Wellington Harbour, a Capital Place for Birds

Forest and Bird, Issue 300, 1 May 2001, Page 9

Wellington Harbour, a Capital Place for Birds

— Marieke Hilhorst.

ffal, it seems, is one bird’s feast but another bird’s famine. Wellington Harbour’s diverse coastal birdlife is changing, the result of sewage and offal outfalls closing down during the past 25 years. Numbers of offal eaters, such as giant petrels, have declined markedly, while spotted shags, which like clean water for fishing, have moved in and begun breeding. The trends in bird numbers and distribution are coming forth from results of a longterm survey carried out by members of the Wellington branch of the Ornithological Society of New Zealand (OSNZ). Three times since 1975, most recently from 1998 to 2000, the volunteers have given up every second Sunday each month for two years to survey their patch of the harbour, rain or shine. Their efforts have recorded the presence of 13 key coastal-bird species, and 32 others, helping

to map changes in numbers and distribution. Hugh Robertson, a Department of Conservation scientist and OSNZ member, has been involved in two of the three surveys. He says the changes in the populations of some birds can be linked to the efforts to clean up the harbour waters over the past 25 years. ‘Clean ups’ include diverting raw abattoir waste into Wellington city’s sewage system, the milli-screening of sewage outfalls, and the closure of the city’s infamous inshore sewage discharge at Moa Point. Not all birds have benefited from the changes. Along with the giant petrel whose numbers have dropped from more than 100 in the early 1960s to virtually none today, reductions in waste discharges may also be responsible for the dramatic drop in red-billed and blackbacked gull populations. Redbilled gulls now number less than 2000 birds, fewer than half

the numbers counted in the original 1975-77 survey. Black-backed gulls are down to one-third of their mid-1970s population. Decreases in the population of black-backed gulls may be due to egg-pricking programmes at the airport and on Somes/Matiu Island, or a decrease of local nesting habitat. Hugh Robertson suspects the main cause is less food in the harbour, while the egg pricking has exacerbated the decrease. The effect of a cleaner harbour on shags is less clear. Two species, the spotted shag and little black shag, both seem to have benefited. Spotted shags are a relatively recent colonist in Wellington Harbour, arriving in the early 1970s. Their numbers have risen to about 50 and they are breeding. Little black shags are a non-breeding visitor whose numbers have soared, from less than five in the mid1970s to more than 100 now. Two shag species that have not fared so well, for unknown

reasons, are the black shag and little shag. Both populations seem to have stabilised at about 30 and 180 birds respectively. Hugh Robertson says the growing database of Wellington Harbour’s coastal birds provides interesting trends and clues about the health of the harbour and its impacts on birds, but the pathways and causes are speculative.