Date: 4/22/26 12:01 pm From: Stephen Broker via CTBirds <ctbirds...> Subject: [CT Birds] Breeding Codes and Safe Dates for our Connecticut Birds - Part One
One of the most enjoyable aspects of the recently completed Connecticut Bird Atlas (2018-2021) has been the field work devoted to gathering evidence of breeding. The preliminary results of the Atlas are on-line at this address:
Atlas species descriptions continue to be written for those bird species for which we have breeding evidence (possible, probable, confirmed). In fact, all 453 or so species on the official state list are listed and will have summary information of their occurrences. For now, many but not all species accounts for our breeding birds include combination maps (block results for Atlas 1982-1986 vs. block results for Atlas 2018-2021), text on distributions of breeders, phenological information, and photographs of breeding evidence. There also are maps of early winter and late winter occurrences for many species.
Two essential tables of information that the Atlas volunteers used during field work were the Breeding Codes and the Safe Dates. Here are two passages of text provided by the Atlas Team that served as guidelines for collecting breeding evidence:
“One of the difficulties with assigning breeding codes is that the behaviors the codes describe are sometimes used by birds away from their nesting sites. Migrating birds are particularly problematic because they often engage in breeding behaviors long before they get to their breeding grounds. When a species is known to not nest in a region, it is easy to recognize that they are not local breeders. But, when some individuals of a species breed in an area, while others migrate through en route to sites farther north, it becomes harder to know how some records should be treated.
“Like many breeding bird atlases, we attempted to address this problem by identifying “safe dates” at the beginning of the project for each species known to breed in Connecticut. Safe dates give the period during which the occurrence of a bird means that there is a high likelihood that it is breeding near to where it was seen, and that it is unlikely to be a migrant or a wandering non-breeder. Safe dates do not identify the entire period in which breeding may occur, and records of birds engaged in breeding behaviors that confirm breeding were accepted even when outside this period. Breeding codes in the possible and probable categories, however, were generally not accepted unless they lay within these dates. For example, if a Wood Thrush was seen building a nest in the second week of May, the record would be accepted because there is no doubt it was nesting. But, if one was just heard singing, even if in the same place that one was heard a week earlier, it would not be accepted because it could have been a migrant.”
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