Hoe het nest van een Kruisbek Loxia curvirostra te vinden

To find a Crossbill nest, it is necessary to be aware of timing and meaning of activities which lead to pair formation, occupation of a territory and nest building (Table 1). Timing of the breeding season varies from year to year, with a tendency in The Netherlands of starting later since 1984 (Bijlsma et al. 1988; Table 1). Flocks disintegrate in February and early March (Fig. 1, based on observations in years following large irruptions, i.e. 1991 and 1994). Pairs settle in semi-colonial groups in the vicinity of feeding areas and water, Full song can be heard throughout the year, but intensifies in February and early March. At that time of the year, it can be used for mapping purposes. Pair formation takes place within the flocks and is typically accompanied by pursuit flights (male chases female). As soon as pairs have settled, males guard their mate against strange males (rather than defending a territory) and the frequency of antagonistic interactions increases, probably to prevent extra-pair copulations. Nearest-neighbour distances can be quite small (less than 30-50 m in years with a high density). Courtship-feeding and copulations precede and overlap with the period of nestbuilding and both activities indicate the presence of a nearby nest. Copulations are seldom witnessed, presumably because they take place very quickly. A helpful indication to find a nest site is a male in the top of a tree, giving his warbling song, interspersed with soft ’tuktuk’ alarm calls. Alarm calling sometimes intensifies if the bird is approached. It is a soft, but nevertheless rather far-carrying song which can be heard over distances of up to 100 m (in quiet weather, but very difficult to hear during adverse weather conditions). These males are accompanying their females during nest-building. Waiting in the vicinity of warbling males is the best method of finding nests, because he follows his mate to and from the nest. Nestbuiding normally occurs in early morning, but short bursts of building can occur at any time during daylight hours. Nest-building takes 5-15 days, and is sometimes interrupted for a few days. Female Crossbills do most, if not all, of the building, including the choice of nest site. The latter can take a while and many spots are tested before a choice is made. Even so, the site can be abandoned after a few days and building resumed elsewhere. Females will fly up to 150 m to collect nesting material, but usually stick to trees within 60 m of the nest site. Females are willing to fly longer distances to collect material for the lining of the nest (Fig. 2), which should be kept in mind when searching for nests. Five females at Berkenheuvel flew on average 57 m to collect nesting material (109 flights, SD=35.6, range = 10-150 m, median distance 50 m), but showed striking individual differences. During egg laying, females are hard to detect and males subdue their vocalisations. When feeding its mate (by male) or the nestlings (usually both parents together), the nest is approached indirectly. The bird(s) land in nearby trees and start giving the contact call (resembling alarm call) which is subdued before visiting the nest. Feeding itself is of short duration. The begging calls can be heard from the ground (and consequently, the nest can be located). Parents eat the faeces of nestlings, but progressively less so with advancing age of the young. After ten days, the young start defecating on the nest rim. These droppings are visible from the ground. A pile of faeces can be found underneath the nest on branches or the ground, indicating successful fledging. Foraging flights of parents can be used to trace the nest, especially when following birds with a full crop. This method is not profitable with long-distance feeding flights (over 100 m) because of logistic problems of following fast flying birds in difficult terrain and low frequency of visiting nests with food (once per hour or less). Cold searching for nests is especially worthwhile in pine and larch forests, but very difficult in forests of fir and spruce. An indication of the location of the nest is necessary before cold searching is adopted.