Definitions

RBBP recommends the adoption of standard terminology for the recording of breeding birds. Details of all breeding categories are given below.

Breeding evidence

The basis for the definitions of ‘Confirmed breeding’, ‘Probable breeding’ and ‘Possible breeding’ is the recommendations of the European Bird Census Council (EBCC) (Hagemeijer & Blair 1997), but for rare breeding species the precise definitions vary according to more detailed criteria which are defined elsewhere on this website.

Confirmed breeding always means that there is evidence that a pair has laid eggs: this could be a nest with eggs or young, recently fledged young or broods of still dependent chicks (e.g. ducks and waders), an occupied nest where the contents cannot be seen, adults carrying food to a nest, intense distraction display indicating that there is a nest nearby (usually one with young; e.g. some waders), or a recently used nest. It is important to note that confirmed breeding is not the same as successful breeding; nests that fail with eggs or with young still fall into the confirmed category. A successful breeding pair is one that fledges at least one young bird from a nesting attempt.

Probable breeding is usually based on the presence of a pair in suitable breeding habitat (but excluding birds known to be on passage, where the birds migrate in pairs (e.g. Garganey)) or a territory being maintained for at least five days. Other evidence supporting probable breeding includes nest-building and visiting a probable nest site.

Possible breeding is normally defined as at least one adult bird in suitable breeding habitat or a bird in song. This definition works well for common and widespread species, where it is likely that there will be a partner or another breeding territory in the vicinity. For rare breeding birds though, the likelihood of that is reduced, so unless pairs are present or a territory is being held for at least five days, RBBP classes these records as Possible breeding. For some species which occur at low density and are prone to being under-recorded, such as Willow Tits and Lesser Spotted Woodpeckers, a single record of one individual in breeding habitat during the breeding season for that species (typically April to July but including February and March for resident, sedentary species) is also classed as Possible breeding.

A special category of Possible breeding is a count of Singing males, which is used for species where the male can be obvious when in song but females tend to remain hidden. For rare passerines, where males can sing from suitable habitat for only short periods while still on migration, we will collect and report data only where the individual remains on site for at least five days.

Breeding status

Rare breeding birds are classified into five categories depending on whether breeding in the UK occurs regularly (most years), less frequently, or has not yet been recorded.

A regular breeder is defined as a species that has bred (i.e. confirmed breeding has been recorded) at least once in the UK and which has bred (or was strongly suspected to have bred) for any five consecutive years within the last 25 (unless the last breeding was more than ten years ago). This definition is the same as used in the Birds of Conservation Concern (BoCC) reviews (Eaton et al. 2015). Regular breeders are further classified according to their population size, based on a 5-year mean (the mean maximum population size calculated for the most recent five years for which data are available):

• Very rare (mean of <30 breeding pairs (bp) per annum) – e.g. Pintail, Wood Sandpiper, Montagu’s Harrier, Savi’s Warbler.
• Rare (30–300 bp per annum) – e.g. Garganey, Black-necked Grebe, Osprey, Black Redstart.
• Scarce (301–1,000 bp per annum) – Common Quail, Common Pochard, Little Ringed Plover, Bearded Tit.
• Less scarce (>1,000 bp per annum) – e.g. Shoveler, Turtle Dove, Little Egret, Woodlark.

An occasional breeder is one which has bred at least once in the UK but is not a regular breeder.

A potential breeder is one which has not bred previously in the UK but, in some years, shows signs that it may do so, e.g. presence of singing males holding territory or pairs in suitable breeding habitat.

A colonising breeder is a new colonist which first bred in the UK in the last five years (i.e. the most recent five including the year being reported on) or subsequently, if known at the time of publication, or one which may have occasionally bred in the past but for which breeding now appears to be becoming more regular.

A former breeder is one which bred regularly in the past but for which there has been no confirmed breeding record in the last ten years.

Published estimates used by RBBP are either the RBBP 5-year mean or, where there has been a recent published national survey, this is used and referenced. Note that if the annual RBBP coverage is poor, the best available national population estimate is used. The unit varies, but is most frequently ‘breeding pairs’ (bp). We acknowledge that, for some species, estimates based purely on RBBP data may be contrary to other estimates, especially where RBBP coverage is moderate or low.

Coverage categories

The degree of coverage (of the total population of a species in the UK) in any particular year is classified as follows:

• Near-complete – RBBP reports present more or less complete annual totals;
• High – a good estimate of the number of pairs breeding annually, though an unknown (but thought to be small) proportion has not been recorded/reported;
• Moderate – a less accurate estimate of the number of pairs breeding annually, which is nonetheless thought to be a significant proportion of the total population;
• Low – the volume of the data received is such a small proportion of the total population that RBBP totals are of little value for conservation or status reviews.

The coverage classification for a species is based on a comparison between the 5-year mean and the most reliable population estimate, where possible, taking into account known factors in the monitoring and detectability of the species. Note that for those species where we only achieve low coverage, maintaining an archive of known sites is nevertheless useful, and this information can be used in the design of future targeted surveys. We strive to improve coverage of each species through encouraging field recording and surveys in less well-covered areas.

Mixed pairs and hybrids

Records of mixed pairs, where one of the adults is a species or race on the RBBP list, are collated and reported on by the RBBP. We do not, however, routinely collect breeding records of hybrid individuals breeding.