Dynamics and viability of a New Zealand robin population reintroduced to regenerating fragmented habitat

Summary

Reintroduction, as a method of increasing population size and range, has been widely used for conserving New Zealand's threatened bird species. Post-release monitoring, following establishment, is important for determining the success and long-term management of reintroduced populations.

The North Island robin Petroica austalis longipes experienced a massive population decline as a result of habitat loss and introduced predators, and are sparsely distributed throughout North Island, and are also present on some much smaller offshore islands. Translocations, to increase their range and bolster the population, took place in 1992 and 1993, to an island undergoing extensive vegetation restoration work, and to which other bird species have already been successfully introduced.

This case study describes a reintroduction, through translocation of wild birds, to the island of Tiritiri Matangi. The robin's disappearance from this island is thought to have been caused by habitat destruction but ongoing restoration work was considered to have led to a suitable increase in the birds' woodland habitat to support once again this endemic bird.

Donor population: North Island robins Petroica austalis longipes were caught for reintroduction to Tiritiri Matangi island (see below), from a wild population inhabiting pine plantations on the Mamaku Plateau (38°2'- 38°6'S, 175°57'-176°3'E), 15 km northwest of the town of Rotorua (North Island)(Armstrong 2005).

Release site: Tiritiri Matangi, located in the Hauraki Gulf, 3.5 km east of Whangaparaoa Peninsula, was chosen as a reintroduction site for New Zealand robins due to the ongoing restoration of the island's flora and fauna, which began in 1983. Introduced Pacific rats Rattus exulans (notorious nest predators of passerines and other bird species) were eradicated in October 1993. Due to habitat degradation (through forest clearance and livestock grazing) small patches of remaining native forest were confined to gullies, ranging in size from 0.1 to 4.0 ha. The remainder of the island consisted of grassy clearings and areas of extensive replanting, which were commonly used by the five other native bird species reintroduced to the island; little spotted kiwi Apteryx owenii, brown teal Anas aucklandica chlorotis, takahe Porphyrio mantelli, red-crowned parakeet Cyanoramphus novaezelandiae novaezelandiae and whitehead Mohoua albicilla.

Releases: Forty-four robins (30 males, 14 females) were translocated between 7 and 12 April 1992, and a further 14 (5 males, 9 females) in June 1993. The released birds and any subsequent fledglings were fitted with colour-ring combinations for identification purposes.

Monitoring: Population monitoring was carried out between 1992 and 1998. New Zealand Robins prefer forest habitat, and it was thought that the release sites in the forest fragments surrounded by open habitat might prevent dispersal into other forest remnants on the island. Thus one objective of the monitoring was to ascertain if this was so. In the remnant forest patches, searches for individual birds were carried out along transects in order to estimate survival and presence/absence. The number of transects increased with increasing patch size, and surveys were concentrated during the breeding season (September to January). Nests and fledglings were located by giving the male a meal-worm, then following the female back to the nest, after having been fed the meal-worm by the male, who had called her off the nest. Fledglings were found in a similar way, by both parents being given a meal-worm and then followed to the fledglings.

Survival: Following the first five months after release, 33 of the 44 released robins (75%) were still alive (September 1992). By September 1993, (a year and five months after release), 27 (61%) of the initial released birds had survived. Population size increased in the four years following reintroduction, and reached a plateau of around 65 birds in 1996 to 1998 (see Table 1, attached). Sex ratios also became more equal over time, with 37 males:29 females in September 1998, compared to the initial male bias of 26:7 in September 1992.

Fecundity: In the first year following translocation, females produced an average of 0.64 young, with an average of 0.57 fledglings in the 1992 to 1993 breeding period, and 0.71 in the 1993 to 1994 breeding period. This increased to an average of 2.4 young per female in the subsequent years following reintroduction.

Juvenile settlement: Of the 69 banded fledglings, 56 (81%) settled outside their natal forest patches between 1993 and 1997, with an average dispersal distance of 441 m. Juvenile settlement was related to the number of territory holders per forest fragment as well as fragment size.

Conclusions: The reintroduction of NewNorth Island robins to Tiritiri Matangi has been successful, with increases in population size to a carrying capacity (given the available forest habitat) of around 65 birds. The apparent isolation of remnant forest patches did not hinder dispersal between forest fragments, with juveniles dispersing distances of over 400 m from their natal forest patch. Females produced more young in the years following reintroduction, suggesting that fecundity in translocated populations may rise with increasing time after release if habitat conditions are suitable.

Population modeling of the Tiritiri Matangi robin population suggests that robins could survive for several decades in patches as small as 2.5 ha. As the forest matures, there will be around 150 ha of suitable habitat becoming available over the next 20 years, compared to the 13 ha which the robins currently occupy. This increased forest cover could support a possible population of several hundred robins.

Due to the rapid growth in population size, established translocated populations may be a more suitable source population for further translocations, rather than taking birds from native populations, and indeed a translocation project using birds from Tiritiri Matangi has now been undertaken.


References
Armstrong D. P. (2005) Effects of familiarity on the outcome of translocations, ІІ. A test using New Zealand robins. Biological Conservation, 71, 281-288.


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