Survival, breeding success and habitat selection of North Island robins Petroica Australis Longipes translocated to the Karori Wildlife Sanctuary

Abstract

In May 2001, 40 North Island robins (Petroica australis longipes) were translocated from Kapiti Island to the Karori Wildlife Sanctuary (KWS), a mainland island surrounded by a mammalian predator- and browser-proof fence. A further 36 North Island robins were translocated from Kapiti in May 2002. These translocations were the first flighted birds to be released into the KWS and are the only known transfers from a predator-free offshore island to a predator-free mainland island. This study assessed the dispersal patterns, survival rates, habitats selected, territory sizes and breeding success of the KWS robins in such a mammalian predator-free environment. The results are compared with up to 18 other studies of robin populations. Data between three different KWS robin cohorts (21 pairs) are also compared. The majority (59%) of the translocated robins settled locally within the KWS, with 70% of these robins establishing territories < 500 m from the release site. The farthest known distance dispersed from the natal site was 2.3 km. The first robin cohort released had the choice of the best available habitats. Their mean dispersal distance from the release site was 366.5 m, and their survival rates to one year was higher (78%) than the nine other translocated mainland island populations. A comparison of the first robin cohort released with eight other translocated robin populations showed a statistically significant relationship between survival percentage to one year, and the mean distance dispersed (m) from the release site. This result indicated that the greater the number greater of robins that settle within a release site the greater the chance of creating a self-sustaining population, and hence a translocation success. Survival of progeny to independence, and then breeding age (59%) was higher than three other mainland robin populations and is the highest rate of recruitment recorded for robins in New Zealand. The mean territory size (2.9 ha) of the KWS robins was relatively comparable with two mainland sites, and was larger than all island sites. The number of fledglings per female in the year of translocation was second to highest for the first robin cohort released compared with five other mainland island translocated populations. The KWS robin population had one of the highest number of young per brood, one of the lowest mean nesting efforts, and was one of the three known robin populations to successfully rear up to four successive broods in a season. The KWS robins also had lower dispersal rates, higher survival rates, larger territory sizes and higher breeding success, than most other robin populations studied. Habitats selected by the KWS robins were generally comparable with those selected by other robin populations. Flat, moist areas with available tree ferns as well as litter cover appeared to be important factors in determining habitats selected. The likely reasons for these results are the combination of suitable habitat, a low population density and a mammalian predator-free environment. The experience and age of robins was found to influence their dispersal rate, habitat selection, survival rate and breeding success. Less experienced young robins dispersed further from natal sites compared with the distance that founder robins moved away from their release site. The younger and less experienced progeny selected a lower variety of nesting trees species (significantly less rangiora and five-finger) and were less diverse in their selection of tree species in their territories (4 c.f. 7), while their territories were on a steeper mean slopes (35.13 c.f. 31.14), had significantly less litter depth (1.42 cm c.f. 2.21 cm), with higher litter moisture (2.26 c.f. 1.93), and significantly less 5 to 9 cm diameter breast height stems (79 c.f. 87) but were within in denser forest compared with the older (founder) robins. They also had lower survival rates to one year (42.5% c.f. 78%) and a lower mean number of fledglings per female per season (3.57 c.f. 4.22). Dispersal rates increased and survival rates decreased for the later robin arrivals due to the general increase in robin numbers and increased competition for resources. The recruitment rate of robins into the population compensated for natural mortality rates, indicating the initial translocation success of the KWS robins. There are many opportunities for further research on the KWS robin population. Dispersal outside of the KWS is inevitable as the population expands. Predator control programmes in forest remnants outside the KWS will be essential for survival of any robins that settle there.