FEIJNEN HR (1976) Food, occurrence and decline of the Rook Corvusfrugilegus in The Netherlands. LIMOSA 49 (1): 28-67.

A survey of food studies of the Rook is given. The Rook prefers animal food (larvae of Coleoptera and Lepidoptera, worms etc.). It also eats grain. It is difficult to assess whether the Rook does more good or harm. The balance to be struck is not between the numbers of weights of grains and insects, but between the potential yield of the grain and the harm which the insects destroyed might have done. The source of both kinds of foods must also be known. Most of the papers reviewed consider the Rook as a predominantly useful bird for agriculture and forestry. Evidence is given that the vegetable part of the food has been overestimated in most papers, due to the methods used. The same conclusion of usefulness of the Rook can be drawn from food studies carried out in the Netherlands by Kloos. The resultsof his 1943-1944 inquiries are published in this paper. In 836 stomachs and 46 collar samples examined there were 20,840 pieces of vegetable remains and 16,780 of animal origin. Per sample 23.6 vegetable remains were found as against? 19.0 animal remains. Most of the vegetable remains were grains (22.1 out of 23.6). However many of them had been eaten off stubble. Most of the animal material (14 out of 19) was of noxious insects. The mean clutch size of the Rook, found during investigations by Van Koersveld in the Netherlands in the years 1948-1951, was 3.7. Clutches of 4 eggs were found most frequently. The breeding-results of the 4-clutches were better than those of the 2-, 3- and 5-clutches. Mortality of the young was found to be high. In the course of IS weeks 93% of the offspring present on the 15th of April disappear. After August the percentage of first-year Rooks is about 10%. The percentage of permanently empty nests (6.8%) fits the percentage of second-year Rooks (7%). The sex-ratio is almost I : I. Several birds of prey and 2 martens are predators of the Rook. Lockie suggests that diseases and parasites are primarily responsible for death. among Rooks. The nematods Diplotriana tricuspis and Syngamus passerum (gapeworm) are important, as is lsopora passerum. Severe winters, cold or stormy springs and sudden storm or fog over sea in the migrating-time are also responsible for deaths. A survey is given of the occurence of the Rook and control measures in several countries. A decline in the Rook population has occurred in Scandinavia, East- and West-Germany, France, Belgium, Austria and Italy. These declines are the result of shooting, disturbance of nests and poisoning. The Rook became a new breeding-bird in Switzerland. In Britain the Rook population increased slightly till about 1945. In Britain about 6 pairs of Rooks occur per km2 ? The largest rookeries occur in Hungary (in 1943 a colony with 16,000 nests). Other large colonies were found in Germany (in 1898 a colony with 9,000 nests) and in Scotland (6,085 nests in 1945). In the Netherlands the Rook was protected till 1942. However before 1942 shooting, disturbance of nests and poisoning were certainly not unusual. In 1942 the large rural fight against Rooks started with disturbance of y., of the total number of nests. Disturbance of nests was repeated yearly till 1950. In 194329.000 nests were disturbed (7'3 of the total). From 1943 till 1949 a premium was paid for shooting adult Rooks. A total of 23,372 Rooks were shot during those years. Most Rooks were shot during the breeding-time, by contrast with other noxious birds like Woodpigeon, Carrion Crow and Magpie. Proportionally more Rooks were killed than Woodpigeons, Carrion Crows and Magpies, although hunters prefer to shoot the last two and although the Woodpigeon is considered to be more harmful. This high proportion is due to the vulnerability of Rooks as colonybreeders. Because of the shooting in the rookeries 58% of the killed Rooks were females (in May the percentage of females was 69.5%). Between 1950 and 1965 the method used during the rural campaigns was as follows. The Rooks were kept from their nests by shooting at the time that the nestlings were small. This was done for 24 to 72 hours and preferably during cold weather. If the Rooks did not leave the colony, the method was repeated 3 weeks later. This method was employed, especially for large colonies, for several consecutive years. Besides rural campaigns, local action in the form of shooting, disturbance of nests and poisoning took place and still takes place. Poisoning was usually carried out wi~h poisoned grain (using strychnine, parathion, etc.). Between April 1969 and March 1970 7260 Rooks were shot or poisoned in the eastern parts of the Netherlands. Rooks also fall victim to 'unintentional' poisoning from e.g. seeddressings. In the Tables 13 and 14 the results of several censuses can be seen. The numbers recorded for 1924 and 1936 are probably too low and the result for 1950 is just an extrapolation. The distribution of the colonies is shown in the Figures 2-5. The preference for areas near water is quite obvious. . Between 1944 and 1970 the Rook population fell to 20% of the 1944 population; In the period 1967-1970 the population fell by 25%. The Rook has disappeared from the western parts of the Netherlands. In 1944 there were still 9 colonies with over 1000 nests (one with 2240 nests). In 1970 the largest rookery had 377 nests; The density of the Rook is now only I pair per3 km2 ? The decline of the Rook in the Netherlands is for the greater part the result of control operations. Disturbance of nests and shooting in the breeding-time have been especially effective. Poisoning (both intentional and accidental) has also played a part. The reduction of numbers of suitable places for rookeries is also responsible. As there is no evidence for harmfulness of the Rook, its protection is recommended Rook Corvus frugilegus

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