Survey effort and timing
Getting the level of survey effort and the number of repeat visits correct is difficult and is likely to be dependent on a range of site-specific factors. Clearly a single visit is unlikely to provide a robust level of information whilst the more visits undertaken the more species you record, until with the law of diminishing returns the species detection curve flattens out.
As standard it is recommended that six bird survey visits be undertaken as part of a survey for breeding birds.
Six visits is considered sufficiently robust to identify over 90% of bird species using lowland deciduous woodland in the breeding season and establish a good understanding of the numbers and distribution of species present. Lowland deciduous woodland is one of the most complex habitats to survey, due to the range of bird species it can support, and the dense vegetation leading to a heavy reliance on vocal encounters. Six visits is therefore considered to be a proportionate survey effort for all terrestrial and freshwater habitats. As species vary in their detectability through the day, at least one of the six visits should be in the evening to pick up species not readily recorded by conventional surveys early in the morning.
As long as a detailed and robust justification is given, consideration could be given to circumstances where an increase or decrease in the number of surveys may be appropriate. Additional survey effort may need to be considered for large-scale projects with the potential to have significant impacts on birds, and/or for high profile, sensitive projects. On the other hand, fewer survey visits may be justified for projects with very limited impacts, or sites with habitats of low value for birds.
Additional, or species-specific surveys, should also be considered based on the above factors and, for particularly large sites, may require several surveyors. Should the number of breeding season survey visits exceed six, consider an early (March) and late season survey visit (August – September).
For wetland/coastal sites, survey design should be built around peak counts of priority species and therefore wintering or passage season surveys may be more appropriate than a breeding bird survey.
Survey timing and seasonality
The bird breeding season is generally acknowledged to occur from late February to early August inclusive. As a general framework breeding bird survey visits should be spread evenly between March and early July to ensure that the survey covers resident breeders which start breeding early and migrant breeders which arrive later are equally covered. Visits in February and early March may also be required if there is potential for early nesting species to be present at the site (see Species-specific surveys section)
However, within this general framework careful consideration should be given to the timing and seasonality of survey work during the bird breeding and this will partly be informed by desk study and scoping.
Factors to consider:
- How has the weather been – has there been a mild winter and good weather in spring so that the resident species have started nesting early. Or has the weather been really bad during the spring and is this likely to have delayed the arrival of migrant breeding species.
- Does your site have seasonably constrained features – such as temporary pools that might only be present early in the season or in wet years. If your survey coincides with a dry year you may underestimate the importance of a site or miss important birds such as lowland waders.
- Do you need data from more than one breeding season?
- Do you have suitable habitat or other reason to suspect that you might have early or late nesting species – for example hobby are a migrant breeder and tend to breed later in the breeding season therefore a visit(s) in August/September may be required, whilst goshawk nest early and are best detected in February when they are undertaking aerial display flights.
- Do you have reason to suspect species that are more readily detected by nocturnal surveys such as barn owl or nightjar?