Atlas Data Entry 301: Understanding Breeding Codes & Timing

By Nick Anich 1 Apr 2015
Sharp-tailed Grouse Tympanuchus phasianellus

Assuming you are now familiar with the basics of entering a checklist (if not, visit our tutorials on Incidental Observations and Complete Checklists), our next tutorial will discuss how and when to use the Atlas breeding codes. Even if you are comfortable with what the codes are we highly recommend you read on to learn more about the appropriate times of year to use them, which can be challenging. 


  • Familiarize yourself with definitions and examples of atlas breeding codes. Most are straightforward with some exceptions and cautions.
  • Knowing appropriate dates to use the codes is more difficult, especially for Possible and some Probable codes AND when surveying outside of the prime June–July atlasing period.
  • Consult the Breeding Guideline Bar Chart to help determine the non-breeding, migration, and breeding periods for each species.
  • Use extra care — and your local knowledge of bird distribution, behavior, and migration phenology — during the “Either” period when an individual of the observed species could be a non-breeder, migrant, or breeder. Patience can be a virtue.
  • Your field observations, especially of Confirmed and higher-level Probable codes, trump the chart — if you see a breeding behavior in a period marked M, mark it down!
  • Please use the details field in eBird to provide comments describing the circumstances of your observation when entering breeding codes that run counter to the Breeding Guideline Bar Chart.
  • When in doubt about how or when to use a code, post your question to the Atlas Discussion Forum and/or consult your County Coordinator.

The breeding codes fall into 4 major categories: Observed, Possible, Probable, and Confirmed. Within each category, there are specific breeding codes tied to behaviors you observe in the field. These are briefly listed below, from highest to lowest level of breeding evidence, and a complete list with detailed explanations and examples can be found here.


NY – Nest with Young
NE – Nest with Eggs
FS – Carrying Fecal Sac
FY – Feeding Young
CF – Carrying Food
FL – Recently Fledged Young
ON – Occupied Nest
UN – Used Nest
DD – Distraction Display
NB – Nest Building
CN – Carrying Nesting Material
PE – Physiological Evidence


B – Woodpecker/Wren Nest Building
A – Agitated Behavior
N – Visiting Probable Nest Site
C – Courtship, Display or Copulation
T – Territorial Defense
P – Pair in Suitable Habitat
M – Multiple (7+) Singing Males
S7 – Singing Male Present 7+ Days


S – Singing Male (during breeding season)
H – in Appropriate Habitat (during breeding season)


F – Flyover
No code used – Species present but without evidence of breeding or outside of its breeding season or suitable nesting habitat

Many of these codes do not present challenges for atlasers. If you observed a Killdeer performing a distraction display, an American Robin building a nest, or a Vesper Sparrow carrying food, it’s quite obvious.


However, one of the most challenging aspects of atlasing is separating birds that may be breeding from birds unlikely to be breeding. This can be especially difficult for the Possible codes and lower-level Probable codes such as S7, M, or C, and is most problematic early and late in the nesting season outside of the prime May–July atlasing period.  Is that May 9 American Redstart singing on territory or just in migration?  Is that July 10 Solitary Sandpiper a local breeder?  What about the Northern Cardinal singing in my neighborhood in February? Red-tailed Hawks perched side-by-side in January? Ruffed Grouse drumming in October? The list of potentially perplexing situations is quite long!

Use the Breeding Guideline Bar Chart… and Your Local Knowledge!

To help you we have compiled a comprehensive species-by-species, week-by-week guidance document – the Breeding Guideline Bar Chart (available as a pdf and as a larger, sortable spreadsheet).  The bar chart is intended as a general guide to when species are expected to be migrants or nonbreeders in your area, or when it’s reasonable to assume a bird you encounter is on a breeding territory.


The abbreviations are as follows:

B = Breeding.  It is generally reasonable to assume birds present during this time window are breeders (see exceptions below where things get tricky).

  • Extremely rare birds (normal breeding phenology is listed, extremely rare birds are bolded)
  • Rare singing songbirds not always likely to pair (White-eyed Vireo, Kirtland’s Warbler)
  • Colonial waterbirds prone to roaming (Am. White Pelican, Double-crested Cormorant)
  • Oversummering terns and gulls (Great Black-backed Gull, Caspian Tern, Little Gull)
  • Shorebirds (many arctic shorebirds do not breed here, but pass through in June and July)
  • Species that wander in summer, early migrants (Tennessee Warbler, Philadelphia Vireo)

M = Migrant.  This is the expected prime migration window for this species.  You cannot safely assume a bird in suitable habitat is a breeder.  For some species, this code also denotes a post-breeding period where birds may not actually be migrating, but observations of breeding activity are no longer likely.

E = Either.  This is a window of overlap between breeding and migration or between breeding and nonbreeding when it is entirely possible that birds are on a breeding territory; however migrants are also moving through, or birds observed may not be on their breeding territory.  Take note of singing birds in suitable habitat but do not consider them as Possible resident breeders until later visits. This is the period in which your local knowledge and careful observation can help determine whether birds you see are resident or migrant.

N = Nonbreeding.  This indicates birds present in what is likely not their breeding season.



