Zes, vijf, drie, twee, nul, of: de teloorgang van twee broedsels van Boomvalken Falco subbuteo
Drentse vogels , Volume 6 - Issue 1 p. 37- 50
Two Hobby broods were closely monitored during 1993. Both pairs settled within 1000 m of former breeding sites, probably during late April and early May, and started laying on old nests of Carrion Crows on 7 June. The laying interval in one pair was two days, in the other probably two days between 1st and 2nd and at least three days between 2nd and 3rd egg. Fresh egg weight was 25.5-26 g (Hertenkamp-pair, Table I). Incubation look 34 days for the 1st egg, 33 days for the 2nd and 32 days for the 3rd egg (Hertenkamp-pair). Weights and measurements of the nestlings are given in Fig. 1 (excluding the runt) and Appendix 1. First down was creamy white, second down (acquired on 8th day) was longer and thicker, pearl-grey on the upper parts (excluding head) and while below. Thighs and belly were naked up to the 8th day. The egg tooth disappeared on the 8th day (lx), 9th day (3x) or 10th day (2x). The mask started to develop from the 12th day onwards, first greyish, later on blackish. Primaries emerged from the sheath on day 12, the tail-feathers one day later. Body mass increased daily by 26-75% during the first six days of life, then by 7-16% up to the 17th day. Aggressive behaviour, such as hissing, calling and presenting claws while laying on the back, was shown from day 7 onwards, but pronounced individual variation was found. Nestlings were able to stand from the 15th day onwards, but rarely did so during the first three weeks. The females brooded their young non-stop until the smallest young had reached its 7th or 8th day of life. Thereafter, brooding was restricted to periods of rainfall. The females stayed at or near the nest during most of the nestling stage, at least up to the 24th day. Males were less often present, on 4 of 15 controls (Hertenkamp) and on 7 of 15 controls (Doldersummerveld). The latter male furiously attacked during visits to the nest. Predators were also relentlessly attacked, especially Goshawks. Despite this anti-predator behaviour, five of the six nestlings were depredated by Goshawks, the other young (a runt) died from starvation during a period of prolonged rainfall (Fig. 2). Nestlings were fed with vertebrates, mostly birds from the open air (Table 2). Of 44 prey items. 64% belonged to Juveniles.
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