Kan een sperwervrouw Accipiter nisus haar nest tegen predatie door een Havik Accipiter gentilis verdedigen?

During a study on the energy expenditure of nestling Sparrowhawks in the northern Netherlands in 2003, 25 nests were checked daily at the time of egg hatching and at least once every five days throughout the nestling stage. At two of these visits feathers of a Goshawk were found underneath the nest; nevertheless all nestlings from both nests fledged successfully. It was not clear whether these feathers were moulted, or lost during skirmishes with the local Sparrowhawk. However, it shows that a Sparrowhawk nest detected by a Goshawk is not necessarily a nest lost. As the chance of feather loss by the Goshawk during a visit to a Sparrowhawk nest is likely quite small, such nest visits by Goshawks may occur even more frequently than observed. Additional anecdotal evidence is presented to back up this conclusion. It was observed that female Sparrowhawks can be quite aggressive at the nest; four out of 25 females in this study eventually physically attacked the observer during climbs to the nest. During these attacks, males were either not present, or watched from some distance (sometimes alarm calling). After having experienced blows to the head by female Sparrowhawk talons, both authors agree that also Goshawks would risk serious injury when trying to raid Sparrowhawk nests. In spite of this, nest predation is the most common cause of failure mentioned on nest record cards of Dutch Sparrowhawks. In this study, during 176 visits to Sparrowhawk nests during the nestling stage, only once was a female not present. None of the nests was depredated. Apparently, females are capable of deterring raiding Goshawks although we have found plucked Sparrowhawk females near the nest in the past (in the egg – or early nestling stage (when brooding females may be more vulnerable to attack). These observations suggest benefits of staying close to the nest for female Sparrowhawks (i.e. lower chance of nest predation). Therefore the “decision” of a female Sparrowhawk to stay at her nest during the nestling period may depend on two factors, i.e. the food provisioning quality of the male (impacting nestling condition, and hence their survival) and the chances of nest predation (by a Goshawk or some other predator). The demand for food varies in relation to the number of nestlings and the proportion of females in the brood. A female may decide to start hunting – in addition to the male’s hunting – when the demand for food is high and/or the male cannot provide sufficient food to satisfy the needs of its brood, thus increasing the chance of nest predation. On the other hand, the female may be able to ”estimate” the chances of nest predation by taking into account the frequency with which she successfully deterred potential predators from the nest (including the observer’s visits). The high frequency of nest visits by the observers (possibly perceived by the female as a high predation risk) may have caused the females in this study to stay at their nest throughout the nestling period, and hence the absence of predation.