Lake Whatuma is home to about 25 per cent of the total Hawke's Bay population, and its accessibility has provided important information, particularly about the male birds.
Supported by volunteers, Ms Williams, who has now attained her PhD and continues her work as a private consultant, has been locating and monitoring the bitterns at Lake Whatuma for five years and last year caught and attached transmitters to 10 males.
This year one of the birds died, with the cause of death being investigated by Massey University. However the lifespan of the transmitters meant only about five continued to be monitored. Work continues with an increased focus on the females and chicks.
Information gathered about the males had been a "massive leap forward" in finding out about their behaviour, she said.
This showed that the birds were loyal to their habitats, leaving the lake during summer and autumn when the water level dropped, consistently heading to the same spots on neighbouring farmland, before returning in August to breed.
Research in the South Island this year revealed that when they leave their breeding grounds they travel a lot farther than previously thought.
"One that we were tracking near Christchurch, a female juvenile, went more than 100 kilometres away."
In terms of the birds' sustainability, it was vital to find out more about the females, she said.
"The females need food and the right water levels throughout the year to get them through - because this isn't consistent we are seeing a steady decline."
Being smaller than the males, and the sole carers of the eggs and chicks, also made them vulnerable.
"The ground-nesting female incubates the eggs and also has to feed herself.
"That's hard on her - she has to come off the nest if food is not close by, leaving the eggs or chicks exposed. On top of that there are not many good places around the country for breeding."
Lake Whatuma was one of the better sites, and is home to about five female birds.
With its fringes of raupo and a good number of small eels and water prey, their food source was fairly consistent.
This was not the case elsewhere where females and chicks had starved to death.
The search for food had also led to birds turning up in urban centres, seen recently in Tauranga and Christchurch.
"This is very unusual - it's never happened before and to have two independent cases is quite bizarre."
The impact of human activity on their natural habitats could not be under-estimated, she said.
Farming practices such as draining and flooding swamp and wetland areas, and other activities that affected water clarity, had the potential to be devastating.
"It's been shown that bitterns are the least adaptable when their feeding behaviour is disrupted, especially because they are so secretive.
"Water levels are particularly important - if it's too deep from being flooded they can't dive for their prey and they have nowhere to hide when foraging."
Bitterns stalk and eyeball their prey before stabbing them, and as such water clarity was important, Ms Williams said.
"We need to make some big changes to the way we manage our wetlands as it's accelerating their decline.
"I think it's crazy that some species like kiwi and kakapo get so much attention but bitterns don't, which also means in terms of funding.
"What bothers me most is that by the time we find out what the problem is the population could be gone.
"We are managing as we go, but there's so much more that needs to be done."
Group effort
Instrumental in finding out more about the bitterns at Lake Whatuma has been the support of volunteers.
One group of about six to 10 adjacent landowners have been carrying out pest control, setting and monitoring traps around the lake boundary.
Group member James Mackie said this work had been under way since 2014, aided by funding and trapping tips from Hawke's Bay Regional Council and DoC.
This year 337 predators including stoats, weasels, ferrets, opossums, hedgehogs, mice and rats were caught, up on 191 the year before, he said.
"There's a lot there - it's the biggest body of water in Central Hawke's Bay with a lot of birdlife."
He said the rise this year could also be because it was a dry year, with the birds not having many places to go and giving better access to predators.
In addition, there was a lot of cropping in the area providing food for predator and prey.
Monitoring was weekly or fortnightly and more volunteers were welcomed to help with the work or donate traps.
The Lake Whatuma Wetland Care Group, made up of various volunteers, has also contributed, with particular attention on preventing willows from encroaching on the birds' nesting spots in the raupo.
CHB Forest & Bird and Hawke's Bay Ornithological Society have also been involved. Other assistance for the bittern project had come from sponsors: Ducks Unlimited, the Birds NZ Research Fund and Arawai Kakariki wetland restoration programme.
