RT14: Birds and agricultural change: past, present and future research for conservation and management

¹British Trust for Ornithology & University of Oxford, UK. Fax 0944 1842 750 030; ²Swiss Ornithological Institute, Sempach, CH-6204, Switzerland, e-mail schifferli@orninst.ch

Siriwardena, G.M. & Schifferli, L. 1999. Birds and agricultural change: past, present and future research for conservation and management. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 3187-3189. Johannesburg: BirdLife South Africa.

In Europe and North America, agriculture is the dominant land-use and elsewhere the proportion of these areas used for farming has increased in recent years. Agricultural landscapes are therefore an important habitat for many bird species for breeding, as a staging habitat during migration and as a wintering ground. Changes in farming practice have affected the fauna and flora of farmland at scales ranging from the field to landscape levels. The consequent conservation and management issues represent international problems, yet the situations in different countries differ markedly. In our Round Table Discussion we wanted to bring together an international forum to assess our current understanding of the relationships between birds and agriculture and to improve future communication between workers interested in different, but related, systems. We aimed to discuss the questions numbered below. Although the discussion was allowed to flow freely and did not follow this fixed agenda exactly, we believe that these questions summarise many of the central issues in the conservation of agricultural birds today. Some points arising from the discussion are summarised after the list of topics.

Have population changes in farmland birds been similar, internationally, in timing and scale?

Are they a general phenomenon, or are they more pronounced or limited to certain species groups, such as migrants versus residents, seed-eaters versus insectivores, or to arable versus pastoral farming regimes?

How can causal historically (and currently) important environmental changes best be identified? How useful are comparisons of contemporary habitats and species' histories?

Are particular species especially well suited for use as indicators of environmental change over wide geographical areas?

Can we apply the methods and lessons of European studies to problems elsewhere?

What are the habitat requirements of species breeding in farmland (habitat quality and area; minimum requirements)?

Are species densities and distribution patterns related to farmland structure, habitat diversity and land use?

How may such correlations between landscape and density be tested for causal relationships? Can we use existing variations in farming practice as natural experiments (e.g. in farmland with annually rotating crop systems, or where grassland is converted to arable crops)?

What is the potential of large-scale field experiments for testing proposed mechanisms for population change in farmland birds: are they useful/feasible and can they replace more extensive, observational work?

What is the role of habitat diversity per se? It is often argued that species living in monocultural farmland have to rely on fewer prey species, perhaps leading to seasonal shortages or bottlenecks during adverse weather conditions; in diverse farmland birds might switch to alternatives.

Populations of birds living in farmland can only be considered as safe in the long-term if reproduction, mortality and survival of adults and juveniles are in balance.

Are fluctuations in population sizes and especially long-term trends (declines or increases) generally linked to changes in breeding success, survival of juveniles (after fledging and/or in winter) and/or the annual survival of adult birds? What is the role of dispersal, immigration and emigration? In Britain, at least, it appears that changes in annual survival rates and not changes in per attempt breeding success have driven most species’ long-term population trends. Post-fledging mortality can be high and the number of breeding attempts possible may be the most likely variable to change in some situations, but neither of these variables are well-monitored anywhere.

How has farming practice affected demographic rates? High intensity farming is characterised by large fields and monocultures, the use of fertilisers and pesticides. Has increased pesticide use had important indirect effects? Or have changes in grassland management or arable cropping patterns (for example) had stronger impacts?

Have recent changes, such as the growth of organic farming and the provision of set-aside, improved farmland for birds?

Do we know enough about the detail of the interaction between farming practices and birds’ habitat preferences (for example the importance of the crop types sown before the current land-use began)?

Where are the major gaps in our knowledge on farmland birds and their habitat? Can we now provide advice for farmers that is detailed enough to give them guidelines for day-to-day or week-to-week management of their land, or are we limited to recommendations for a broad farmland type or management strategy?

Management of the problems caused to agriculture by birds: are conflicts between the needs of conservation and pest management (involving the same or different species) a problem? If not, will they become one in the future?

Participants in the discussion stressed that there are vast differences in farming practices and food production between continents, ranging from over-production to famine. Consequently, the effects of land-use on farmland plant and animal communities and the priorities given to conservation differ as well. In most continents trends in populations and distribution and relationships with agriculture are still poorly known. A major determinant of the kinds of conservation problems posed by agricultural change is the age of the pattern of land-use found in a country: field landscapes in Europe can be several hundred years old and support a specialised avifauna whereas younger systems such as that in North America are characterised by a relict, impoverished fauna derived directly from that present in the indigenous habitat. This difference has led to a divergence between the conservation strategies generally applied: recommendations in Europe tend to have focused on the extensification of the broader agricultural landscape, whilst the protection of pristine, uncultivated areas within intensive farmland seems to be the preferred option in North America.

In western Europe, where agriculture has become very intensive, farmland features a higher proportion of species declining markedly than any other habitat, and farmland birds form the majority of endangered species; the same holds for other taxa. Whereas ‘generalist’ species have persisted in intensively farmed landscapes and even sometimes become ‘pests’, farmland specialists tend to have declined in abundance and range. Political changes leading to intensification in eastern Europe, where many breeding birds declining in the west are still wide-spread, might soon be followed by similar population trends.

Data on patterns of population change within and across species and on trends in agricultural practice may reveal what the important agricultural factors are. In this way, trends in bird populations can also act as indicators of the overall health of the agricultural environment. Indicators for farming intensity, studied over a large area seem very attractive, but care should be taken to select suitable species (and it should be accepted that a suitable species may not exist). While recent trends in breeding bird populations have been attributed to the intensification in agriculture, detailed ecological studies investigating the effects of alternative farming systems in Europe may not be sufficient to interpret the complex interactions between habitat and bird communities, since many changes are interlinked and occur simultaneously. Participants stressed the need for experimental approaches, but the problems in obtaining funding for sufficiently large-scale designs were acknowledged.

It is still unknown to what extent recent European schemes intended to reduce overproduction and to improve farmland as a habitat for wildlife have benefited birds, and more work is needed to assess the usefulness of these measures. Set-aside and ecological compensation schemes, for example, tend to produce areas which are chosen by birds for nesting or foraging, but there is currently little information available on consequences at the population level. The distribution of a given area of set-aside land, for example, in the agricultural landscape may be critical to its effectiveness as a conservation measure: interactions between population density, food use and predation pressure are likely to depend on the size of the available patches of preferred habitat.

One major consequence of intensive farming is the loss of incidental habitats associated with agriculture and hence reduced biodiversity. To reverse this trend, hedges and other habitats have been re-established in farmland in some countries. However, such schemes require subjective decisions as to how much of each of a range of desirable habitats should be re-introduced. International co-operation and co-ordinated ecological studies are needed to prevent the favouring of certain species guilds (such as woodland birds) to the detriment of others (such as birds of open landscapes).

Thanks are due to all those who attended the RTD or who suggested topics for discussion and thus helped make the session interesting and fruitful. Thanks also to Ian Henderson for his help with this extended abstract. GMS thanks Richard Bradbury and the University of Oxford, the BTO, Baxter Healthcare plc and Thermos plc for supporting his attendance at the IOC.