Voous K.H. (1950) Klapeksters, Lanius excubitor L., met één en twee vleugelspiegels in Nederland. ARDEA 37 (3-4): 169-172

In addition to the paper by Finn Salomonsen (1949) on the European hybrid population of Lanius excubitor L., I have made an investigation of the racial status of this species in the Netherlands. Both single. and double-barred birds have been collected in these countries, but certain breeding birds had never been examined. I have gathered material containing 61 Dutch specimens of the Great Grey Shrike of which 45 (= 74 %) had two wing bars and only 16 (= 26 %) one bar, Not more than 6 specimens had been taken during the breeding season and in very likely breeding localities. In addition, 3 not yet fully fledged young birds have been examined. Only one out of these 9 birds showed one wing bar (May 11, 1881, Harderwijk, province of Gelderland) in addition to slightly barred under-parts. The remaining birds showed all characteristics of the race galliae. It thus appears that Dutch breeding populations of Lanius excubitor still contain an extremely slight Eastern element, though they are strongly tending towards the Southwest European race galliae. The rivers Rhine and Maas probably represent suitable boundaries between the hybrid populations of excubitor and the pure race galliae. Dutch breeding birds have the following wing measurements, which are not materially smaller than those from Scandinavia. Wing ? 111 and 113 mm; ? 107,109, 111, 116 mm. The majority of the single-barred Grey Shrikes collected in the Netherlands out of the breeding season must be migrants from the North.