Bird Study

ISSN: 0006-3657 (Print) 1944-6705 (Online) Journal homepage: http://www.tandfonline.com/loi/tbis20

The influence of weather conditions on the detection of birds during Common Birds Census fieldwork R. J. O'Connor & Roger K. Hicks To cite this article: R. J. O'Connor & Roger K. Hicks (1980) The influence of weather conditions on the detection of birds during Common Birds Census fieldwork, Bird Study, 27:3, 137-151, DOI: 10.1080/00063658009476672 To link to this article: http://dx.doi.org/10.1080/00063658009476672

Published online: 23 Jun 2009.

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Date: 18 January 2017, At: 19:39

The influence of weather conditions on the detection of birds during Common Birds Census fieldwork by Raymond J. O'Connor and Roger K. Hicks While most species examined showed statistical correlation with at least one weather factor, these effects were too small to bias CBC results, due to extended bird breeding seasons and CBC fieldwork period in Britain. IN HIS CLASSIC PAPER ESTABLISHING the mapping method of censusing bird populations Enemar (1959) suggested that weather conditions during the census would be of particular importance to census effectivity, through its effect on the intensity of bird song. Enemar noted that this would be particularly important for the mapping of singing territorial males, and standardized his census visits to those conducted in clear sunny weather, noting that low temperatures (in particular) were depressive of bird song. Armstrong (1963) subsequently reviewed the relationship of bird song to weather and noted that low temperatures and mist were especially significant in retarding song output. In principle, therefore, census visits conducted under these conditions should be relatively poor in providing registrations of birds, a point which might require modification of the standard treatment of visit maps within the BTO Common Birds Census programme. In practice the CBC is not based exclusively on contacts with singing males, but includes all registrations evidential of a territory. Sightings thus play a larger part in the determination of the eventual total of clusters than did the essentially aural location method of Enemar (/c. cit.). Thus it is not clear what the overall effect of changing weather in the course of the census season might be. Moreover, because the CBC scheme depends on the voluntary efforts of BTO members it is not possible to omit censuses conducted under less than ideal weather conditions (the practice adopted by Enemar) without falling below the minimum number of visit maps for analysis; the vagaries of British weather in any event preclude raising the threshold number of visits asked of participants without reaching an unacceptably high level. The present paper therefore presents the results of a systematic investigation of the effects of the weather prevailing at the time of censusing on the total registrations obtained in the visits. MATERIALS AND METHODS The analyses reported here were undertaken within a wider-ranging examination of Common Bird Census methods and assumptions (O'Connor and Marchant in prep.). The fieldwork was conducted to a schedule determined by the broader remit of that study, within which examination of weather effects on censusing efficiency was secondary. The full details of methods of data collection and analysis are presented in that report, and only a summary of relevant points are presented here. In brief, each of four observers paid ten census visits to a CBC plot on Aston Rowant National Nature Reserve in the Chiltern Hills, Buckinghamshire. Each worker conducted his census independently of the others and followed normal CBC methods throughout. Bird Study 27:137-151, September 1980

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The observers generally tried to avoid censusing the plot on the same date as another worker but on nine occasions duplicate or (in one case) triplicate censuses were conducted; three of these involved observers on the site at different times of day. One observer consistently conducted his census work between 0600-0900 hrs BST, the other three between 0900-1200 hrs BST (approximate times). Censuses were conducted in 1977 and in 1978 but only the results for the latter year have been used in the present study. Census work in 1978 began on 8 April and ended on 12 July and visits were spread fairly evenly through the intervening period. For the present study the 40 visit maps showing the positions of all registrations were examined on completion of the census work and the total numbers of registrations for each of 27 species established for each visit. These totals made no distinctions as to type of registration obtained. When the registrations from the ten visits of each observer were plotted onto species maps, statistically significant differences in the resulting totals of clusters were present (O'Connor and Marchant in prep.). These differences between observers were reflected in the frequencies of registrations obtained by each, but as the trends apparent in the four observers' results were mutually consistent no attempt was made to correct the data for observer identity. The data on the weather prevailing on each census visit were obtained from the meteorological station at RAF Benson, 11 km WSW from Aston Rowant. Temperature data, however, were corrected to local conditions on the census plot, by reference to irregularly (approximately weekly) gathered data collected at Aston Rowant. Observations made simultaneously at the two sites were plotted against each other and the resulting graph used to correct Benson temperature to local values where these were not directly measured at Aston Rowant. The mean temperatures for the period 0600-0900 his BST or 0900-1200 his BST were used as appropriate to the observer concerned. Rainfall was recorded at Benson station for the 24 hour period beginning 0900 GMT but was attributed to the calendar date, ie. including the early morning censuses. Cloud cover was recorded in eighths of the sky covered at all levels; for coding purposes values such as 25 20 (6) 15

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