Nestbouw en -constructie bij Buizerds Buteo buteo

During spring 1977, a Common Buzzard pair was observed for 142 hours during the prenuptial period and afterwards, mainly to obtain data on nest building and copulation behaviour. This territory in coniferous woodland became occupied by the male in mid- summer 1975 (taking over from a failed Goshawk Accipiter gentilis pair). He attracted a first-year female in late February 1977. The first egg was laid on 20 April, the second on 22 April. Between 11 March and 24 April, 29 copulations were recorded during 4350 observation minutes, i.e. on average 0.4 copulations hour1 but increasing to an average of 1 copulation hour' during the ten days before and during egg laying (Fig. 2). The last copulation was witnessed on 24 April, i.e. two days after the last egg was produced. The nest used for egg laying crashed on 10 June, and was dissected in order to analyse its structure. Between 22 February and 13 April, the pair consecutively “repaired” a Red Squirrel Sciurus vulgaris drey (22 February – 7 March) and built two new nests (respectively 9 March – 4 April and 5-13 April). The male initiated building on the first two nests, but the female incited building on the third nest that was used for egg laying. Nest building was typically performed in the early morning hours, except the first nest where building was recorded until noon with some activity even in the afternoon (Fig. 3). The majority of twigs and sticks involved dead material taken from the ground (Table 1: 84-97%), either by swooping down and grasping potential nesting material in flight, or by walking around and inspecting dead sticks by nibbling, tugging and grasping. Part of the material was discarded after screening, or lost during flight. A minority of material was taken from the upper half of trees where dead branches were broken by simply grabbing in flight (trying to break it by sheer mass and speed) or by dancing with flapping wings to dislodge or break dead and fresh twigs and branches. Fresh material was taken from trees (mostly larch and Scots pine) in the latter part of the nest building cycle, as witnessed in Nest C (and apparently added during incubation as evident from the fresh greeneries found on Nest C in early June). Building activity of the female, expressed as the sex-specific proportion of building flights towards the nest, increased in the course of the pre-laying period, from 4.4% in Nest A (N = 68 building trips of male and female together) to 27.5% in nest B (N = 69) and 64.8% (N=179) in nest C (Fig. 3). The latter nest was used for laying, after a frenzied building bout of only 9 days. For all three nests, the majority of the nesting material was collected within 100 m of the nest (Fig. 4). Nest C, which had fallen down on 10 June 1977, weighed 4.7 kg and was built on two dead branches of a larch Larix leptolepis (circumference of supporting branches near trunk amounted to 5 and 7 cm, with lengths of resp. 229 and 191 cm). The nest measured 134 x 94 cm, with a height of 27 cm. The nest cup was almost circular, 33 x 32 cm, and had a depth of 21 cm (Fig. 4). The outer layer consisted of448 dead sticks of larch Larix leptolepis, the nest cup of 955 dead sticks of larch, 24 live sticks of Scots pine Pinus sylvestris and 4 dead roots of heather Calluna vulgaris (Table 2). The nest cup was lined with fresh shoots of Scots pine and Deschampsia flexuosa. Fresh greeneries from Scots pine (N = 3), larch (N = 6), Prunus serotina (N = 3) and Quercus robur (N = 2) were found on the nest rim. The nest cup held an assortment of bark flakes from larch, beech, oak and Scots pine (N = 43, Table 3). Based on the observed division of labour at Nest C (female responsible for 64,4% of all material), the average length of collecting trips (39.7 m per oneway trip), the composition of the nest (N = 1426 twigs used for outer layer and nest cup) and the fact that only a single twig was transported per flight, it is calculated that the female covered 72.8 km (two way trip) just for collecting nesting material at nest C.