The number of successful hunting attempts by male and female New Zealand falcons was similar between stand edges and interiors (Table 4.5). Females made fewer successful hunting attempts during the breeding season than males. Males made most successful hunting attempts within stand interiors less than four years old, more than 20 years old and along stand edges between these two classes of pine stand. Females made the most hunting attempts in stands less than four years old and along stand edges between stands less than four years old and more than 20 years old, but in contrast to males had very few successful hunting attempts within stands over 20 years old.
Table 4.5. Locations of successful male and female New Zealand falcon hunting attempts in each pine stand class during the 2004 and 2005 breeding seasons in Kaingaroa Forest.
Stand Classes (years) Total Interior Total Edge Less than 4 yearsa 4 to 9 yearsa 10 to 19 yearsa More than 20 yearsa 4-9<4 / b 10-19<4 / b 20+<4 / b 10-194-9 / b 20+4-9 / b 10-19 / 20+ b Female 16 10 15 0 0 1 1 1 8 0 0 0 Male 55 64 19 8 5 23 7 8 29 7 10 3 TOTAL 71 74 34 8 5 24 8 9 37 7 10 3
astand interior classes, bstand edge classes.
Male New Zealand falcons made more successful hunting attempts in every stand age-class than females and hunted in a wider variety of pine stand age-classes. Both male and female delivery rates declined over the breeding season, while the number of prey deliveries made by males always remained far higher than those made by females (Fig. 4.2).
Figure 4.2. Mean number of prey items delivered to the nest by male and female New Zealand falcons during the 2004 and 2005 breeding seasons in Kaingaroa Forest.
4.5 Discussion
The New Zealand falcon home range size of 9 km2 recorded in Kaingaroa Forest, is considerably smaller than the 75 km2 previously estimated for New Zealand falcons in indigenous forest (Fox 1977). Although the accuracy of previous estimates is low because of a lack of available radio telemetry data, it remains likely that home ranges in indigenous forests are much larger than in pine plantations because prey is more abundant in the latter (Clout and Gaze 1984). If nest site availability is not limited, this suggests that the highest densities of this species could now be located in exotic pine plantations.
Prey species are more available to merlins (Falco columbarius) and black
sparrowhawks (Accipiter melanoleucus) along plantation edges (Parr 1991, Malan and Robinson 2001). It has also been suggested that prey is more available to New
Zealand falcons along habitat boundaries (Fox 1977, Barea 1995). In pine plantations prey availability is enhanced by very high densities of prey congregating along pine stand edges (Chapter 2). New Zealand falcon preferred several pine stand edge
classes, suggesting that they are choosing areas characterised by high prey abundance, and that optimal hunting conditions are located along these habitat boundaries.
Prey availability is also high in open stands where prey is more vulnerable to
predation (Suhonen et al. 1994), and open habitat species are commonly selected for in the diet of New Zealand falcon in pine forests (Chapter 3). Although habitat use will always be biased toward nest sites, (which are often located in pine stands less than four years old) (Stewart and Hyde 2004, Addison et al. 2006), a high proportion of prey was taken by both sexes in stands less than four years old. Indicating that young pine stands provide more than just potential nest sites. The selection by both sexes for stand edges bordering stands less than four years old, also indicates the suitability of these young stands, with the older stand providing the necessary cover for New Zealand falcons from which to launch hunting attempts out over younger more open pine stands. Large prey species such as rabbit (Oryctolargus cuniculus), which are only taken by females and can contribute significantly to the diet(Fox 1977), are more abundant in young open stands (Gibb 1961), which may also contribute to their selection.
Male New Zealand falcon also preferred pine stands that were more than 20 years old. This behaviour is similar to other raptor species, including European sparrowhawk (Accipiter nisus), grey goshawk (Accipiter novaehollandiae) and brown goshawk (Accipiter fasciatus) (Marquiss and Newton 1981, Burton and Olsen 2000b), where the males’ smaller size makes them more adapted to hunting under a canopy than females. Male New Zealand falcons are more manoeuvrable than females (Fox 1977), and the broad variety of stand classes that males were successful hunting in indicates that males are better adapted to hunt in a wider variety of stand ages than females. The size and species of prey delivered by each sex is likely to differ accordingly (Fox 1977, Marquiss and Newton 1981, Burton and Olsen 2000b).
Raptor home ranges vary over the breeding season depending on a variety of factors, including food availability and the energy requirements of offspring (Village 1982, Bloom et al. 1993). The size of male New Zealand falcon home ranges peaked after all the chicks had fledged, and then declined along with prey deliveries, as fledglings became more independent (Chapter 6). Prey density increases at the end of the breeding season (Chapter 2), and this may allow males to forage over a smaller range (Village 1982, Bloom et al. 1993). Female home ranges increased over the breeding
deliveries decreased as young became self sufficient. Throughout this time there is very little, if any, overlap in home range between adjacent pairs, which confirms that New Zealand falcon pairs are very territorial during the breeding season. However, we suggest that only a proportion of the overall home range is vigorously defended as very few aggressive territorial interactions were observed.