Home > Contents > Methodology
> Nest Record Scheme
Nest Record Scheme
The BTO's Nest
Record Scheme is the largest, longest-running and most highly
computerised of such schemes in the world and possesses the most
advanced and efficient techniques of data gathering, data capture
and analysis (Crick et
al. 2003). There are now more than a million nest records
held by the Trust, of which 35% are computerised.
The primary aim of the Nest Record Scheme is to monitor the breeding
performance of a wide range of UK birds annually as a key part of
the BTO's data collection. Periodic reports are published in BTO
News (e.g. Leech
& Barimore 2008) and the significant results communicated
immediately to JNCC. Another primary aim is to undertake detailed
analyses of breeding performance of species of conservation interest
(e.g. Crick et al.
1994, Brown et
al. 1995, Peach
et al. 1995a, Crick
& Crick 1999, Siriwardena
et al. 2001, Crick
et al. 2002, Chamberlain
& Crick 2003, Freeman
& Crick 2003, Browne
et al. 2005, Tryjanowski
et al. 2006, Douglas
et al. 2010).
The Nest Record Scheme gathers data on the breeding performance
of birds in the UK through a network of volunteer ornithologists.
Each observer is given a code of conduct that emphasises the responsibility
of recorders towards the safety of the birds they record and explains
their legal responsibilities. These observers complete standard
nest record cards for each nest they find, giving details of nest
site, habitat, contents of the nest at each visit and evidence for
success or failure. When received by the BTO staff, the cards are
checked, sorted and prepared for input and analysis. Data are prioritised
for computer input according to their potential for population monitoring
and for specific research projects. Those for Schedule 1 species
are kept confidential. (These are species protected from disturbance
at the nest by Schedule 1 of the Wildlife
and Countryside Act 1981: they are generally rare species and
the location of their nests may need to be protected from egg collecting
(an illegal activity for every wild bird) or other potential disturbance.
A special licence is required to visit any nest of a Schedule 1
species.) Computer programs developed by BTO check the data for
errors and calculate first-egg date, clutch size, nest loss rates
at egg and chick stages.
Currently the BTO collects a total of more than 30,000 records
each year for around 180 species. Typically, there are more than
150 records for 55 species and more than 100 for a further 10–15
species. The quality of records improved substantially in 1990 with
the introduction of a new recording card, which promotes greater
standardisation and clarity in the information recorded by observers.
The general distribution of completed Nest Record Cards is patchy
at the county scale but is more even over larger regions of the
UK. Overall, Northern Ireland and parts of Scotland (southeast,
Western Isles) and parts of England (West Midlands, southwest) have
relatively low coverage, often reflecting observer density. A major
analysis of trends over time in various aspects of breeding performance
found relatively few differences between major regions in the UK,
when analysed using analysis of covariance (Crick
et al. 1993). The scheme receives records from all
the UK's major habitats. Most records come from woodland, farmland
and freshwater sites, but the scheme also receives data from scrub,
grassland, heathland and coastal areas.
Five different variables were analysed for this report: laying
date (where day 1 = January 1); clutch size; brood size; and daily
nest failure rates during egg and nestling stages, calculated using
the methods of Mayfield (1961,
1975) and Johnson
(1979) (see Crick et
al. 2003 for a review).
To minimise the incidence of errors and inaccurately recorded nests,
a set of rejection criteria was applied to the data: laying date
included only cases where precision was within ±5 days; clutch size
was not estimated for nests which had been visited only once, for
nests which were visited when laying could still have been in progress,
or for nests which were visited only after hatching; and maximum
brood size was calculated only for nests which were observed after
hatching. The last variable is an underestimate of brood size at
hatching, because observers may miss early losses of individual
chicks; it differs from clutch size because some eggs may be lost
during incubation or fail to hatch.
Daily failure rates of whole nests were calculated using a formulation
of Mayfield's (1961,
1975) method as a logit–linear model with a binomial
error term, in which success or failure over a given number of days
(as a binary variable) was modelled, with the number of days over
which the nest was exposed during the egg and nestling periods as
the binomial denominator (Crawley
et al. 1997, Aebischer
1999). Numbers of exposure days during the egg and nestling
periods were calculated as the midpoint between the maximum and
minimum possible, given the timing of nest visits recorded on each
Nest Record Card (note that exposure days refer only to the time
span for which data were recorded for each nest and do not represent
the full length of the egg or nestling periods). Each calculation
assumes that failure rates were constant during the period considered.
Violations of this assumption of the Mayfield method can lead to
biased estimates if sampling of nests is uneven over the course
of each period. It is unlikely that any such bias would vary from
year to year so, although absolute failure rates may be biased,
annual comparisons should be unaffected (Crick
et al. 2003). In this report, therefore, we present
only temporal trends in daily nest failure rates.
As the combined influence of concurrent trends in these individual
breeding parameters on overall productivity is difficult to assess,
the estimates produced are used to derive an annual mean estimate
of the number of 'fledglings produced per breeding attempt' (FPBA)
according to the equation below (Crick
et al. 2003):
FPBA = CS × HS × (1 – EF)EP
× (1 – YF)YP
where CS represents clutch size, HS represents hatching success,
EF and YF represent egg- and chick-stage daily failure rates and
EP and YP represent the length of the egg and nestling periods.
Standard errors were derived using the formula given by Siriwardena
et al. (2000b).
Statistical analyses of nest record data were undertaken using
SAS programs (SAS 2009). Regressions
through annual mean laying dates, clutch sizes and brood sizes were
weighted by sample size. Nest survival was analysed by logistic
regression. Quadratic regressions were used when the inclusion of
a quadratic term provided a significant improvement over linear
regression. These are described as 'curvilinear' in the tables on
species pages. Significant linear trends are described as 'linear'.
The best-fitting regressions (i.e. quadratic or linear) are presented
on the figures in this report. Where neither regression is significant,
the linear regression line is shown for illustrative purposes.
Results are presented only if the mean sample size of records for
a particular variable and species exceeds ten per year, and are
presented with a caveat for small sample sizes if the mean number
of records contributing data was between ten and 30 per year.
Next section – 2.8 The
Back to Methodology Index
HERE to go to the NRS section of the main BTO website