Daily census data held by the observatories for Corn Crake, European Turtle Dove, Eurasian Wryneck and Spotted Flycatcher, 1946-2000.

Summary

The report analyses the data for the four species on the basis of bird days recorded in the daily logs of 15 observatories. Following the work of the Game Conservancy on the phenology of Turtle Dove migration, using these data, the mean arrival date for Turtle Dove and Spotted Flycatcher has been examined. The data have been adjusted and re-examined by eliminating possible sources of error due to the presence of breeding birds in an Observatory's recording area or the probability that some observatories are counting birds that are not breeding in Britain.

The data for Corncrake show a clear trend in accordance with the known decline of the species in Britain. Although this is a species that is rarely recorded at Observatories it was much more frequent in the 1940s and 1950s than in the following decades but still occasionally recorded. In spite of the rarity of this bird at the observatories the data track the decline of the species very well.

The same is true of the data for Turtle Dove where the index calculated from the observatory data is closely correlated with the CBC index. The Turtle Dove data are capable of showing a clear earlier autumn migration in accordance with the reduction in nesting attempts found by the Game Conservancy. There is no indication of any change in the spring migration.

The data for Wryneck is scanty and difficult to interpret. It is concluded that if this species is no longer breeding in Britain, the birds now seen at the observatories are continental birds.

Spotted Flycatcher data from the observatories indicates a very different pattern of population trend than the CBC. Numbers at the observatories have only shown any decline since about 1995 whereas the CBC index has shown a consistent decline since about 1970. The numbers recorded at the observatories are skewed by the large numbers seen at the Observatories on the west coast. Ringing evidence suggests that many of these birds are migrating to and from Ireland. One interpretation of these differences may be that until recently the species has not shown as dramatic a decline in Ireland (poorly monitored by the CBC) as in southern England. There is no evidence of any change in the phenology of migration of Spotted Flycatcher.

The use of data from the observatories for population monitoring is discussed with reference to work in Scandinavia and North America. It is concluded that the variation in the observatories' data is such that these data are not statistically robust enough for them to stand alone as population -monitoring indices. Nevertheless they are useful confirmatory data or can point to possible inadequacies in other monitoring indices or provide insight into species at present unmonitored.

The full report (35pp, 2 Tables, 18 Figures) can be obtained from Peter Howlett, Dept. of BioSyB, National Museums & Galleries of Wales, Cathays Park, Cardiff, CF10 3NP - £5 including p&p.