Birds are important constituents of any ecosystem. But they are also valuable monitors of environmental conditions — if their populations begin to decline, we need to know why.
There is no doubt that some populations of farmland birds are in decline in both Europe and North America1. But what is the cause of such declines? This question cannot usually be tackled experimentally, but writing in the Journal of Applied Ecology Anders Møller2 describes an instance in which he has been able to do so. The bird concerned is the barn swallow (Hirundo rustica, Fig. 1). The place is Denmark, where changes in land use have allowed assessment of the effects of declines in dairy farming on swallow populations. Møller finds that the breeding success of swallows depends strongly on the presence of cattle.
Most investigations into population changes in farmland birds are based on correlations, usually relating observed changes in bird density to factors such as the intensity of farming. Correlations are then explained by reference to other factors, such as the availability of insects or seeds as food, resulting in the development of models in which cause and effect can be unravelled. But such reasoning from correlation can all too easily lead the unwary into error.
Møller's approach avoids such pitfalls. He has been able to conduct paired statistical testing of a hypothesis by comparing farms in which land-use changes have taken place with unchanged control farms. He has also been able to take a detailed look at differences in the physical condition and breeding success of his test organism, the barn swallow, under the two sets of conditions.
Barn swallows are still abundant in both the Old and New Worlds. Their population in Europe has fallen markedly in recent decades, however, possibly by as much as half since 1975. Observations that swallows seem to be more abundant on dairy farms led Møller to the view that a decline in dairy farming could be responsible. To test this hypothesis, he selected 15 farms in which dairy farming had been abandoned and compared them with 15 control farms in which all other factors, such as the availability of nesting habitats, remained unchanged. He checked for alterations in insect food abundance by regular sweep netting, and recorded the number of swallow broods and clutch size, as well as the physical characteristics and growth of nestlings and ringed adult birds, which were regularly netted and then returned to the wild.
Farms where dairy farming had ceased lost around 50% of their insects, and showed an average fall in swallow populations of 48%. The likely reason for this is the loss of tussocky grassland when arable farming is introduced, along with the loss of the dung associated with cattle. The direct cause of the swallow decline was the failure of one-year-old birds to return to the site; recruitment had crashed. The adult birds on both sets of farms did not show any significant changes in body mass, parasite load or any other of the many variables measured, so the problem would appear to lie with the production and survival of young. Nestlings examined on farms where dairy farming had been abandoned were of notably poorer quality, in terms of both size and immune response, than those on control sites. Clutch sizes were also smaller in the absence of cattle. These declines could be detected over the period of change in land use, while unchanged farms showed no such alterations in nestling number or quality.
This thorough and elegant piece of work establishes a clear connection between the presence of dairy cattle and the maintenance of high swallow abundance, operating through the effect of cattle on the density of insects in grassland. Although adult birds can cope with the change, their reproductive success and subsequent recruitment are badly affected. The implications are evident. If the decline in dairy farming in Europe continues, which is likely in view of the current pattern of European Community subsidies that is intended to reduce surplus milk production, then we can expect further declines in swallow populations. A proposal from the Netherlands3, backed by experimentation, suggests that farmers could be encouraged to conserve their bird populations by financial rewards linked to the number of breeding pairs of certain species supported on their land. Perhaps swallows need help of this kind. Equally, birds are sensitive indicators of environmental conditions. When their populations begin to decline — or in some cases rise — unduly, it is usually a sign of changes in the environment.
Meanwhile, in Britain, where the combined effects of BSE and foot-and-mouth disease have led to severe losses in cattle herds, an unexpected repercussion may well be a fall in swallow populations. Other insectivorous birds, such as yellow wagtails, could also be affected. As in Denmark, here is an unsought experimental set-up which, if adequately monitored, could further our understanding of the complex interactions between land use and bird populations.
Hagenmeijer, W. J. M. & Blair, M. J. The EBCC Atlas of European Breeding Birds (Poyser, London, 1997).
Møller, A. P. J. Appl. Ecol. 38, 378–389 (2001).
Musters, C. J. M., Kruk, M., de Graaf, H. J. & ter Keurs, W. J. Conserv. Biol. 15, 363–369 (2001).
About this article
Effects of environmental and management factors on the structure of bird communities in the grasslands of northeastern Tochigi, central Japan
Grassland Science (2014)
Biological Conservation (2011)