How the Checklist of Birds of Northern Virginia Was Developed

The Checklist of Birds of Northern Virginia was developed using data from eBird, an online real-time checklist program,, supported by the Cornell Laboratory of Ornithology at Cornell University.  The steps in creating the checklist are documented below.

The checklist was developed by a team led by Dixie Sommers, with input from Tom Blackburn, Greg Butcher, Greg Fleming and Laura McDonald. 

Step1: Download eBird Data. The first step was to download eBird species bar chart data into an Excel spreadsheet. The data are for the following dates and areas:

  • Period of 2005-2015 (11 Years)
  • For the combination of the following counties/cities which constitute the ASNV territory:
    • Alexandria City
    • Arlington County
    • Fairfax City
    • Fairfax County
    • Falls Church City
    • Fauquier County
    • Fredericksburg City
    • Loudoun County
    • Manassas City
    • Manassas Park City
    • Prince William County
    • Rappahannock County
    • Stafford County

The eBIrd data are frequencies for each species for 1 week periods each month.  The file included 401 taxa (species, hybrids and “sp.”). Data on sample size (number of checklists) is also included.  Notes on the calculations are available here

Step 2: Assign Seasons. On the data spreadsheet, each week was labeled:  Jan 1, Jan 2, Jan 3, Jan 4, Feb 1, Feb 2, etc.

Each week was also labeled as to season:  WI, SP, SU, FA. The assignment of weeks to seasons follows the pattern used by USFWS on National Wildlife Refuge checklists: 

  • Spring – March through May
  • Summer – June through August
  • Fall – September through November
  • Winter – December through February

Step 3: Calculate frequencies by Season.  The frequency for each week was calculated by eBird by dividing the number of checklists for the week on which the species was reported divided by the total number of checklists for the week.  To calculate the frequency by season, the following calculations were made using Excel:

  1. For each species and week, compute the number of checklists on which the species was reported:  multiply the total number of checklists for the week by the ebird frequency for the week.
  2. Sum the number of checklists on which the species was reported for all weeks in the season.
  3. Sum the total number of checklists for all weeks in the season.
  4. Divide the total number of checklists on which the species was reported for the season by the total number of checklists for the season.

In addition, the total number of checklists on which each species was reported during the 11 years was computed.

Step 4: Assign AOU checklist order. The data resulting from Step 3 were loaded into an Access database, and matched to the American Ornithology Union (AOU) checklist, 7th edition through the 57th supplement. (Click here for changes in the 57th supplement issued in July 2016.) This step assigned the AOU checklist order to the ASNV area eBird data. 

The matching process eliminated the hybrid and “sp.” taxa on included on the original eBird data. Also, Orange-cheeked Waxbill was manually deleted from the list. This species was reported on 1 checklist, and its range does not include North America.

The results from this step were copied to a new Excel spreadsheet, which shows the species in AOU order and the frequency for each season. 

Step 5: Assign Abundance categories. Species with 11 or fewer eBird observations (rounded data) during the 2005-2015 period were placed in a short list following the main checklist, without seasonal occurrence codes.  The short list represents accidental and vagrant species for Northern Virginia.

In some instances, species with more than 11 observations were added to the short list based on the checklist team’s review.  The team looked at specific eBird observations to determine whether there were multiple observations of the same few individual birds.  Black-capped Chickadee was also added to the short list. Finally, species identified as “Accidental” for Virginia on the most recent Virginia Society of Ornithology checklist were also placed in this short list. There are 56 species on the short list.

The 266 remaining species constitute the main checklist. Each species was assigned an abundance designation for each season using the following criteria determined by the checklist team:

  • Abundant (A): Reported on 20% or more of checklists
  • Common (C): Reported on 5% or more and less than 20% of checklists
  • Uncommon (U): Reported on  1% or more and less than 5% of checklists
  • Rare (R): Reported on 0.1% or more and less than 1% of checklists
  • Very Rare (VR): Reported on  0.01% or more and  less than 0.1% of checklists
  • No frequency data (-): Reported on less than 0.01% checklists 

The following explanation of the occurrence assignment is shown on the published checklist:

A     Abundant – nearly always found, sometimes in large numbers

C     Common – likely to be seen or heard in the appropriate habitat

U     Uncommon – possibly present but not always seen or heard

R     Rare – infrequently observed and not every year

VR   Very Rare – not expected to be present

Step 6: Assign other indicators. An “Introduced” notation (I) was assigned to species identified as Introduced on the Virginia Society of Ornithology 2016 checklist.

Species on the Virginia Avian Review Committee (VARCOM) list for review by the committee are annotated with an asterisk *.  The species annotated are those reviewable either statewide or in the Coastal Plain.  Checklist users are encouraged to report these species to VARCOM at 

Step 7: Final review. The resulting checklist and seasonal assignments of occurrence codes was manually reviewed by the checklist team.