Blauwe Reigercensus 1970-1976

Numbers of nests of Grey Herons in the Netherlands were obtained by organized censuses from 1973 onwards. The pre-1973 data were obtained by making inquiries in later years. The numbers presented relate to the maximum occupation of the colonies. Extreme care was taken to use uniform and reliable methods. The data were checked by comparison with a closely studied sample-group of colonies of different sizes. For these reasons the figures given are not entirely comparable to previous censuses, either in the Netherlands or abroad. More than 2000 observations from about 300 observers were collected and on average nearly half of the colonies were counted twice in every season. After a period of continued increase a maximum of 10.890 occupied nests and 232 settlements was reached in 1975, followed by a decline of 18 percent of the number of nests in 1976. The numbers for 1975 are the highest ever recorded in the Netherlands, though higher, but uncounted, numbers may have occured in the first half of this century. In the absence of the need to recover from a hard winter the population showed a striking capacity to increase at an exceptional rate. Possible causes, as well as the function of colonies of different sizes and the colonisation of new areas in this growth pattern, are discussed. The international importance of the Dutch Grey Heron population is stressed and illustrated by comparing it with recent European census data. In all cases the Dutch figures were considerable higher than the others. The maximum density is 3 times higher than the highest German figures and 8 times those from Engeland and Wales. This material will be used for further analysis to estimate the influence of the errors usually involved in this type of census work and to construct a reliable sample model for future heron censuses. A.A. Blok & M. Rods, p/a Instituut voor Taxonomische Zoologie, afdeling Vogels, Plantage Middenlaan 53, Amsterdam-C.