Terugplaatsing en adoptie bij een boomvalkpaar Falco subbuteo

In 2013, a Hobby pair was studied in farmland in the central Netherlands. The pair used an old crow’s nest situated at a height of 13.3 m in an oak Quercus robur of 18 m. The observations, usually from distances of 150-300 m, were distributed throughout the pre-laying and incubation period (19 h), nestling stage (66 h) and post-fledging stage till departure (101 h). Based on the behaviour of adults and plumage characteristics of the chicks, it was estimated that egg-laying had started on 11 June. The nest contained three chicks, of which the youngest fell from the nest when five days old. It was replaced the next day (well-fed) and was successfully raised together with its siblings. Fledging took three days of branching (difference between first and last chick), when the chicks were at least 32 days old. None of the chicks ever returned to the nest after fledging, but instead frequented electricity pylons in the vicinity where an old nest of Carrion Crows Corvus corone was used as resting place. With increasing age, the fledglings used a progressively larger area (Fig. 1), but up till departure on 2 October (last day recorded) covered a home range of only c. 50 ha centred around the nest site. The birds were fed by both parents, but the female was not recorded anymore after 5 September. The male was recorded (sometimes indirectly, via prey transfers that were heard only) through 24 September. Food delivery remained rather constant throughout the nestling stage and first three weeks after fledging (0.4-0.8 prey deliveries/h), then started to decline to nil after 24 September. Of 38 identified preys, 2 were voles (1x Microtus arvalis) and 36 were birds. The latter included 9 swallows (8x Hirundo rustica), 3 sparrows (1x Passer domesticus, 1x P. montanus) and 1 Linaria cannabina. The fledglings started hunting when 44 days old, but targeted insects only (dragonflies, crane-flies, beetles). Just after fledging, a fourth chick was released at the nest site. This bird had been found elsewhere on 14 August, fed and nurtured in captivity till 41 days old (wing length 245 mm), and released on 29 August. It was a week younger than the fledglings of the Binnenveld nest, but was seen in company of the three older chicks within two days of release. Although it seemed in a lesser condition than the three original chicks, it was accepted and often seen in company of the three other juveniles (although it is doubtful whether it obtained any vertebrate prey delivered by the adults). This bird was frequently seen hunting for insects on foot (more often than the other three chicks), mainly targeted at crane-flies Tipula paludosa and beetles. It was less successful that the other fledglings (one insect/min whilst foraging in grassland, compared to two insects/min for the juveniles involved in aerial insect hunting). It stayed in the area till 2 October, i.e. some days longer than the three juveniles from the original nest. The nesting site with fledgling Hobbies was visited by a Red-footed Falcon Falco vespertinus on 15 September (other observations of presumably the same bird nearby ranging from 8-21 September), without eliciting agonistic behaviour from the Hobbies.