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Abstract from BTO Research Report No 214:

Field, R.H. & Gregory, R.D. (1998)

Habitat bias in actual and ideal transect lines in Breeding Bird Surveys 1994-1997.

Executive summary

1. The BTO/JNCC/RSPB Breeding Bird Survey (BBS) involves volunteers making bird counts in randomly selected Ordnance Survey (OS) 1-km squares using a line transect method. Within each 1-km survey square volunteers are asked to establish two parallel line transects oriented north-south or east-west. The line transects should be 500m apart and 250m from the edge of the square. The 2-km of transect are divided into two 200m sections and bird and habitat types are recorded in these units.

2. In reality, it is rare for the 'ideal' transect routes to be followed and observers are forced, or may choose, to deviate their routes. Here we assess the degree to which deviation from the 'ideal' route might bias coverage of habitats and hence bird populations.

3. Habitat surveying has been very consistent over the first four years of the BBS whilst coverage has increased by nearly 40% since 1994. Despite frequent deviation from the prescribed line transect routes, observers still surveyed the intended habitat types in nearly 90% of all transect subsections.

4. Where habitat surveyed was not that of the intended 'ideal' transect, the majority of bias was towards broadleaved woodland and away from farmland, possibly reflecting restriction of access by landowners and the proximity of woodland as alternative routes. This bias was consistent across all four years. The cause of this bias may be clarified in future years if the reasons for partial exclusion from proposed transects was reported. A smaller bias away from coastal habitats was probably a function of physical accessibility.

5. Linear and boundary feature bias was apparent in human sites, towards roads, but the situation was not clear in other habitat types since complete information was not available.

6. The biases identified are consistent across years and probably have little effect on the interpretation of BBS results.

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