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2.4 Heronries Census

As predators at the top of the freshwater food chain, Grey Herons are excellent indicators of environmental health in the countryside. The aim of this census is to collect annual nest counts of Grey Herons Ardea cinerea from as many sites as possible in the United Kingdom. The Heronries Census began in 1928 and is the longest-running breeding season bird monitoring scheme in the world. Volunteer observers make counts of apparently occupied nests at heron colonies each year. Changes in the numbers of nests, especially over periods of several years, provide a clear measure of the population trends. In recent seasons, observers have counted also the nests of Little Egrets Egretta garzetta, which are now appearing in a number of southern heronries.

Coverage is coordinated through a network of regional organisers. A core of birdwatchers and ringers monitor their local colonies annually, providing a backbone of regular counts. Around two-thirds of the heronries in England and Wales are currently counted each year, with major censuses carried out in 1929, 1954, 1964 and 1985. Rather few counts are made of heronries in Scotland and Northern Ireland. Counts are submitted to the BTO on cards and the data are entered onto computer at BTO headquarters. The number of heronries cards submitted each year is around 450.

Data Analysis
Population changes are estimated using a ratio estimators approach derived from that of Thomas (1993). Essentially, the ratios of the populations in any two (not necessarily consecutive) years of the survey are estimated from counts at sites visited in each of those years. These ratios can be used to estimate the counts at sites that were not visited, and hence build an estimate of the total population. Further modifications have been made to allow for the extinction of colonies and the establishment of new ones, resulting in the graph as shown (Marchant et al. in press). A report containing simple chain estimates of change for the latest year is published annually in BTO News.

The trend is presented graphically with annual estimates in blue and their 85% confidence limits in green. A smooth trend line in red is based on a non-parametric regression model, using thin-plate smoothing splines with 24 degrees of freedom.

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This report should be cited as:
Baillie, S.R., Crick, H.Q.P., Balmer, D.E., Beaven, L.P., Downie, I.S., Freeman, S.N., Leech, D.I., Marchant, J.H.,
Noble, D.G., Raven, M.J., Simpkin, A.P., Thewlis, R.M. and Wernham, C.V.
(2002) Breeding Birds in the Wider
Countryside: their conservation status 2001. BTO Research Report No. 278. BTO, Thetford. (http://www.bto.org/birdtrends)

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