BIJLSMA RG (1982) Crossbills Loxia curvirostra on the SW. Veluwe. LIMOSA 55 (3): 85-92.

Common Crossbill Loxia curvirostra In this paper the establishment of a small population of Crossbills Loxia curvirostra on the Southwest Veluwe in the centrc of the Netherlands during the seventies is described. During January 1974 to December 1980 two large invasions were recorded, viz. from June 1976 onwards and from July 1979 onwards with a peak in October (figure 1, lower graph). In this graph counts of wandering and foraging Crossbills are shown (n = 40000). Systematic counts during peak passage in 1976 resulted in an average of 1550 migrating Crossbills per hour (12 hours) while ten observation hours in October 1979 revealed 35 510 migrants. The 1979 invasion was accompanied with several tens of Two-barred Crossbills Loxia leucoptera (records of Loxia leucoptera not accepted by the Rarities Committee). Most pairs bred in old coniferous woodland dominated by Scots Pine, with a lot of clearings and an undulating underground. Inter-nest distances were mainly less than 500 m in this habitat (figure 2, 3). Scots Pines constituted also the most important feeding places during late winter, spring and early summer (figure 4). During 1975-80 the start of laying in 164 clutches could be calculated with an accuracy of half a month (figure 5). Most pairs started egg-laying in February (57%) and March (35%) so that most fledglings were found in March and April. It is unknown whether late clutches in May and June were first broods or related to second or repeat layings. Start of laying hardly differed from year to year. Even after the severe winter of 1978179 the first clutches were laid in February. Several females were found brooding with night temperatures as low as _8?C. Despite frost and snow all these nests resulted in hatched eggs. To sum up, data of 65 nests with eggs, 97 nests with young and 164 pairs with recently fledged young were included. Mean clutch size was 3.72 (n = 65, sd = 0.75) and slowly decreased during February and March. The same pattern was detected in nests with young (x = 3.14, n = 97, sd = 0.84). Mean number of fledglings per pair decreased from February up to the first half of May and thereafter increased again (x = 3.16, n = 164, sd = 0.90) (table I). However, sample size was very small in the latter part of the breeding cycle. Breeding success was calculated on the basis of 8 layings in 1976, 17 layings in 1977, 15 layings in 1979 and 12 layings in 1980. Of th'ese 52 layings, seven were lost during the egg stage and four during the nestling period. The remaining 41 nests resulted in one or more fledglings. In sum 189 eggs were laid of which 155 hatched (82%) and 137 fledged (72%). Breeding success was highest in C/3 (81%, n = 25), lower in C/4 (68%, n = 21) and still lower in C/5 (63%, n = 6). However, the absolute number of fledglings per laying was highest in C/5 (x = 3.2 young/nest), followed by C/4 (x = 2.7) and C/3 (x = 2.4). From visits to other parts of the Veluwe it is concluded that in the period 1973-77 at least 325 pairs were present on the Veluwe (1200 km2) and probably 350-400 in the Netherlands as a whole. This is much more than the estimate of 25-75 pairs in Van Dijk (1979). Furthermore, the Veluwe population probably can exist without being supplemented by regular invasions. In contrast to the province of Drenthe (van Dijk & Swart 1979) no relation was found between invasions and the size of the population in the next breeding season (figure 1). As compared with other finches on the Southwest Veluwe Crossbills had a small clutch size, a high breeding success and an early breeding season (table 2). Theoretically the Crossbill is capable to. produce several clutches per year, especially when succeeding cone crops of Larch, Scots Pine and Norway Spruce are exploited.

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