Report from the Sub-comittee on the environment and health

Birds are particularly interesting because a breeding bird index is
prepared each year on <strong>the</strong> basis of counts all over <strong>the</strong> country. Birds are
thus <strong>the</strong> only group of organisms for which we have monitoring data that
can be related to pesticide consumption over time. It is known that birds
are affected by a number of o<strong>the</strong>r agronomic variables as well and also
exhibit climatically determined population fluctuations. Some species are
migratory birds, which are affected in <strong>the</strong> winter period by <strong>the</strong> conditions
in o<strong>the</strong>r countries, including <strong>the</strong> use of pesticides. To summarise, <strong>the</strong>
relationship between <strong>the</strong> breeding bird index and <strong>the</strong> use of pesticides is
complicated, and <strong>the</strong> use of pesticides can be regarded as one of many
factors in a multifactorial complex. That means that qualitative or
quantitative changes in <strong>the</strong> use of pesticides are hardly likely to be
directly reflected in <strong>the</strong> year’s index but may be reflected in <strong>the</strong> trend
over a number of years.
The population fluctuations of sixteen species of bird in <strong>the</strong> period 1976-
1996 have been described by ma<strong>the</strong>matical models as a function of a
number of climatic variables, land use, treatment frequency for
pesticides, size of population in <strong>the</strong> previous year and several o<strong>the</strong>r
factors (Petersen, Jacobsen 1997). For three species – wood pigeon,
sparrow and yellowhammer, <strong>the</strong> model indicated that <strong>the</strong> use of
herbicides and/or insecticides had a negative impact on <strong>the</strong> population
size. For <strong>the</strong>se three species, a simulation of <strong>the</strong> effect of a reduced
treatment frequency index (<strong>the</strong> action plan’s goal) on population size was
carried out. For wood pigeon and sparrow, <strong>the</strong> simulation indicated a
considerably larger population, whereas <strong>the</strong> effect on <strong>the</strong> yellowhammer
population was negligible. It is not certain how <strong>the</strong>se results should be
interpreted, firstly because <strong>the</strong> method is based on a number of
assumptions and, secondly, because it is not clear how much importance
should be attached to <strong>the</strong> size of <strong>the</strong> previous year’s population. The
breeding success of <strong>the</strong> skylark was thus seriously reduced by <strong>the</strong> use of
pesticides in <strong>the</strong> individual year but no clear link to population
development has yet been established.
Table 5.2 shows <strong>the</strong> bird species in farmland that are included in <strong>the</strong>
scenario calculations in this report (see 10.3.1). The table gives <strong>the</strong> size
of population and development of <strong>the</strong>se species in <strong>the</strong> period <strong>from</strong> 1976
to 1996. The development has now stabilised and some farmland birds,
such as <strong>the</strong> crow, have shown some progress. One of <strong>the</strong> most vulnerable
and specialised farmland birds – <strong>the</strong> corn bunting – is not included
because <strong>the</strong> available data are thought to be insufficient (Petersen, Jensen
1998).
Table 5.2
Characteristic farmland birds for which sufficient data are available for
modelling <strong>the</strong> population development in <strong>the</strong> different scenarios. The
table also shows <strong>the</strong> latest estimate of <strong>the</strong> size of each species population
in Denmark and <strong>the</strong> population trend in Denmark <strong>from</strong> 1976 to 1996,
using <strong>the</strong> following symbols: -: decline, +: increase, 0: unchanged (+/-
20%), 1: 20-50% change, 2: >50% change (Based on Petersen, Jensen
1998).
Species Number of breeding pairs in
Denmark
Trend
67