How are NRS data used?
What do nest records tell us about productivity?
The major reproductive parameters that can be calculated from Nest Record Scheme (NRS) data are:
- Clutch sizes
- Brood sizes
- Laying dates
- Nesting success
While many recorders are able to record precise counts of eggs and young, laying dates must often be back-calculated using information about the timing of hatching and fledging. The estimation of nesting success is more complicated, as the outcome of a breeding attempt is often unknown. To overcome this problem, the failure rates of nests per day of observation are calculated during the egg and nestling stages. Data from all nests that were visited more than once can be used in this process, regardless of whether the ultimate outcome of the attempt is known.
Monitoring UK Bird Populations Using Nest Record Data
The population size of a species is dependent on survival rates, movements of individuals and levels of productivity. The Ringing Scheme monitors changes in survival rates and movements. The Nest Record Scheme is able to supply the other piece of the puzzle, identifying trends in breeding success. Together the information collected by these schemes can be used to identify the causes of apparent population declines. This is essential if conservation measures are to be effectively and efficiently put into action.
Trends in breeding performance are currently calculated each year for approximately 95 IPM species. Fifteen of these species are included on The Population Status of Birds in the UK Red List of rapidly declining species, including Skylark, Spotted Flycatcher, Bullfinch and Corn Bunting. A further 25 species, including Peregrine, Barn Owl, Swallow and Willow Warbler are included on the Amber List, indicating that they are of medium conservation concern.
Currently, the BTO has issued NRS Alerts about declining or low nesting success of Reed Bunting, Linnet, Moorhen, Greenfinch, Lapwing, Meadow Pipit, Ringed Plover, Yellowhammer, Sedge Warbler, Dunnock, and Willow Warbler. Productivity trends and trends in abundance produced using data from other BTO monitoring schemes, are published annually in the BirdTrends Report. This report should be the first port of call for anybody wanting to access the most up-to-date information about a species' conservation status.
Understanding changes in Productivity
The first scientific paper to use NRS dta was David Lack’s classic study of the Robin in 1946, in which he investigated seasonal, geographical and annual variation in clutch size, the timing of breeding and hatching success. Since then, NRS data have been used as the basis for more than 270 scientific papers, reports, conference proceedings, local bird reports and books. For the full list of papers that have utilised data, see 'References'.
a) Impact of Pesticide Use
The detrimental impact of organochlorine pesticides on raptor breeding success during the 1960s and 1970s was supported by evidence from the NRS dataset. These data revealed an increase in egg failure rates as the use of these types of agrochemicals increased. These compounds caused the thinning of eggshells, which meant that the eggs broke when the adults tried to incubate them.
NRS data also suggest that some species of songbird experienced similar reductions in breeding success. Fortunately, following the ban of such chemicals, most species have exhibited a full recovery.
b) Effects of agricultural intensification
Several recent studies have used NRS data to investigate the problems faced by declining farmland birds, such as Yellowhammer, Corn Bunting and Tree Sparrow. Many of these species have suffered severe population declines as agricultural practices have intensified over the last 50 years. For most species, breeding performance has not been affected, indicating that the factors causing population declines are linked either to survival or to the number of breeding attempts initiated. NRS data help scarce conservation resources to be targeted at the key stages of the life cycle for these birds.
However, data collected by nest recorders over the last 40 years have shown that Linnet nest failure rates have increased significantly since the 1960s, as shown in the graph below taken from BirdTrends. This reduction in productivity is of sufficient magnitude to have driven its population decline. Encouragingly, analysis of the most recent NRS information suggests that failure rates have started to fall rapidly over the last five years - could this be a response to the introduction of government agri-environment schemes?
Agricultural Intensification has also affected species breeding in upland pastures. NRS data suggests that higher stocking densities have led to an increased incidence of both desertion and nest destruction for Lapwing and Golden Plover.
c) Influence of global warming
Nest recorders in the 1970s could not have foreseen how valuable their data would be. As atmospheric temperatures continue to rise, NRS data have been used to investigate the effect of climate change on the timing of egg laying clearly demonstrating the value of maintaining a long-term historical dataset of this type. When data for 65 species over the period 1971-1995 were analysed, it was found that 20 species had significantly advanced their laying dates, on average by 8.8 days. This trend was not restricted to one taxonomic group, but occurred across a broad suite of species, including waders, resident and migrant insectivores, corvids and seed-eaters. A further study indicated that the laying dates for 31 of 36 species investigated were significantly related to either mean monthly temperatures or rainfall.
Although earlier laying appears to be advantageous as it permits a longer breeding season, changes due to climate change may lead to a lack of synchrony between the peak nestling food requirements and peak prey availability. This asynchrony could potentially result in reduced breeding success. The impact of climate change on breeding success is currently being explored, so watch out for future findings.
Making the most of BirdTrack data
We have been working to produce useful summaries for bird reports using data from the millions of annual BirdTrack records.