Bal C. (1950) De nestbouw van Sperwers, Accipiter n. nisus (L.), in Nederland. ARDEA 38 (1-2): 19-34

Nest-building behaviour in the Sparrow-Hawk. During the spring of 1944 a pair of Sparrow-Hawks (Accipiter n. nisus (L.)) were observed while building their nest, in the central part of the Netherlands (8 miles W. of Arnhem). These Sparrow-Hawks built a completely new nest themselves, in a Scotch pine tree. The building process lasted about 3 weeks (15-IV-8-V), the last week being one of little activity. Both the male and the female built, each of them making 10 % and 90 % respectively of the flights observed. The sticks were collected from neighbouring trees, at distances of about 30 yards in all directions. All sticks were broken off, not one was picked up from the ground, or collected elsewhere. The hen had three methods of collecting: 1. She broke twigs off with her beak (fig. 2, 3). This she did in about 98 %of the cases. 2. She let herself fall on a twig, which then broke under her weight (fig. 4). 3. She grasped a twig with her talons, and then tried to turn round on her own axis. Thus the twig was broken off in horizontal plane (fig. 5). The cock only practised the first method. The pieces of bark did not get into the nest by accident (Niethammer), nor were they collected from the ground, but the hen actually picked them from the stems of trees (fig. 6) and took them to the nest with her beak (as the birds also do with most of the sticks). In actual building, the birds moulded the nestcup by 'scraping' with the talons, in quite the same way as many ground-breeding birds do (fig. 9). Twigs were fastened into the nest by means of persistent trembling movements. The flight from the trees to the nest was made at a very high speed, the return flight from the nest to the trees where sticks were collected, was made with partly closed wings, after some preliminary wing beats (fig. 10). During the period, when the young were between 10 and 21 days old, the hen again showed nest-building behaviour. This was not real building, however. It is best interpreted as a displacement-reaction (see N. Tinbergen 1940, 1942a and N. Tinbergen & J. J. A. v. Iersel 1947).