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Black Swan

The black swan (Cygnus atratus) is New Zealand’s largest wetland bird, around 1.2 metres long and weighing 5-6 kilograms. It is probably one of the most recognisable wetland birds found in Pāuatahanui Inlet; a large black bird with very long neck often seen floating on the water. 

On land, black swans walk slowly and are ungainly. 

When flying, pure white flight feathers are visible, which are black-tipped in immature birds. The bill is a bright red with a terminal white band, the eyes are red, and the legs are dark grey. Both sexes are alike, but the female is noticeably smaller than the male. 

A young bird, called a cygnet, is grey with a black bill, while a newly-fledged cygnet is a dull greyish-brown with a brown eye.

The swan call has been described as “a musical bugling”, used mostly as a contact call between mates and often accompanied by flicking head movements. An adult is very protective of its young, hissing when people approach too closely.

Adult Black Swan

The black swan is considered a native New Zealand species. It was thought to be present here at the time of first human settlement, becoming extinct prior to European contact, possibly due to predation by Polynesian colonisers. The 1860s saw a deliberate re-introduction of 100 birds from Australia and by the late 1800s there were once again considerable numbers throughout the country. The rapid spread and increase in swan numbers was far greater than expected and suggests that natural re-colonisation may have occurred, something that may still be happening today. There are now estimated to be at least 60,000 black swan in New Zealand but this is below a peak population of 100,000 in the 1960s. This reduction has been put down to the Wahine storm of 1968 which had disastrous effect on the vegetation of Lake Ellesmere. It caused a population of 70,000 swan that lived there to crash to just 10,000 by 1978. Another possible cause for population decline was farm silt run-off that began in the 1970s, killing off the water plants that the swans depend upon. Starving birds began to eat grass, leading to culls in which many thousands were killed. 

In other areas, where the swan’s preferred water plants persist, such as in Pauatahanui Inlet, black swan numbers remain strong. 

Most regions of the country still allow limited swan hunting to take place. Black swan are a popular target for many hunters as it has a delicious taste and texture to the meat. 

Black swans are herbivorous, feeding mainly on submerged vegetation of various species. In Pāuatahanui Inlet this is almost exclusively seagrass (Zostera muelleri), preferring to crop sub-tidal and intertidal areas when low tide is cycling between mid-ebb and mid-flow. They require freshwater and favour Zostera beds in close proximity to incoming streams carrying strong freshwater flows at low tide. Using their long neck to advantage they eat both leaves and rhizomes.

Breeding begins at 2-4 years old and, while they do not mate for life, a mating pair may stay together for more than one breeding season. It has been estimated that in large colonies only about 25% of the population breed in any one year. Breeding occurs between July and September. If food is abundant, a pair may nest again in summer. (This has been seen in the lower lake at Whitby). On average 6 eggs are laid. Cygnets hatch in 4-6 weeks and fledge in about 4 months, although the family may remain together for several months longer. Eggs and young cygnets are vulnerable to predators, especially rats and black-backed gulls. If successful into adulthood, total lifespan can be up to 30 years.

Affects on the habitat

Research in Tauranga Harbour and Golden Bay shows that swans can have a significant impact on the physical sea floor environment and on biodiversity.

Cropping of sea grass leaves directly impacts the food chain by diminishing the food source of herbivorous fish while reducing the surface area for growth of minute algae and fungi that are food to small crustaceans and juvenile fish – the base of the food chain for larger carnivorous fish.

Potentially more serious is the effect of grubbing up rhizomes on local biodiversity. This denudes the sediment making it less suitable for several invertebrate species. In Tauranga Harbour black swans frequently create barren patches of sand equivalent to a circle of 30 cm diameter. Because full recovery can take up to three years, persistent overgrazing must severely degrade sea grass meadows with permanent adverse effect on biodiversity of the area. Further research is needed to assess the impact of grazing swans on seagrass viability in Pāuatahanui Inlet.

In addition, black swans are not very efficient feeders. A Golden Bay survey estimated food intake at around half a kg/swan/day but, with only about 14% assimilated, most is ejected as faeces. This wasteful feeding regime has an upside, releasing large quantities of finely broken down plant material into the environment to be consumed by other bottom dwelling species, e.g. cockles. However, a potential adverse factor is the impact of swan faecal material (high in nitrogen and phosphorus) on nutrient levels in the Inlet. This can affect water quality and clarity (a process known as eutrophication), impeding photosynthesis by sea grass and algae.

For Pāuatahanui Inlet it is not known how significant the swans’ contribution to nutrient levels is compared with rural and urban runoff.