Willem van Manen (2024) Voluntary or forced urbanisation of Rooks Corvus frugilegus in the Netherlands. LIMOSA 97 (2): 79-87.

In the Netherlands, rookeries have been systematically counted by volunteers since 1980. In 1980 about 75% of all colonies were counted, in 2022 an estimated 95%. Overwintering rooks were counted at points (5 minute counts) in the second half of December, starting with 780 points in 1978, 2500 in 1980, over 5000 from 1983 and about 14 000 in recent years. The rookery counts and the winter counts (PTT) are organised by Sovon Dutch Centre For Field Ornithology.
In the Netherlands, urban areas and forests have doubled since 1950, heathland and peat bogs have been reduced by a factor of five and the area of arable land has barely changed from 75% to 65%. Crops such as oats and rye have almost disappeared in favour of maize. The number of farms has fallen eightfold, farming has become more intensive, plots have become larger, water tables have fallen and recently cattle are kept more and more indoors. During the period 1978-2023, breeding Rooks preferred urban areas, wintering rooks were observed on farmland more than expected, while water, woodland and other natural habitats were avoided in both seasons (Tab. 1).
After a low in 1970, due to the use of herbicides, breeding numbers increased in the 1980s and 1990s, reaching a maximum around 2000, after which numbers declined again (Fig. 1). The decline in wintering numbers is much more pronounced and began a decade earlier, as breeding populations became resident in Eastern Europe, as evidenced by declining numbers of migrating birds observed along the eastern border of the Netherlands (Fig. 2). It is likely that wintering in the Netherlands by non-native Rooks ceased completely around 2010.
The decline in breeding birds started in the 1990s in the southern part of the Netherlands and was not observed in the north of the country (Fig. 3a). The decline in wintering birds was much stronger in the southern than in the northern half of the country (Fig. 3b).
Colony size decreased almost from the beginning of the counts in 1980 and colonies with more than 500 nests became rare after 2000. The decline in colony size was greater in rural than in urban colonies (Fig. 4a), and the proportion of nests in urban habitat increased from 20 to 75% between 1980 and 2023 (Fig. 5). At the beginning of the study, colony size correlated positively with the amount of arable land within one km of the colony, but this was reversed by the end of the study period (Fig. 6). There was only a modest decrease in the average number of birds counted at the points in December (Fig. 4b).
It is discussed that the urbanisation of the rook is merely a result of the deteriorating conditions in Dutch farmland compared to the profitable conditions in built-up areas. A stronger decrease in the southern half of the Netherlands coincides with more intensive farming in this part of the country.

[pdf only for members] [dutch summary]