Native bird monitoring

Native forest bird abundance monitoring is undertaken in September each year to track how the birdlife is recovering. This monitoring is undertaken by Mainly Fauna Ltd.


Initial monitoring was undertaken in September 2014 before any pest control was undertaken. Since then annual monitoring has been done.

36 transects are monitored, 18 on each side of the valley, which can be seen in blue on the map below.

Consistency is a critical aspect of native bird monitoring, so steps are taken to eliminate as many of the potential variables as possible:

  • data is collected at the same time each year, in fine weather and during pre-set daylight hours;
  • transects are walked at a slow and constant speed;
  • notes are taken recording temperature, breeze and cloud cover.

This is still an inexact science; we look at trends over the longer term rather than individual results as in indication of progress.

What is monitored?

Bird counts are conducted using two methods:

  • line-transect based distance sampling to estimate population densities and monitor density trends of five key forest bird species, and
  • encounter rates (mean number of detections per km) to monitor gross changes in population size and composition for all forest bird species.

Five species are monitored more intensively as they represent important guilds for the healthy functioning of forest ecosystems, and are also vulnerable to predation by possums, rats and stoats, and therefore are good species to indicate the success of predator control.

These five key forest birds are:

  • Bellbird/korimako (Anthornis melanura),
  • Wood pigeon/kererū (Hemiphaga novaeseelandiae),
  • South Island rifleman/titipounamu (Acanthisitta chloris chloris),
  • South Island tomtit/ngirungiru (Petroica macrocephala macrocephala) and
  • Tūī (Prosthemadera novaeseelandiae)

Bird monitoring was conducted on some lines in September 2015 and all lines in 2016, 2017, 2018 and 2019. This is in accordance with the reports by Iris Broekema, Density estimates and encounter rates for forest birds prior to predator control in the lower Hollyford valley, Fiordland prepared in June 2015 for DOC and in subsequent reports for the Trust.


The results of the 2019 annual bird survey demonstrate that intensive predator control carried out by the Trust between 2014 and 2019 has resulted in a significant increase of many native forest bird species at Martins Bay, in particular those species that are most vulnerable to predation and competition from introduced predators.

Bellbird densities in particular have skyrocketed since predator control started in the area; with estimated densities increasing from 1.2 bellbirds/ha in 2014 to 6.1 bellbirds/ha in 2019. Mainly Fauna conduct annual bird surveys at many South Island sites and have reported that the bellbird density at Martins Bay is higher than at any of the other sites they have surveyed.  This can be attributed to the richness and diversity of the forest and the abundance of food sources found at Martins Bay. Mixed beech/podocarp forests contain a large variety of tree species, and therefore there is excellent availability of food year-round, which in turn means the forest is able to support a much higher density of birds than other less diverse forests in other areas.

This fertile forest and abundance of food is also a good food source for rodents, making this area particularly high in rats, which prey on our native species. In mast years the abundance of food causes rodent numbers to explode, which places even great pressure on our native bird species, particularly during the breeding season when they are sitting on nests and eggs and baby birds are vulnerable to predation. However, when predators are controlled to low numbers the food sources are able to be utilised by native birds and other native animals, and this is when we can see incredible changes to our native birdlife. The project run by the Hollyford Conservation Trust at Martins Bay is an excellent example of how bird numbers can rapidly increase to impressive levels with sustained pest control.

Mainly Fauna has also noted that riflemen were rare at Martins Bay when the first bird surveys were first undertaken in 2014. In 2019 their densities have not only increased, but they are much more widely distributed across the project area. Other native species such as tūī, kererū, kākā and kakariki have also become a lot more abundant.

Overall, the annual bird surveys have shown that sustained pest control in this area has allowed forest birds to recover. 

Population density estimates for bellbird, kereru, rifleman, tomtit and tui at
Martins Bay between September 2014 and October 2019.

We want to see a healthy and thriving native ecosystem in the lower Hollyford Valley, with diverse and abundand bird life free from the risk of predation by possums, stoats and rats.

Our Vision

To protect, enhance and restore all native flora and fauna for the enjoyment of all, today and for generations well into the future.

About The Trust

The Hollyford Conservation Trust was formed in June 2014 to establish a ‘mainland island’ sanctuary in the lower Hollyford Valley, in partnership with the Department of Conservation (DOC) and other stakeholders.

Pest control

This project was established in 2014 after local landowners realised the impact introduced pests were having on the Lower Hollyford Valley – the birdsong was gone, and the canopy was collapsing, with the more palatable species such as southern rātā and fuschia being browsed extensively to the point of dieback.

Bird translocations

One of the key long term aspirations of the project is to return species that once lived in the project area but have become locally extinct.

Cliff Broad Tawaki Project

The Cliff Broad Tawaki pest control project is a trapping line along the rocky coastline to the North of Martins Bay to protect Fiordland crested penguins/tawaki during the breeding season.  This project is funded by a generous donation from the Broad family in memory of their father Cliff Broad, who had a special bond with the Hollyford Valley and Martins Bay.

Latest News

2019 annual bird monitoring

2019 annual bird monitoring

The results of the 2019 bird monitoring show birdlife in the valley has increased significantly since intensive pest control began in 2015.

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February 2020 floods in the Hollyford Valley

February 2020 floods in the Hollyford Valley

Early February brought record rainfall to the Hollyford Valley, causing signficant damage to the Hollyford and Milford roads, as well as causing large slips and damage to tracks, huts, and the Trust's own infrastructure. 1,000mm of rain fell in a 60 hour period in...

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