Stephen R. Baillie

Long-term datasets in biodiversity research and monitoring: assessing change in ecological communities through time

The growing need for baseline data against which efforts to reduce the rate of biodiversity loss can be judged highlights the importance of long-term datasets, some of which are as old as ecology itself. We review methods of evaluating change in biodiversity at the community level using these datasets, and contrast whole-community approaches with those that combine information from different species and habitats. As all communities experience temporal turnover, one of the biggest challenges is distinguishing change that can be attributed to external factors, such as anthropogenic activities, from underlying natural change. We also discuss methodological issues, such as false alerts and modifications in design, of which users of these data sets need to be aware.

ANALYSIS OF POPULATION TRENDS FOR FARMLAND BIRDS USING GENERALIZED ADDITIVE MODELS

Knowledge of the direction, magnitude, and timing of changes in bird population abundance is essential to enable species of priority conservation concern to be identified, and reasons for the population changes to be understood. We give a brief review of previous techniques for the analysis of large-scale survey data and present a new approach based on generalized additive models (GAMs). GAMs are used to model trend as a smooth, nonlinear function of time, and they provide a framework for testing the statistical significance of changes in abundance. In addition, the second derivatives of the modeled trend curve may be used to identify key years in which the direction of the population trajectory was seen to change significantly. The inclusion of covariates into models for population abundance is also discussed and illustrated, and tests for the significance of covariate terms are given. We apply the methods to data from the Common Birds Census of the British Trust for Ornithology for 13 species of farmland birds. Seven of the species are shown to have experienced statistically significant declines since the mid-1960s. Two species exhibited a significant increase. The population trajectories of all but three species turned downward in the 1970s, although in most cases the 1980s brought either some recovery or a decrease in the rate of decline. The majority of populations have remained relatively stable in the 1990s. The results are comparable with those from other analysis techniques, although the new approach is shown to have advantages in generality and precision. We suggest extensions of the methods and make recommendations for the design of future surveys.

The use of constant effort mist-netting to measure between-year changes in the abundance and productivity of common passerines

The Constant Effort Sites (CES) Ringing Scheme of the British Trust for Ornithology aims to provide annual measures of change in the abundance and productivity of common breeding passerines in scrub and wetland habitats in Britain and Ireland. Changes in the sizes of the annual catches, from a set of standard mist nets operated during 12 summer (May–August) visits, are combined across sites to produce estimates of the percentage change in adult and juvenile numbers. The proportion of juveniles in the catch is used as a relative measure of breeding productivity. Methods are presented for calculating standard errors of between-year changes in both adult and juvenile catches, and changes in the proportion of juveniles. Present levels of precision are summarized and predictions are made concerning likely improvements in precision from a larger CES Scheme. For most of the species considered there was little evidence that between-year changes in catches of adults at CE sites have differed between habitats, regi...

Evaluating the Breeding Bird Survey for producing national population size and density estimates

Capsule The BBS has potential for producing better estimates of habitat-specific densities and population sizes for many UK bird populations than those available previously. Aims To examine the use of the Breeding Bird Survey (BBS) in providing unbiased national population and habitat-specific density estimates of British birds. Methods Line transect data collected by volunteers in 1998 from 2287 1-km squares across the UK were analysed using distance sampling methods to calculate habitat-specific density and abundance estimates. For each species, the habitat-specific decline in detectability with distance from a transect line was modelled and applied at a regional level to incorporate variation in sampling intensity in different areas of the country. Results National population and density estimates calculated here were at a magnitude expected for at least seven species in this study. However, national population size estimates were higher than expected for Starling Sturnus vulgaris, House Sparrow Passer domesticus, Blackbird Turdus merula, Greenfinch Carduelis chloris, Jackdaw Corvus monedula, Whitethroat Sylvia communis, Woodpigeon Columba palumbus and Linnet Carduelis cannabina and lower than expected for Skylark Alauda arvensis, Dunnock Prunella modularis, Song Thrush Turdus philomelos and Corn Bunting Miliaria calandra. These differences are likely to be related to differences in sampling design and survey methods. For example, Starling, House Sparrow, Blackbird, Jackdaw, Greenfinch and Wood Pigeon, which have considerable populations in urban areas, were undoubtedly underestimated by the Common Birds Census (CBC). The counts of species that flock during the breeding season, or are not strongly territorial (e.g. Linnet, Jackdaw and Wood Pigeon) could be biased if detectability is strongly related to flock size. National population estimates of Skylark and Corn Bunting are lower than those based on CBC, but higher, or similar, to targeted national surveys of these species. Possible reasons for the differences between these estimates are considered. Conclusions This study highlights the strength of the BBS over previous data sources in producing national estimates of density and abundance at the habitat and national level. More research on the sex ratio and status of birds counted during surveys, and on the reliability of the detectability functions derived from distance sampling is needed to improve the interpretation of population estimates derived from BBS data.

