BIJLSMA RG, RODER FE DE & BEUSEKOM R VAN (1988) The year 1984: an apocalyptic breeding season for Crossbills Loxia curvirostra in the Netherlands. LIMOSA 61 (1): 1-6.

Common Crossbill Loxia curvirostra A massive irruption of Crossbills swamped the Netherlands in 1983, resulting in unprecedented numbers of breeding pairs in the next spring. Based on large scale mapping in different parts of the country, it was calculated that the Dutch population in 1984 consisted of at least 5000 pairs (tab. I). Considerable fluctuations in numbers of breeding pairs were found in a woodland plot on the SW-Veluwe during 1974-87 (fig. 3); the five- to ninefold increase in 1984 is probably typical for the Netherlands as a whole. The highest densities occurred in 5x5-km squares with large areas of coniferous forest in Drenthe, Salland, Veluwe, and Utrechtse Heuvelrug (fig. I). Few territories were found in coniferous forests south of the Rhine. In the areas first mentioned, coniferous forests are older, more extensive and less fragmented than south of the Rhine, which presumably accounts for their attractiveness to Crossbills. The sequence of behaviour, leading from pair formation via territorial aggression, nest building and egg laying, to the production of fledglings, showed a delay of at least one month in 1984 as compared to normal years (1975-80, fig. 2). Although clutch size in 1984 did not differ significantly from the clutch size in 1975-80, viz. respectively 3.8 (n = 21) and 3.7 (n = 65), breeding success was exceedingly poor in 1984. Only 28 out of 156 pairs succeeded in raising one or more young (18%), compared to 79% in 52 pairs in 1975-80. Moreover, the average number of recently fledged young per successful pair was lower in 1984 than in 1975-80, being respectively 2.6 (n = 34) and 3.2 (n = 164). Ample evidence was gathered to prove that the poor breeding performance in 1984 was caused by the delay in the start of breeding. Rising temperatures in mid April made the coniferous trees to shed their seed, thus confronting Crossbills with an abrupt food shortage. An exodus of Crossbills followed, most pairs abandoning their nests with eggs or young. Few pairs stayed behind, feeding their young on a mixture of buds, decayed wood, and bark. It is argued that the wrong timing of the breeding season in 1984 might have been the result of inexperience (most birds being in their first year), partial depletion of the seed stock during winter (making it harder for the females to achieve a good breeding condition), and a genetically fixed start of breeding (most birds originating from Siberia, where a later start of breeding is suspected).

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