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2.6
Constant Effort Sites
The Constant
Effort Sites (CES) Scheme uses changes in catch sizes across a network
of more than 100 standardised mistnetting sites to monitor changes
in the abundance and breeding success of common passerines in scrub
and wetland habitats. At each constant effort site, licensed ringers
erect a series of mistnets in the same positions, for the same
amount of time, during 12 morning visits between May and August.
Yeartoyear changes in the number of adults caught provide a measure
of changing population size, while the proportion of young birds
in the total catch is used to monitor annual productivity (breeding
success). By monitoring the abundance of young birds between May
and August, the CES method should integrate contributions to annual
productivity from the entire nesting season, including second and
third broods for multibrooded species. Betweenyear recaptures
of ringed birds can also be used to calculate annual survival rates,
although this requires specialised analytical techniques (e.g. Peach
1993) and is not considered further here. Further details
of the CES Scheme and methods of analysis are presented by Peach
et al. (1996).
The CES Scheme
began in 1983 with 46 sites and now has nearly 150. The distribution
of CES sites tends to reflect the distribution of ringers within
Britain and Ireland. The majority are operated in England, and there
are small numbers in Scotland, Wales, Northern Ireland and the Republic
of Ireland. The CES is able to monitor the populations of 28 species
of passerines in scrub and wetland habitats.
Data Analysis
Annual estimates of the abundance of adults and young are separately
assessed through application of loglinear Poisson regression models,
from which fitted yeareffects are taken as annual relative abundances,
compared to an arbitrary value of unity in 2000. 85% confidence
limits are based on the corresponding asymptotic standard errors.
At sites where catching effort in a year falls below the required
12 visits, but a minimum of 8 are completed, annual catch sizes
are corrected according to experience during years with complete
coverage (see Peach et
al. 1998 for full details). Sites with fewer visits
in a given year are omitted for the year in question.
Annual indices
of productivity (young per pair) are estimated from logistic regression
models applied to the proportions of juvenile birds in the catch,
the yeareffects then being transformed to measures of productivity
relative to an arbitrary value of unity in 2000. As above, catch
sizes are corrected for small numbers of visits missed where necessary.
It should be noted that these indices are relative, and are not
estimates of the actual numbers of young produced per pair. Full
methodological details are provided by Freeman
et al. (in prep).
Data are presented
graphically with annual estimates in blue and their 85% confidence
limits in green. Methods and software for the optimal fitting of
smoothed trends to CES data remain in development. Here, we also
present a nonparametric regression model fitted to the calculated
annual indices of abundance and productivity (via thinplate smoothing
splines with 5 degrees of freedom), to provide a simple smoothed
picture. This is the red smoothed line on the CES graphs on the
species pages. A caveat is provided for "small samples"
when the average number of plots per year is between 10 and 19.
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Section  2.7 Nest Record Scheme
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