The demography and age-specific annual survival of song thrushes during periods of population stability and decline

1. The British song thrush (Turdus philomelos Brehm) population suffered a marked decline between 1975 and 1993, falling by 65% at an average rate of 5.7% per annum on farmland. We used ring recoveries to investigate whether this could have been brought about by changes in annual survival probabilities of fully grown birds. 2. The annual survival probabilities of first-year song thrushes averaged 0.484 prior to the population decline (1962-75), but dropped to an average of 0.405 in the years 1975-93. A long-term non-linear decrease accounted for 32% of the variance in the logit-transformed estimates of first-year annual survival probability. Adult annual survival probabilities did not show a similar decline; they increased then decreased over the same period, with 28% of this variance being described by a quadratic equation. 3. The changes in first-year annual survival probabilities alone were sufficiently large to account for the observed changes in population size. 4. First year annual survival probabilities were lower in years with cold (r 2 = 11%) or dry (r 2 = 20%) winters and together with the long-term decrease these effects explained 49% of the variance in the logit-transformed estimates of first-year annual survival probability. Adult annual survival probabilities were lower in years with cold winters (r 2 = 34%), and together with the long-term trend, this accounted for 51% of the variance. These relationships with weather conditions explained part of the variation about the long-term trends but did not explain the trends themselves. 5. This study demonstrates that an Integrated Population Monitoring Programme can not only alert conservationists to declining species but also elucidate the demographic processes underlying population declines on a national scale.

The UK Nest Record Scheme: its value for science and conservation

Capsule A review of its methodology, analytical procedures and uses. Aims To provide a comprehensive review of the UK Nest Record Scheme (NRS), its methodology and value to science and conservation. Methods We reviewed the history of the development and current methodology used in the analysis of NRS data in the scientific literature and unpublished documents and from our personal knowledge. Results The British Trust for Ornithologys (BTOs) NRS is the largest, longest running and most highly computerized such scheme in the world. Advanced and efficient techniques of data gathering, data capture and analysis are used. It was founded in 1939 to provide information on the breeding biology of birds, but has since developed into a key component of the overall monitoring strategy for birds in the UK. A range of specialized analytical methods is available for NRS data and potential biases need careful treatment. More than 250 scientific publications have used NRS data to describe aspects of basic breeding biology and performance, to study the population dynamics of bird populations and to investigate the demographic causes of bird population declines in the UK. Conclusions Extensive long-term monitoring schemes, such as the NRS, that collate large historical data sets will become increasingly valuable for monitoring the impact of environmental change. While aspects of the NRS, its recording methods, data capture and analysis can be developed further, the scheme will continue to advance our understanding of both the impacts of new environmental stresses in the UK and the effectiveness of new conservation measures in the wider countryside. This has been amply demonstrated by its use for exploring the impacts of global climate change on the UKs avifauna.

The widespread declines of songbirds in rural Britain do not correlate with the spread of their avian predators

During the last 30 years, there have been marked declines in the populations of many British songbirds breeding on farmland, while two of their main predators, sparrowhawk (Accipiter nisus) and magpie (Pica pica), have spread back into areas from which they had disappeared. The causes of the songbird declines remain unclear but given the coincidence in timing, it might appear that increased predation could be responsible. Although many studies have failed to find links between changes in the populations of breeding songbirds and mortality from avian predators, previous work has, with few exceptions, involved only short–term studies on small spatial scales. Here we use large–scale, long–term data from a national bird census scheme to examine whether magpies and sparrowhawks could have depressed the rates of year–to–year population change in 23 songbird species. Our results indicate that magpies and sparrowhawks are unlikely to have caused the songbird declines because patterns of year–to–year population change did not differ between sites with and without these predators.

The influence of female age on breeding in the Eider Somateria mollissima

Some female Eiders breed when they are two years old, and most do so by the time they are four. Young birds lay later, have smaller clutches and are lighter in weight at the start of incubation than older females.

Long-term changes in the abundance of passerines in Britain and Ireland as measured by constant effort mist-netting

Changes in the abundance of 28 species of common passerines in scrub and wetland habitats were assessed from changes in annual catch sizes on Constant Effort Sites between 1983 and 1995. At sites where annual catching effort fell below the required level, annual catch sizes were inflated according to experience during years with complete coverage. Changes in abundance were assessed through application of log-linear Poisson regression models, which allow tests for temporal trends in abundance and for differences in trends in abundance between habitat types. For most species, long-term changes in adult catches were similar to changes in territory counts on Common Birds Census plots, suggesting that standardized mist-netting is a reliable method for assessing extensive changes in songbird populations. Changes in catch sizes tended to be more positive and less negative in wetter habitats than in drier habitats. Catches of most insectivorous resident species either increased or remained stable, while catches o...

Modeling large-scale dispersal distances

We present an approach to analyze dispersal distance data. This approach allows one to take into account accuracy of the recorded dispersal distances. Three distributions were used, all assuming continuous space; a maximum likelihood approach was used for parameter estimation and model selection. Numerical simulations showed that our method is statistically consistent since it selected the correct model with increasing frequency when sample size increased. Ringing data on two species of tits (Parus caeruleus and Parus major) in Britain and Ireland were used to illustrate the potentialities of our method. In both species, adults dispersed significantly further than juveniles. The differences between species within an age-class were not statistically significant. In all species and age-classes, the model finally selected was the one assuming a heavy-tailed half-Cauchy distribution where long-distance dispersers are predicted to be more frequent than in the exponential model. The proposed methodology can potentially be applied to any organisms, and the model selection procedure can be used with any model of the distribution of dispersal distances (DDD). Several extensions are presented in the discussion, such as generalized linear modeling of the dispersal parameters, or interfacing with capture–recapture models.