Macquarie Island is home to a sizeable population of subantarctic (or brown) skuas. These birds are related to gulls, and are basically a seagull “on steroids” which have evolved into hawk-like birds and occupy a similar ecological niche here to small hawks. Subantarctic skuas scavenge and predate upon other seabirds and their young, eggs, fish, molluscs, crustaceans and small mammals. They are particularly fond of elephant seal placenta and milk, which they opportunistically sneak direct from the teat, or as leaked milk off the beach sand during pup feeding. They breed during summer months and will typically lay two eggs. They will often nest on elevated grasslands or in sheltered rocky areas adjacent to penguin colonies and will defend their territories vigorously against all intruders including other skuas and human visitors. Some birds establish solitary nesting sites on the elevated island plateau.
Prior to the Macquarie Island Pest Eradication Programme (MIPEP) commencing in 2011, rabbits, particularly the newly emerged youngsters, formed a large part of the skua diet. They were effective rabbit predators, but obviously not sufficiently effective to keep the rabbit population in check. Whilst planning for the MIPEP phase of rabbit poison baiting from helicopters, the skuas were a cause for much concern due to the anticipated outcome of considerable ‘non-target species’ deaths of skuas (and giant petrels) as a result of scavenging ingestion of poisoned rabbit carcasses. This was one reason for distributing rabbit calicivirus (rabbit hemorrhagic disease virus) immediately prior to the poison baiting phase, as a pest rabbit bio-control agent, to reduce the number of rabbits prior to poison baiting. Unlike the baited carcasses, the rabbits killed by the virus were not dangerous to the skuas and giant petrels to ingest. This was very effective, with the virus killing somewhere around 95% of the rabbits in a short time. For a few months in the 2010–11 summer, the skuas had a field day and couldn’t believe their good fortune with freshly dead rabbits everywhere! The skuas demonstrated their intelligence by ‘refrigerating’ rabbit carcasses by depositing them in the cold water streams running off the plateau, for later consumption. Nevertheless, there was still a significant number of skuas killed during and immediately after the helicopter bait drop.
For this reason, it is particularly important now to monitor the skua population. Apart from the skua deaths due to unintentional ‘non-target species’ poisoning during MIPEP, there is also the question of whether or not the skua population size prior to MIPEP was sustained at unnaturally high numbers by the presence of the rabbits as a readily available food source.
Our TASPAWS rangers, wildlife ranger Mike, ranger in charge Chris, and summer ranger Andrea, last week completed an island wide census of the breeding skua population. They were ably assisted in this demanding task by albatross project researchers Kate and Jarrod, and station expeditioners including Ian, Benny, Scotty, Greg and Ivor. For the station people, it was a welcome opportunity to spend a few days roaming the coastal featherbed or the island plateau with the wildlife experts, and contribute to an important wildlife project. Ian, Scotty, Greg and Ivor enjoyed several days of somewhat unseasonal snow and blizzard whilst assisting Mike to count nesting skuas across a large chunk of the plateau, an ‘off-the-walking-tracks’ area that we do not normally visit.
Final numbers are not in yet, but we are happy to report that the skua population is still very secure on Macca, although numbers appear to be a little down on the pre-MIPEP numbers. This possibly represents a return to a more natural rabbit-free population size. The locations of all nests were recorded using GPS, to enable a return visit later in the summer to assess breeding success rates.
