Bittern Annual Surveys in Britain 1990-2019

Quality Control

Method steps

1. Essential Information To Collect • The dates and times of visits to sites to assess listening effort. • The start and stop dates of booming males. • The mapped positions of booming males made on each visit to a site when males are heard. • Descriptions of the rate of booming of each male on each visit and the “quality” of the sound of the boom. Only those males that are known to have boomed for a week or more are counted in the minimum figures for the year. Where a site or area holds, or is thought to hold, more than one boomer, it is important to confirm the number of boomers actually involved. This can be achieved by hearing different boomers at the same time, and by comparing the booming periods of each male to confirm that they overlap. Visits Visits by recorders began at the key sites at the beginning of March, and by the end of March visit these sites about once a week. Key sites are those at which Bitterns have bred consistently for the last five years. Sites that have previously been occupied by Bitterns, but are not considered key should be visited fortnightly. For all other sites, recorders aimed to visit once a month during the main booming period, allowing for about three visits at most. The best time to hear booming males is in the two hours around dawn. The time you are most likely to hear a male is about half an hour before sunrise. But it was recommended recorders start listening at a site at least one hour before sunrise, and continue listening until it is light. Although booming may continue during daylight hours, it is unpredictable. The second best time to hear booming males is in the two hours around dusk. Recorders were advised they should start listening at least one hour before sunset and continue until it is dark. A male that is not booming very consistently is more likely to be heard at dawn, so infrequent visits to new sites should entail dawn visits as this is the best way to be sure of whether males are present or not. Males are less likely to boom (and be heard) in very windy conditions. Recorders advised if it was raining a little, it is still worth listening as long as it is not too windy. Methods Recorders advised to listen from within 250m of all potential habitat. Some Bitterns will be heard from several kilometres away, but equally some can only be heard at close quarters. Depending on the size of the site and the number of listening positions necessary, recorders advised they should listen for a minimum of 10 minutes at each position, regardless of the time spent walking between positions. Recorders advised to record the date, site, weather and the time listening started. On hearing a Bittern, recorders advised to record in their notebook the time of the boom and what it sounded like (i.e. strong, weak, close, distant). Recorders also advised to try to note down the times of each boom series they heard from each bird or note enough to say whether the male boomed twice a minute or once every ten minutes etc. Compasses should be used to take a bearing on the direction in which the male boomed from. Recorders then advised to triangulate the positions of known males as often as possible, as this helps to build up a picture of their home range through the season. When taking a bearing on a booming male position, the following information had to be recorded: recorders position (marked on a map); the ID of the male (an appropriate name for the bird); the time; the bearing; whether it is a good or poor grunt or boom; whether it is distant or close. Where possible, at least three bearings were taken on each male. Recorders advised not to just assume they knew where the male is, even if it has previously been heard from a similar position. A note of boom quality that records whether the individual notes within a boom were “good” or “poor” should be notated on the form as “b” for boom and “g” for grunt. At the end of each visit, recorders instructed to fill in their “Site Visits Summary” sheet, regardless of whether any Bitterns were heard. If they only surveyed a part of a larger site, they were advised to make a note of the area covered in the “Site coverage” box, under the “Booming activity” column.
   If males were heard, recorders advised to translate the information from their notebook to the “Booming Summary” sheet and to a site map. Therefore, at the end of each visit recorders should have a completed row in the “Site Visits Summary” sheet, and if applicable, a map with male booming positions marked on it and a “Booming Summary” sheet.
   Only those males that are known to have boomed for a week or more will be counted in the minimum figures for the year.