This graphic illustrates the pulses of spring migration, breeding, and fall migration. Note that the E window is the most confusing, because the first birds in breeding mode typically overlap the window where there are still migrants around. Assigning breeding codes during the E period requires local knowledge, careful observation, and a sense of this year’s migration/breeding phenology.

These guidelines are designed to provide a general impression of the typical phenology for each species. However, because these are average estimates subject to latitudinal and annual variation, your field observations take precedence over this chart — if you see a breeding behavior in a period marked M, mark it down! Beware than an early or late spring can affect when migrants move through and when birds settle on territory.  Birds may also arrive across the southern end of the state several weeks earlier than in the north, depending on the weather.

We hope this spreadsheet will provide the most guidance for the lower breeding codes that are generally the most uncertain (e.g., birds in suitable habitat and singing males) and outside of prime breeding season for a species.  The “Either” periods are especially subjective and will demand extra care and conservatism from atlasers. Your local knowledge of bird distribution, behavior, and annual migration phenology will be critical to distinguishing migrants and non-breeders from birds likely on territory.


Let’s look at an example – say you encountered Sandhill Cranes on March 28. This is in the “E” for Either window, where birds could be exhibiting either breeding or migration behavior. If you saw a flock of birds standing in a field, it’s probably too early to code anything, they could be migrants. If you saw a pair of birds standing in a field with no prior information about the situation, it is probably still too early to call, they could be migrants or birds searching for a territory. The situation where you might code them as H, S, P, T, or C might be if you had birds nesting in a marsh across from your house in past years, and you feel it’s the same pair that has returned and has been present for a number of days. Note that you want to be careful about using S7 too early in the season if there is any chance that your initial observation could have been of a different migrant pair, or a pair not established on a territory yet. Whenever you are submitting records during the M or E periods, it would help us greatly if you would insert comments in the details box in eBird explaining what you saw and what your interpretation of it was.  Your comments will help us in refining our understanding of phenology and guide revisions of the Breeding Guideline Bar Chart in future years.

It helps to be aware of patterns of bird distribution.  A Black-throated Blue Warbler singing near Madison at the end of May is likely not setting up a territory, but one near Rhinelander might be.  Good references for expected breeding ranges include Species Maps by month at or the maps from the first Atlas.  But of course if these ranges were still all the same we wouldn’t be doing another Atlas — so please use these maps as general guidance of what to expect when, but let your field observations take priority over what you “should” be seeing. If you’ve got a Black-throated Blue singing for 3 weeks in June in Madison, mark it down as S7.  Tricky records can and will be reviewed by County Coordinators and Atlas staff at the end of each field season.

When in doubt, be patient before entering codes and make every effort to visit again at a later time to relocate the bird or species in question and upgrade its breeding code for the block.

Note that you can take the spreadsheet version of the Breeding Guideline Bar Chart and sort it, which is a very handy way of knowing what species to seek for breeding behaviors at any given time of year. What’s breeding in the first week of April? Take a look:


To create the above list, simply sort the spreadsheet version of the Breeding Guideline Bar Chart. To sort, you will want to select all of the rows except for the top 2 header rows and then sort by the column corresponding to your week of interest. For the first week of April, we will sort by column U.


Other Assorted Tricky Things

Brown-headed Cowbirds don’t make nests of their own but rather lay their eggs in the nests of other species, a behavior known as brood parasitism. Cowbirds are often a bonus confirmation in a block in that if you find a nest with host eggs and cowbird eggs, you can enter NE for both species. The trick here, if you don’t actually see any adult cowbirds on that visit, enter the number “0” into eBird for Brown-headed Cowbird, then breeding code NE. It’s also useful to note in the comments field which species was parasitized. On the other hand, an adult of the host species feeding a fledged cowbird young is also a double-confirmation (FY) but in this case the number entered would be greater than zero and equal the total number of cowbirds observed.


In the last atlas, the code M – multiple singing males (7+) was used at the block level. This still applies, even though we are advising you to break your checklists up within a block to specific properties, when feasible. So in certain instances, you might report 5 birds on a checklist and code it M. In this case, please make a comment on the checklist like “I had 4 other singing males in block at locations, X, Y, Z”. This is important to help us ensure this is not an error.

UN – Used Nest is a code that you should be careful using. It is intended for the rare instance where you discover a nest that is already done with for the year. In certain cases, (e.g. Baltimore Oriole) species make fairly distinctive nests. But in many cases, it’s tricky to be able to confidently pin down the builder of a nest with no birds around, or even the year in which it was built in some cases. Please use this code with extreme caution and do not guess at the identities of the makers of random stick nests within your block. This is also a code that you might use with a “0” in the count field for eBird, if you encountered no individuals of this species on your visit.

The S7 code is used to indicate a singing male has been present for the same location on at least 2 occasions 7 or more days apart within the same breeding season. The thing to watch out for here is that the initial observation must definitely represent a resident breeder. If you have a singing American Redstart on May 9 and you have one in the same vicinity on May 18, that could still just be two migrant American Redstarts!

A Resource to Help

What should you do if you’re unsure about whether it’s time to use a code, or if you’re using the correct code?  Your go-to resources should be the Atlas Discussion Forum. Sign in, post a question, and atlasers and atlas staff will chime in to help. The system may be initially intimidating, but with practice, it will quickly become second nature.