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2.6
Constant Effort Sites Scheme
The Constant Effort Sites (CES) Scheme uses changes in catch sizes
across a network of 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 mist nets in the same positions, for the same
amount of time, during 12 visits evenly spaced between 1 May and
31 August. Yeartoyear changes in the number of adults caught provide
a measure of changing population size, while the ratio of young
birds to adults 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, but will also include
a small component of mortality during the immediate postfledging
period. Betweenyear recaptures of ringed birds can also be used
to calculate annual survival rates of adult birds, although this
requires specialised analytical techniques (e.g. Peach
1993) and is not considered further here. Further details
of the CES Scheme are presented by Peach
et al. (1996) and methods of analysis are detailed
in Peach et al. (1998)
for abundance measures and Robinson
et al. (2007) for productivity measures.
The CES Scheme began in 1983 with 46 sites and now has around 120.
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 routinely monitors
the populations of 25 species of passerines in scrub and wetland
habitats.
Data analysis
Smoothed trends in the abundance of adults and young are separately
assessed using a generalised additive model (GAM), with 85% confidence
intervals calculated by bootstrapping (
Fewster et al. 2000). At sites where catching
effort in a year falls below the required 12 visits, but eight or
more visits have been completed, annual catch sizes are corrected
according to experience during years with complete coverage, by
incorporating an offset into the GAM (see
Peach et al. 1998 for full details). Sites with fewer
than eight visits in a given year are omitted for the year in question.
Annual indices of productivity (young per adult) 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 100 in the most
recent year. As above, catch sizes are corrected where small numbers
of visits have been missed. It should be noted that these indices
are relative, and are not estimates of the actual numbers of young
produced per adult (Robinson
et al. 2007).
Data are presented graphically with the smoothed trend in blue and
their 85% confidence limits in green. A caveat is provided for 'Small
samples' when the average number of plots per year is between 10
and 20.
Annual estimates of adult survival are derived from a form of the
standard CJS capture–mark–recapture model (Lebreton
et al. 1992) modified to account for the presence of
transient birds. Transients are birds passing through the site,
or perhaps living on its periphery, and which therefore have a much
lower probability of capture than resident birds living in the vicinity
of the nets. The presence of transients thus tends to decrease the
estimated survival rates. We allow for this by introducing an additional
'survival period' in the year of first capture (Hines
et al. 2003). As with our other schemes, we assume
survival probabilities vary annually in a similar fashion across
all sites, though mean survival probabilities may differ between
sites. Because of the standardised capture protocol, we assume that
recapture probabilities are sitespecific, but constant through
time. For each bird we also insert an additional period after the
first capture, indicating whether the bird was caught subsequently
in the same season. The probability of surviving this period can
be regarded as the probability that the bird is resident on the
site (that is the probability that it is available for recapture).
The survival and recapture probabilities for this initial period
are assumed constant across years and sites. Note that the annual
estimates of annual survival presented are in fact the probability
that adult birds return to the same CE site the following year;
this will be lower (to a small but unknown extent) than the true
survival rate. We do not estimate survival rates for juvenile birds,
because of their much greater propensity to disperse.
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