[CT Birds] Some Comments on Connecticut Bird Atlas Safe Dates, Part V (Last)

Stephen Broker stephenpaulbroker at gmail.com
Tue Apr 10 11:50:53 EDT 2018

On April 6 I advocated a slow and deliberate approach to breeding bird atlasing with emphasis on close observation of bird behavior, multiple trips to the same sites, observing proper birding etiquette, and careful use of breeding bird codes. I feel that this is the most rewarding form of birding (not "required form" as spellcheck decided to print it).

The discovery of a Barred Owl nest in a woodland in Atlas Block 80D Mount Carmel required a dozen trips from the February 28 observation of a pair of Barred Owls to the March 29 experience of seeing the adult female entering, then leaving a cavity in a tall dead deciduous tree.  How do these observations fit in with the CT Bird Atlas published safe dates?  Connecticut Bird Atlas safe dates for this species are April 1 through July 15.  I shared all my eBird posts from this woodland with CTbirdatlas, maintaining that the February 28 observation constituted Probable breeding due to a pair in suitable habitat.  The observation of both birds on March 6 led to a shared post of Probable breeding, courtship.  On March 29, March 31, and April 2 the observation of both adults vocalizing and the female within a nest cavity led to a definitive call of Confirmed breeding, occupied nest.

One could maintain that I was pushing the evidence of probable breeding by a month, although hearing the female calling from the nest cavity on March 31 and April 2 certainly clinched the report of confirmed breeding.  So, how accurate are the safe dates we are using?  Published safe dates are important to keep in mind, but they have their limits, and one result of the current atlas project we can expect is future revision of some safe dates.  An important caveat about safe dates from the Atlas website is:

“Breeding codes, especially those in the Possible and Probable categories, generally should not be used outside the safe dates for a given species. Confirmed breeding codes, are less likely to cause errors, but even confirmed breeding should be well documented (e.g., a detailed description of the bird species and the behaviors that confirm breeding) if it happens outside the safe dates for a species because these records are likely to be of particular interest."

Here’s where more information might benefit us but also might lead us astray.  The Second Atlas of Breeding Birds of New York State (2008) lists Barred Owl egg dates from March 20 to May 4.  The Massachusetts Breeding Bird Atlas (2003, based on field work of 1974 to 1979) lists Barred Owl egg dates from February 25 to May 13.  The Second Atlas of Breeding Birds of Vermont (2013) lists Barred Owl safe dates from March 15 to July 15.  The Atlas of Breeding Birds in Pennsylvania (1992) lists Barred Owl egg dates from March 1 to April 8 and safe dates from January 15 to August 30.  This one example of breeding data on a resident owl in the Northeast indicates that there can be a lot of play in the defining of safe dates and how they relate to field observations of egg dates as well as nestling and fledgling dates.  In the above description of observing a pair of Barred Owls in atlas block 80D, the safe dates window of April 1 to July 15 for this species works quite well.   

Example No. 2 Peregrine Falcon.  Connecticut safe dates are May 15 to August 1.  I’ve been observing nesting peregrines in south-central Connecticut each year for the last twenty years.  Peregrines begin nest scrape preparation a month before egg laying begins, in late February and early March.  The first egg of each year’s clutch typically is laid between April 3 and April 10, and the earliest date for the start of a peregrine clutch at this site is March 29.  Peregrine nesting on the Travelers Tower in Downtown Hartford is well monitored and widely known, but peregrines nesting on bridges, other buildings, and cliff faces may not be detected until the adults are feeding fledged young.  The comparatively small number of nesting peregrines in Connecticut may justify a safe date period that is fully two months after nest preparation and one month after the beginning of egg laying.    

Example No. 3 Common Raven.  Connecticut safe dates are March 20 to July 20.  The nesting ravens I’ve been observing since 2002 typically lay their first egg or begin incubation by March 7.  This year the raven pair laid the first of five eggs on February 25.  Raven courtship, including tandem flying, allopreening, and a fascinating spreadwing spiral freefall with male and female facing each other and clasping bills begins in mid to late January.  Nest rebuilding takes place over two to three weeks in February.  I’ve seen ravens copulating once, on March 4.  (They are not the exhibitionists that peregrines are.)  Ravens build prominent nests consisting of large sticks and smaller branches, with a central bowl lined with redcedar bark peelings, mosses, mud, and finally deer hair.  A March 20 start to the safe date period may be two to three weeks later than the actual commencement of egg laying, but it’s a pretty good measure for most opportunities for confirming breeding of ravens in Connecticut.

Example No. 4 Winter Wren.  Connecticut safe dates are May 1 to August 5.  The state’s first Breeding Bird Atlas confirmed breeding of Winter Wren   in just 10 atlas blocks, mostly in northwestern Connecticut, with another twenty or so reports each of probable or possible breeding, again concentrated in the northwest.  The Second Atlas of Breeding Birds of New York State lists Winter Wren egg dates (5/22-7/29), nestling dates (5/7-8/4), and fledgling dates (4/23-9/4), indicating the difficulty of locating nests with eggs for this species.  Birds of North America describes Winter Wren nest sites as occurring in roots of upturned trees, under creek banks, in decaying logs, in moss hanging on trees, in stumps.  Given that it’s highly unlikely to find the nest of a secretive Winter Wren, a start safe date of May 1 is very reasonable.

To summarize, our safe dates are not perfect, but they provide a very good measure for when most individuals of each species can be expected to breed.  It’s now April 10, and confirmation of breeding for a greater number of species is taking place in atlas blocks statewide.  Best advice on breeding codes is that they “generally should not be used outside the safe dates for a given species”, but breeding codes for confirmed breeding should be used when the evidence is obtained, with provision of “a detailed description of the bird species and the behaviors that confirm breeding”.

Steve Broker


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