Thursday, August 12, 2010

Eradication of Common Mynas from Denis Island, Seychelles, 2010 (Courtesy Prof. Chris Feare)

An attempt to eradicate Common Mynas Acridotheres tristis from Denis Island, Seychelles, began in late May 2010. The justification for the present control is evidence that mynas are directly and indirectly impacting negatively on the populations of some of the introduced endemic birds. An earlier attempt had been made, mainly using Starlicide (a toxicant that demonstrates some specificity to certain bird families, including mynas) in the early 2000s but this attempt was abandoned after a contemporaneous rat eradication attempt had failed; a subsequent rat eradication was successful. Now that four species of Seychelles endemic bird have been introduced to Denis, this toxicant is considered too dangerous for the endemics to be used as a primary control measure and the current attempt to remove mynas is centred on trapping.
In the initial stages two kinds of trap were built and tested: funnel traps, which rely on attractive food to entice mynas into the traps, and decoy traps, which use a live decoy myna in a central compartment to attract nearby mynas. Mist nets were also used in initial trials where flocks of mynas concentrated, as were fine nylon nooses placed on the ground among bait. In trials, only decoy trapping proved effective and the eradication attempt now relies almost entirely on decoy traps, although other techniques may be used later in the project.
Achievements to date

1. Demonstration of the effectiveness of decoy traps in Denis Island’s habitats (and of the poorer performance of funnel traps, mist nets and ground nooses).
2. In the first two and a half months of the project over 500 mynas have been caught, out of a population estimated at about 1000 birds before trapping began (Jildou van der Woude).
3. Discovery that when groups of traps are placed together, catch rate among traps is variable. Experimental manipulation showed this not due to trap location or possible variation in trap structure or operation, but was due to presence of a specific decoy myna. Elucidation of what constitutes a “super-attractive” myna could improve trapping success.
4. Discovery that iris colour changes as juveniles mature, and that it is variable in adult mynas. Whether this variation represents continuing maturity, and is therefore related to age, or whether it is part of an annual cycle, or reflects mood of an individual, is unknown. Data are being recorded routinely among other features of all mynas caught. If a relation is found between iris colour and attractiveness as defined in section 3 above, this could be a pointer towards selecting attractive decoys.
Throughout May, at the beginning of the eradication, adult mynas were in wing moult, indicating that they were not breeding. In June wing moult was completed and breeding began. This was accompanied by a reduction in the rate at which birds were caught in decoy traps. A number of possibilities could explain this: requirement for different foods by nesting birds, reduced attraction to decoys and traps due to over-use of the decoys and lack of trap maintenance and cleaning, and reduced motivation of the trapper. Decoys were replaced with new birds and traps were overhauled, leading to a recovery in trapping rate. It appears that food choice of the birds did not change and that reduced motivation of the trapper led to lack of attention to the detail of trapping.
6. Comparative failure of funnel traps due largely to occupation of these by turtle doves (especially where food concentrations occur – see below) and crabs, mainly hermit crabs, when traps placed near the beach crest. Denis Island’s populations of Madagascar turtle doves, and also moorhens and mynas, are very high, indicating that the island provides abundant sources of food for them. The main food source concentrations identified are the pig farm, the food store on the northern coast, and the food offered to birds in the garden at the restaurant. Limitation of these food sources to birds would help to reduce the bird populations and would also help to reduce food wastage and its costs.
Lessons learned
We have within two and a half months removed almost half of Denis Island’s population of mynas and have additionally learned much new information about mynas and their behaviour in relation to trapping, including some insights into improvements that may be made. This success suggests that a major reduction in myna numbers on the island is feasible and that eradication should not be ruled out as a possibility and should remain the long-term goal. Decoy trapping is likely to remain the main tool but other techniques may need to be employed as numbers decline, with shooting at the nest being particularly important in the later stages.

1 comment:

Bart said...

Hats off to you Denis Island and Prof Feare. As far as I know you are the first island in the Seychelles to take a public stance against this invasive and harmful species. Cousan and Cousine have kept it low profile. Going public will boost awareness. I encourage you to use the local press to educate the public on the negative affect these birds have on the environment. Your endemic bird species will benefit. Regards, Bart