These are the BTO breeding codes. We’re focused on confirmed breeding and the codes highlighted below. Young and adult birds on the nest or young that have recently fledged and hanging out around the nest, which they will do until they are confident and competent at flying.
| Non-breeding | |
| F | Flying over |
| M | Species observed but suspected to be still on Migration |
| U | Species observed but suspected to be sUmmering non-breeder |
| Possible breeder | |
| H | Species observed in breeding season in suitable nesting Habitat |
| S | Singing male present (or breeding calls heard) in breeding season in suitable breeding habitat |
| Probable breeding | |
| P | Pair observed in suitable nesting habitat in breeding season |
| T | Permanent Territory presumed through registration of territorial behaviour (song etc) on at least two different days a week or more part at the same place or many individuals on one day |
| D | Courtship and Display (judged to be in or near potential breeding habitat; be cautious with wildfowl) |
| N | Visiting probable Nest site |
| A | Agitated behaviour or anxiety calls from adults, suggesting probable presence of nest or young nearby |
| I | Brood patch on adult examined in the hand, suggesting Incubation |
| B | Nest Building or excavating nest-hole |
| Confirmed breeding | |
| DD | Distraction-Display or injury feigning |
| UN | Used Nest or eggshells found (occupied or laid within period of survey) |
| FL | Recently FLedged young (nidicolous species) or downy young (nidifugous species). Careful consideration should be given to the likely provenance of any fledged juvenile capable of significant geographical movement. Evidence of dependency on adults (e.g. feeding) is helpful. Be cautious, even if the record comes from suitable habitat. |
| ON | Adults entering or leaving nest-site in circumstances indicating Occupied Nest (including high nests or nest holes, the contents of which can not be seem) or adults seen incubating |
| FF | Adult carrying Faecal sac or Food for young |
| NE | Nest containing Eggs |
| NY | Nest with Young seen or heard |