Persecution and statutory protection have driven Rook Corvus frugilegus population dynamics over the past 120 years in NW-Germany

Abstract

In 1898, the first complete survey of breeding Rooks Corvus frugilegus in all state and municipal forests under the administration of the state of Lower Saxony, NW-Germany, revealed a population of 26,444 pairs. The population in its entirety may be estimated at approx. 30,000–35,000 pairs. With the help of eclectic data corrected according to observation effort, it is possible to trace the development of the population for the subsequent period until 1954, and from 1955 to 2017 to quantify it annually on the basis of complete surveys. The picture emerging from this is of a population decimated by 1976 to 6.5% [2104 ± (SE) 308 pairs] of the 1898 value coupled with considerable diminution of the species’ range. Placing the species under protection from 1 January 1977 resulted in a sustained increase of the population to about 86.4% of the initial value by 2017 [28,064 ± (SE) 1103 pairs]. The period of sharp decline is mainly attributable to the unrestrained persecution (shooting, poisoning, destruction of nests and clutches) to which the species was at that time exposed, a factor which, however, also obscured a number of other contributory biotic and abiotic factors. There are clear differences between the period of persecution and the period of protection with respect both to the ratio of colony number to colony size and to the population development of newly established colonies as well as to their growth rates in the initial phase. There is no doubt that persecution was at the root of the sharp decline, or that statutory protection was the mainspring of the population recovery. The population has developed well since 1977, despite a 45.1% reduction in the proportion of permanent grassland area throughout the country and the conversion of existing grassland to industrially run intensive farmland. At the same time, there has been a considerable increase in the area under maize (silage and energy maize), removing further suitable foraging grounds in the breeding season. This suggests that across Central Europe as a whole the Rook is much more flexible than previously assumed. Statutory protection will, however, remain crucial for the conservation of the population. To retain political support for this protected status, Rook management concepts entailing the establishment of toleration zones for breeding sites are urgently required for urban settlement areas.