Status Categories
The Density status of all birds on the BOS Systematic List has been assessed since the 1960s using the BOS records and a statistical grouping. This provides a standardised way to assign a status, allowing changes in status to be assessed over time.
The Occurrence of the species is assigned based on its breeding or passage status. For some species, a distribution term is also provided as a further clarification of its status.
Some species fall into both categories (breeding and passage), so for them an additional set of statuses is provided, with the primary status given first.
Numerical Density
The numerical density status is a broad measure of the abundance of the species within the BOS, enabling a comparative assessment between species to be assessed. The density status is assessed using two scales – one for non-breeding species and one for breeding species, as follows:
-
Non-breeding occurrence is one of: Over-wintering, Winter visitor, Passage migrant, Casual visitor, Regular visitor, Vagrant
-
Breeding occurrence is one of: Resident or Summer visitor
Non-breeding Species Density
This classification applies to species which only occur as visitors during the winter period, as passage migrants or as vagrants to the BOS area. The occurrence of a number of birds together in one locality on one day is termed a unit record. However, when the same individuals are reported on many occasions the number of localities is used as the unit record. A locality is usually defined as a 1km square or water, or exceptionally, a site such as the Mid-Cherwell Valley. The number of different localities/records in which a species was recorded during a ten-year period is then given a status term according to its place on the scale below.
| Status Term | Records / Localities |
| Rare | 1-2 |
| Very occasional | 3-9 |
| Occasional | 10-29 |
| Fairly frequent | 30-99 |
| Frequent | 100-299 |
| Very frequent | 300+ |
Breeding Species Density
The breeding status of residents and summer migrants is determined by one or more of three methods:
-
Breeding density established by a survey such as the SRSS
-
Counting the number of records occurring during the breeding season
-
Counting the number of occupied 1km squares during the breeding season
| Status Term |
Method 1 |
Method 2 |
Method 3 |
| Rare | 0 - 0.1 | 0 - 4 | 1 – 4 |
| Scarce | 0.1 - 0.3 | 5 – 24 | 6 – 24 |
| Not scarce | 0.3 – 3.0 | 25 – 124 | 25 – 124 |
| Fairly numerous | 3.0 – 30 | 125 – 624 | 125 – 499 |
| Numerous | 30 – 300 | 625 – 3,124 | 500 – 999 |
| Abundant | 300+ | 3,125+ | 1,000 – 1,200 |
Occurrence Status
The occurrence status is based on it being a resident or when it is a visitor.
| Occurrence | Term Definition |
| Resident | Present as a breeding species and recorded throughout the year but not necessarily as the same individuals. |
| Summer visitor | Species which arrive in the spring, breed in the area and depart before winter. |
| Over-wintering | Once regarded as summer visitors, some species can now be seen all year round, with wintering birds thought to be mostly visitors from central Europe rather than British breeders that have stayed on. |
| Winter visitor | Species which visit the area during the winter period but breed elsewhere. |
| Passage migrant | Species which visit the area only when moving to some other area, normally on passage from winter quarters to breeding grounds or vice versa. |
| Casual visitor | Species which have occurred in less than 50% of the last 10 years and which can occur in most months of the year. |
| Regular visitor | Species which have occurred in more than 50% of the last 10 years and can occur in most months of the year but do not breed in the area. |
| Vagrant | Passage migrants off their normal route, wanderers or species which are present for some unknown reason. |
| Escape | Birds which are thought to have escaped from private collections. |
| pre 1952 only | Only recorded prior to the society's formation in 1952. These are verified sightings, as detailed in the various BOS 10 year reports. |
Distribution Status
| Distribution Term | Area/Density Ratio |
| Extensive | 10+ records per 100 sq. km |
| Very widespread | 5 - 9 records per 100 sq. km |
| Widespread | 1 - 4 records per 100 sq. km |
| Local | Recorded in 5+ 10km squares |
| Restricted | Recorded in 1-5 10km squares |