Factors affecting the population density of the corncrake Crex crex in Britain and Ireland

1. The population of the corncrake Crex crex (L.) in Britain and Ireland has been declining for more than a century. Corncrakes breed in hay and silage meadows and other tall vegetation. Losses of nests, chicks and adults occur during mowing of meadows. Previous studies indicate that the mechanization of mowing, earlier mowing and loss of habitats with tall vegetation have been contributory causes of the decline. 2. Singing male corncrakes were counted and land cover mapped in 1993 in 218 1-km squares throughout Britain and Ireland. Corncrakes had been present in all of the survey squares during a census in 1988. 3. Mean dates of mowing of hay and silage were determined from recent weekly survey data. 4. The number of singing corncrakes per 1-km square ranged from 0 to 10, with a mean of 0.6. 5. Multiple linear regression indicated that corncrake population density was positively related to the area of tall marshland vegetation (Iris pseudacorus, Phragmites australis and Phalaris arundinacea) in spring, and grass taller than 20 cm in summer, but that these effects were only apparent where the mean date of mowing was in late July or later. 6. Corncrakes occurred in all of 22 1-km squares which contained more than 2 ha of Iris, Phragmites and Phalaris, more than 10 ha of grass taller than 20 cm in summer and where the mean mowing date was later than 22 July. In these squares an average of 1.4 ha of Iris, Phragmites and Phalaris and 7.6 ha of tall grass was present per singing male. 7. It is likely that neither restoration of the preferred hay-meadow and tall marshland habitats nor late mowing will prevent the decline of corncrake populations if pursued on their own. Both of these objectives should be implemented over large areas of land.