New Zealand's common fish n chip lovin' seagull increasingly prefers the Otago coast as its home more than anywhere else in New Zealand, a University of Otago study has found.
And coincidentally it appears that the conservation efforts related to another bird, the Albatross, could be helping to make the Otago coast such a viable habitat for the New Zealand endemic red-billed gull (Larus novaehollandiae scopulinus), and a variety of other species, including the Stewart Island Shag.
The red-billed gull was designated as a 'nationally vulnerable' species in 2008 after substantial population declines at large breeding colonies, particularly at Kaikoura.
The latest study to check on their progress, by Dunedin ornithologist Lyndon Perriman and marine science researcher at the University of Otago, Dr Chris Lalas, published in the New Zealand ornithological journal Notornis, shows that the gull population has increased substantially along the Otago coast.
Nest numbers from seven surveys from Moeraki to Nugget Point on the Otago coast showed the number of nests more than tripled from less than 1500 in 1992 to 4600–4700 in 2011, with an average annual increase of 6–10% throughout those 20 years.
Sixteen of the 26 red-billed gull breeding locations from Moeraki to Nugget Point were on the mainland and the other ten were on coastal islets.
The researchers found that Taiaroa Head at the entrance to Otago Harbour is now the most popular nesting place for the endemic gulls, with the proximity of Taiaroa Head to a concentrated source of red-billed gull prey, plankton, a possible explanation for this.
The instigation of rat control by poisoning around gulls at Taiaroa Head was followed by a rapid increase in red-billed gull nest numbers, as well as a tendency for scattered smaller groups of gulls to form a single large colony. Rat control ceased in 1999 when the gull population exceeded 500 pairs.
"Rats are still known to be present in the area, although it is unknown if the increased number and density of gulls has lead to a reduction in loss of eggs," says Dr Lalas.
"Red-billed gulls respond to potential diurnal predators with communal aerial attacks so we suggest that size matters; larger colonies may be more viable because more gulls are available, especially to drive off avian predators such as black-backed gulls and swamp harriers. Human disturbance is not a problem at Taiaroa Head because, together with a variety of other seabirds, they benefit from fencing to exclude people from the royal albatross colony."
A previous study attributed the decline in gull numbers at Kaikoura to a decline in prey availability linked to changes in oceanographic indicators, including sea surface temperature (SST).
Fluctuations in SST at Otago and Kaikoura are negatively correlated and may explain these trends in gull populations, with the population rising in Otago, but dropping in Kaikoura.
They recommend that future conservation management at Taiaroa Head and at other key mainland breeding locations is necessary to ensure the viability of red-billed gulls at Otago. This could include further surveys of nesting populations.
"Future monitoring will help establish whether the recorded declines at the main colonies either reflect an actual overall decline in the New Zealand population or these declines are compensated by increases at Otago", the researchers write.
"Above all we regard control of predators and people at important mainland locations as key to the viability of red-billed gulls at Otago."
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