2.6 Nest Record Scheme
The BTO's Nest Record Scheme is the largest, longest running and most highly computerised such scheme in the world and possesses the most advanced and efficient techniques of data gathering, data capture and analyses. There are currently more than 1,000,000 records held by the Trust, of which 35% are computerised.
The primary aim of the Nest Record Scheme is to monitor annually the breeding performance of a wide range of UK birds as a key part of the BTO's data collection. Annual reports are published (e.g. Crick et al. 2000) 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. Brown et al. 1995, Crick et al. 1994, Crick 1997, Peach et al. 1995).
The Nest Record Scheme gathers data on the breeding performance of birds in Britain and Ireland 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 filed away ready for analysis. Those for Schedule 1 species are kept confidential. (These are species protected from disturbance at the nest by Schedule 1 of the Wildlife & 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) and others. To visit the nests of these species a special licence is required.). 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. Data are computerised according to priorities for population monitoring and for specific research projects.
Currently the BTO receives a total of more than 30,000 records each year for around 180 species. Typically, the BTO receives 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 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). Habitat coverage is broad since 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.
In order to minimise the incidence of errors and inaccurately recorded nests, a set of rejection criteria was applied to the data: laying date only included 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 only visited 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 eggs may be lost during incubation and hatching success may be incomplete.
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 day over which the nest was exposed during the egg and nestling periods as the binomial denominator (Crawley 1993, Etheridge et al. 1997, Aebischer 1999). Number of exposure days during the egg and nestling periods was 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 & Baillie 1996). In this report, therefore, we present only temporal trends in daily nest failure rates.
Statistical analyses of nest record data were undertaken using SAS programs (SAS 1990). Regressions through annual mean laying dates, clutch sizes, 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. Linear regressions are presented on the figures in this report, even when statistically non-significant, for illustrative purposes.
Results are only presented if the total sample size of records for a particular variable and species exceed 300 (i.e. mean >10 per year), and are presented with a caveat for small sample sizes if the number of records contributing data was between 300 and 900 (i.e. if mean is between 10 and 30 per year).
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2000 British Trust for Ornithology