Some effects of Dutch elm disease on nesting farmland birds

Abstract

Certain arboreal bird species have benefitted in the short term from Dutch elm disease, though others appear to have suffered. This review of nest record and census data attempts to assess the impact and see what are the alternatives to elms. SINCE 1969 BRITAIN HAS SUFFERED a huge loss of elm trees U/mus spp. through infection with Dutch elm disease (DED). This disease is caused by the fungus Ceratocystis ulmi and is spread either by beetles of the genus Scolytus or through the roots of adjacent trees. The natural history of DED has been described by Mitchell (1973) and by Gibbs (1974). Early attempts at disease control by sanitation felling (i.e. felling the infected trees) were unsuccessful and the national campaign against DED was abandoned by the end of 1972 (Burdekin and Gibbs 1974). Further attempts at control concentrated on trees of special value, using fungicides and insecticides at great cost; but after field trials insecticides were also found to be unsatisfactory for general use (Scott and Walker 1975). By 1978 it was calculated that from a total of 17.1 million elms in selected counties, 10.6 million were dead or dying and 5 million had been felled since the start of the epidemic; DED was still the major tree health concern in Britain after nine years (Elkington 1978). This huge loss of trees was totally unpredicted because, although DED was first recorded in Britain in 1927, it had remained at a low level after a flare-up in the 1930s (Peace 1960). Surprisingly, little attention has been paid to the possible effects of DED on birds. Even in North America, where DED has been rife since the 1930s, more concern has been expressed about the use of insecticides for disease control (eg. Hunt 1960, Wurster 1965) than about the loss of habitat. Mitchell (1973) was probably the first to state the problem in Britain but little has followed. Wilkinson (1978) thought DED was not an ecological catastrophe but presented little evidence, while Pollard et a/ (1974) hoped that DED would draw attention to the general loss of trees in Britain and invited more research on hedge removal and birds. (Elm loss is a special case of hedgerow destruction and DED provides a natural experiment to study.) Throughout the 1960s and 1970s much research was done on hedgerow loss (see Pollard eta! 1974) but the conclusions drawn were conflicting and remain unresolved. Moore (1968) stated that the elimination of hedges would greatly reduce many vertebrate populations, while Terrasson and Tendron (1975) considered hedges essential for maintaining ecological balance on farmland. However, Pollard el al (1974) saw hedges as unessential for the survival of species in the country as a whole, and Murton and Westwood (1974) called them "suboptimal ... red-herrings", irrelevant to the welfare of birds in Britain. This paper is the first in a series reporting research aimed at answering some of the questions about the effects of DED and hedgerow loss on birds. Although no bird is solely dependent on elms for nesting, several species use them a great deal. Their most important features are the cavities which form in both limbs and trunk as a result of fungal decay, often caused by Rigidoporous ulmarius, Polvporous squamosu.s. and Pleurotus ulmarius. MacKenzie (1950) and Haartmann (1957) have noted that both intra-and inter-specific competition for nest holes can be severe because Bird Study 29: 2 16, March 1982 DUTCH ELM DISEASE 3 sites are limited. Elm cavities are often large and such sites are particularly scarce, so that any reduction in their abundance through DED might be expected to lead to population declines in such species as the Kestrel*, Stock Dove, Barn Owl, Little Owl, Tawny Owl and Jackdaw. The elm's susceptibility to fungal decay might also make it useful for nesting by Green, Great Spotted and Lesser Spotted Woodpeckers. Connor eta! (1976) found that