommended that visits should be at least four weeks apart. The average visit time is around 90 minutes. Volunteer counters are asked to begin their counts between 6 am and 7 am so that they coincide with maximum bird activity, but avoid concentrated song activity at dawn (Bibby et al. 1992). Volunteers record all the birds they see or hear as they walk methodically along their transect routes. They are encouraged to - 106 -
pause, listen and scan for birds as they walk along their transects. Birds are noted in three distance categories (within 25m, between 25-100 m, or over 100 m to either side of the line) measured at right angles to the transect line, or as in flight. Recording birds in distance bands is important because it gives a measure of bird detectability in different habitats and allows relative population density to be estimated using DISTANCE sampling (Bibby et al. 1992, Buckland et al. 1993). Note that the estimates of absolute densities derived using this method are based on a number of key assumptions. Birds are noted in the field on specially designed forms using two-letter species-codes, which were developed within the CBC (Fig. 4). Observers are encouraged to summarise this information on count summary forms as soon after the fieldwork as is convenient. Header information on the forms includes the observers' name, address, telephone number, the square reference, county code, date, whether the bird count was early or late in the season, weather conditions, and the starting and finishing times of the two halves of the transect (Fig. 4). Counties are recorded using 4-letter codes which are used across BTO schemes. Weather codes describe cloud cover, rain, wind, and visibility, on arbitrary three-point scale. We discourage bird counts in heavy rain, poor visibility, or strong winds. Figure 4. An example of a BBS field recording form and summary sheet (see text for details). - 107 -
Page 1 and 2: R. GREGORY, S. BAILLIE & R. BASHFOR
Page 3 and 4: COMMON BIRD CENSUS The Common Bird
Page 5: Survey methods BBS fieldwork involv
Page 9 and 10: The range of species recorded in 19
Page 11 and 12: Table 2. Habitat composition of 200

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