Hoe verhoudt door predatoren veroorzaakte (jongen) sterfte zich tot andere factoren die de afname van de Boomvalk als broedvogel in het westen van Noord-Brabant kunnen verklaren?

In 2009, three out of five Hobby nests in a study plot in the SW-Netherlands near Roosendaal were (partially or completely) depredated. Goshawk and Buzzard were suspected in two cases, in another instance the capture of the only fledgling of a Hobby nests in a hedgerow with Quercus robur was witnessed from nearby. The fledgling, nor the adult female, had shown any response as to the approach of the Goshawk; the latter started alarm-calling and dive-bombing only after the Goshawk flew away with the victim. In this part of The Netherlands (87,000 ha), as elsewhere, the Hobby seriously declined as a breeding bird; in the mid-1990s still 30-35 pairs had been recorded. This decline was evident throughout the region, although it started on the breeding grounds on sandy soil and later engulfed the territories situated on clay (the latter regio had been colonised at a later date than the Pleistocene region). In synchrony with the decline, clutch and brood size declined, egg laying was delayed and an increasing number of pairs refrained from laying. Nowadays, some territories have been deserted, but many others still hold single, often male. Hobbies. The present local population is estimated at 10 pairs. Neither predation nor food shortage is thought to have played a key role in the decline. Rather, a change in behaviour has been noted throughout the nesting cycle, in which many parents gradually have lost their vigour in nest defense. This change in behaviour may have made the species more prone to predation.