Understanding Bird Breeding Evidence Codes
A plain-language guide to bird breeding evidence codes used in eBird and breeding bird atlases: what the categories mean, how to choose the right code and how to observe nesting birds responsibly on the prairie.
Contents
Breeding evidence codes are short labels that birders attach to a sighting to record how strongly a bird's behavior suggests it is nesting nearby. Breeding bird atlases and eBird checklists use them. Each code belongs to one of four levels of certainty: observed, possible, probable or confirmed. A bird singing in suitable habitat counts as weak evidence. An adult carrying food to hidden chicks counts as proof. The code records which behavior you saw, so other people can judge how much weight the sighting deserves.
Why the codes matter
A plain species list shows that a bird was present. It does not show whether the bird was raising young there or only passing through. On open prairie and around wetlands such as those near Medicine Lake, many birds are migrating, wandering or feeding far from their nests. Breeding codes separate those birds from local breeders. When many observers report codes over time, atlas projects can map where species actually nest, see how breeding ranges shift and identify habitats that support reproduction rather than brief stopovers.
The four categories of breeding evidence
| Category | What it means | Typical kind of behavior |
| Observed | The bird was seen, but nothing suggests it is breeding at this location | Flyovers, birds clearly migrating, birds outside suitable habitat |
| Possible | The bird is in suitable nesting habitat during its breeding season | A bird present in fitting habitat, or a male singing or drumming |
| Probable | Behavior strongly suggests a breeding attempt, but there is no direct proof | A pair together, territorial defense, courtship, visiting a likely nest site, agitated alarm behavior, a male singing at the same spot over a long period |
| Confirmed | Direct evidence that the species is nesting here | Carrying nest material to a nest, a distraction display, food carried to young, recently fledged young, a nest with eggs or young |
Common codes and what they record
The letter codes below follow the general pattern used in eBird and many North American atlases. Individual atlas projects, including state and provincial ones such as the New York and Québec atlases, may define a few codes differently or add their own. Always check the list in the app or protocol you are submitting to.
| Code | Behavior | Category |
| F | Flyover; the bird was only passing overhead | Observed |
| H | Present in appropriate nesting habitat during the breeding season | Possible |
| S | Singing male (or drumming, or another breeding sound) in suitable habitat | Possible |
| S7 | Singing male present at the same location over an extended period, about a week or longer | Probable |
| M | Multiple singing males of the same species heard together in one area | Probable |
| P | A pair observed together in suitable habitat | Probable |
| T | Territorial defense, such as chasing off rivals of the same species | Probable |
| C | Courtship, display or copulation | Probable |
| N | Visiting a probable nest site repeatedly | Probable |
| A | Agitated behavior or alarm calls that suggest a nest or young nearby | Probable |
| B | Building a nest of a type that may not be used for eggs, such as wren or woodpecker cavities | Probable |
| CN | Carrying nesting material | Probable or confirmed, depending on the atlas and context |
| NB | Nest building at an actual nest site | Confirmed |
| DD | Distraction display, such as a feigned broken wing | Confirmed |
| UN | Used nest found from this season | Confirmed |
| ON | Occupied nest, where an adult enters or sits but contents are not seen | Confirmed |
| CF | Adult carrying food for young | Confirmed |
| FS | Adult carrying a fecal sac away from the nest | Confirmed |
| FY | Adult feeding young | Confirmed |
| FL | Recently fledged young that could not have traveled far | Confirmed |
| NE | Nest containing eggs | Confirmed |
| NY | Nest containing young | Confirmed |
How to choose the right code
- Record the highest level of evidence you actually saw. If a bird sang and then carried food, the food-carrying code is the one to use.
- Code only what you observed. Do not upgrade a sighting based on what the bird probably did after you left.
- Make sure the date fits the species' breeding season. Singing and pairing often begin during migration, so early-season song from a passing bird is not evidence of local nesting.
- Make sure the habitat fits. A grassland songbird singing from a fence line in open prairie is meaningful. The same species in an unsuitable spot may be a migrant.
- Be careful with birds that carry food for themselves. Some species, such as birds that cache food or adults feeding each other during courtship, can be mistaken for adults feeding young.
- Use a fledgling code only when the young are clearly too undeveloped to have come from elsewhere, for example short-tailed, downy or still being fed.
- Add a brief note describing what you saw when a code could be questioned. Reviewers rely on these details.
Examples from a prairie setting
The following are illustrative examples of how codes apply. They are not reports of specific sightings.
- A grassland songbird performs a high song flight over native prairie in early summer. Recorded once, that is singing in suitable habitat (possible). If the same bird is still singing in the same field on visits spread over a week or more, it supports a probable code.
- A Killdeer runs ahead of you dragging one wing and calling loudly. That is a distraction display, which counts as confirmed breeding. Back away instead of looking for the nest.
- A duck hen leads a line of small downy ducklings along a wetland edge. That is recently fledged young (confirmed), because ducklings that small have not traveled far.
- A pair of shorebirds circles you while giving persistent alarm calls near a marsh margin. That is agitated behavior (probable). It becomes confirmed only if you see something more definite, such as chicks or an adult carrying food, without disturbing the birds.
- A flock of the same species passes high overhead. Record it as a flyover (observed), even if the species nests in the region.
Getting more confirmations responsibly
Most confirmations come from patiently watching adult behavior from a distance, not from finding nests. Watching from a vehicle, a trail or a spot with good sightlines often shows adults carrying food or nest material back and forth. Mid to late breeding season tends to produce food-carrying and fledgling observations, which are among the easiest confirmations to obtain. A distraction display, alarm calls or repeated flushing mean you are too close and should move away. Do not use playback to create territorial or agitated responses during nesting season. Stay on established routes, and follow posted closures and land-access rules on refuges and private grassland. A probable code recorded without disturbing the birds is worth more than a confirmed code that put a nest at risk.
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