298 mature individuals
Restricted to Little Mangere Island for over 80 years, the deteriorating condition of the small area of forest available to the birds and their resulting population decline saw the seven remaining birds transferred to a larger patch of bush on adjacent Mangere Island in 1976-77. After reaching a low of just 5 birds in 1980, intensive and innovative management combined with the extraordinary efforts of ‘Old Blue’, the only breeding female, saw the population increase to around 80 birds in 1989 with robins also having been established on Rangatira Island in 1983. At that time intensive management stopped, but intensive monitoring continued to monitor the growth of the population up until 1999, by which time the population had reached around 200 mature individuals. Seemingly secure on the pest-free island homes, monitoring was scaled back to allow work on other species.
The growing population allowed the transfer of forty birds to be moved from Rangatira and Mangere islands to a predator-proof enclosure within the Ellen Elizabeth Preece Conservation Covenant (a.k.a. Caravan Bush) on Pitt Island between 2001 and 2005. Despite two breeding attempts the robins failed to thrive and the last bird ‘Spike’ disappeared late in 2007. The reasons for failure are not well understood, however the abundant mouse population within the covenant may have seriously depleted the robins’ insect and spider food source.
Understanding population trends
Throughout the 2000’s population estimates were made using a sampling method which over time appeared to show a decline in the population on both Mangere and Rangatira Islands. As concern grew over the apparent population trends, researchers from Canterbury University fortuitously started working black robins and other species Rangatira in 2007 and this enabled full population counts to be resumed. Initially, the low census counts obtained indicated that the population had declined on Rangatira, but no obvious reason cound be found for a decline aside from seasonal changes in productivity and survival. Over subsequent seasons the population on Rangatira has grown from 120 mature individuals in 2008 to 210 in 2012. While the robins have certainly had a couple of good years in recent times, it appears that the largest factor in the ‘growth’ of the population is increased observer effort.
Counting black robins is a tricky business. Those who have encountered them will remember robins boldly approaching on the lookout for food, either invertebrates you have disturbed or perhaps a mealworm proffered in attempt to lure the bird into the perfect photo position. If you’re in the business of undertaking a distance sampling type population estimate, having robins chase you about the forest invalidates the method. However, if you’re trying to count every robin on the island then having the robins come to you when you enter their territory is rather handy. It seems that in the period of low monitoring effort, the robins became progressively harder to detect as they encountered humans and their mealworm handouts less frequently. Once the researchers arrived robins were again offered handouts in order to keep them friendly the detectability of the robins increased. Along with increased effort put into searching infrequently visited areas of the island, the ‘feed and count’ methodology has again provided us with a robust population estimate. Unfortunately, this method is quite labour intensive and it takes two or three years to make the population sufficiently friendly.
Mangere Island has also had its ups and downs but probably for different reasons. The mid-90’s saw 50+ birds on the island before declining to the mid-30’s by 2005 and then increasing again to 46 birds in 2012. Rather than varying observer effort influencing population estimates it is likely that the carrying capacity of the small area of forest has been reached and the population is significantly influenced by seasonal conditions.
Keeping black robins secure on their island homes is the highest priority, hence the Department of Conservation maintains strict access and biosecurity restrictions on the islands to prevent incursions of rodents or other pests. The next highest priority is providing more habitat for the robins to occupy. On Mangere providing more habitat has meant the planting of over 100,000 trees and shrubs over the last 30 years. Unfortunately, trees take a long time to grow on windswept islands like Mangere and still have a bit of growing to do before the robins can move in.
Over on Rangatira a ‘hands off’ policy has relied on natural regeneration of the formerly farmed island to fill the gaps between forest remnants. Again the process of growing has been slow and consideration is being given to transplanting some of the natural seedlings along track margins into areas of muehlenbeckia and bracken fern in order to speed up the joining of forest fragments. Undertaking an all-out planting assault like on Mangere would be inappropriate on this island given the number of seabird burrows present and the biosecurity risks of bringing large numbers of plants onto the island.
Moving black robins back to Pitt Island is desirable, but keeping a sufficiently large area of forest mouse-free presents a significant challenge and the current predator-proof fence would require extensive modifications. Increasing the density of seabirds within forested areas like Caravan Bush may also increase improve the robins outlook as the increased fertility the seabirds bring benefits invertebrates.
Habitat quality on Little Mangere Island has improved markedly over the last 30 years and while it would do little to boost the population (there is room for only 6-10 pairs), it would complete the circle and return robins to their source. Complicating a return is the very difficult access to the privately owned island. Regular monitoring is required to ensure the long-term success of the transfer and the lack of a helicopter in the Chathams currently makes this impractical.
Restoring black robins to Chatham Island presents an even larger challenge. Again a predator-proof enclosure could be used, but again it needs to be around a sufficiently large area of forest to support a self-sustaining population, but ideally be isolated from adjacent forested areas that birds could disperse into. There are few such areas on Chatham Island and currently the Henga Scenic Reserve leads the list of possibly suitable candidates.
The black robin population currently stands at 298 mature indivduals and is moderately secure on predator-free islands. Population growth is limited by habitat availability and a third breeding site capable of supporting at least 50 mature individuals is very desirable.