State of NZ Garden Birds 2023 | Te Āhua o ngā Manu o te Kāri i Aotearoa
What are our garden birds telling us?
Birds act as backyard barometers – telling us about the health of the environment we live in. They are signalling significant changes in our environment over the last 10 years. We should be listening.
Manaaki Whenua – Landcare Research researchers use supercomputers to distill a substantial information base – bird counts gathered by New Zealanders from more than 45,000 garden surveys since 2013 – into simple but powerful metrics
Thank you to all the citizen scientists who participated in the survey in 2023. It was a great turnout.
Key findings from the 2023 survey
Key signals continue for four native species:
- Kererū counts show a shallow increase over 10 years (48%), compared to the moderate increase seen previously, and now show a shallow decline over 5 years (12%).
- We now see a shallow increase in fantail counts (pīwakawaka) over both the long term (43%) and short term (12%).
- Tūī (kōkō) counts continue to show a shallow increase over 10 years (23%) but now show little or no change over 5 years nationally. Their regional long-term trends continue to show a rapid increase in Canterbury (210%).
- The long-term shallow decline in silvereye (tauhou) counts continues (16%), but for the first time we see a moderate decline in the short term (26%).
Key signals for introduced species that also act as environmental indicators:
- Myna counts continue to show a trend of little to no change nationally and shallow increases in four regions over 10 years. There is some evidence that the rapid increase in their counts in Wellington continues in both the long and short term (126% and 122%, respectively) along with a moderate increase in the short term in Manawatū-Whanganui (30%).
- For the first time house sparrow counts suggest a shallow decline over the long term (10%) and show a moderate decline over the short term (16%).
- Starling, goldfinch, chaffinch, and dunnock counts show a shallow decline over 10 years, with dunnock counts showing a shallow decline over 5 years (11%), and goldfinch and chaffinch counts showing a rapid decline over 5 years (30% for both species).
If you’re keen to know more about what’s happening with garden birds in your region, reports for individual regions are available, or check out out the full State of NZ Garden Birds 2023 | Te āhua o ngā manu o te kāri i Aotearoa report.
What more needs to be done to care for birds?
Since 2021 we’ve asked New Zealand Garden Bird Survey participants what more needs to be done to care for the birds in Aotearoa New Zealand. In 2023, 3684 participants responded to the question. We’ve analysed the responses, and they reflect the five themes below.
How we calculate NZ Garden Bird Survey results
The general idea behind the survey is to convert the individual bird counts people contribute into meaningful estimates of wider population changes over time. Counts from every garden surveyed are linked to their location within a neighbourhood, suburb, district and region to calculate how bird counts change over time at each of these spatial scales.
We expect a species whose population is increasing over time to show an increase in counts and vice versa. While this is simple in principle, because there are different numbers of gardens in different regions, and different people decide to participate in some years, it might appear as though bird count numbers have changed, when it is just the proportion or location of survey returns that have changed.
Cleaning ‘noisy’ data
The type of garden surveyed (rural or urban), whether people feed birds, and the total number of gardens surveyed in a region also add complexity. Statisticians refer to this as ‘noisy data’ because there are so many variables to take into account.
To reduce this noise, we use cutting edge statistical techniques to account for these variables. Following bootstrap analysis and bias-correction of the modelled data, estimated trends in bird populations over the past 5- and 10-year periods are summarised nationally and regionally according to their direction (decline or increase) and size (rapid to shallow).
These long-term trends are called ‘signals’ and show a persistent increase or decrease in the abundance of a species. We categorise these signals in the following way:
When using samples to estimate wider populations, we need some way of measuring whether the sample actually reflects the wider population. We used 80% confidence intervals for the average percentage change in each species’ counts to evaluate confidence in our estimates. We generally have more confidence at the regional level than district or suburb.
The easiest way to improve this confidence at the district or suburb level is to increase the number of bird counts done. The more people who participate, the greater the strength of our evidence for what’s happening to garden birds at the local scale.
How have bird counts changed?
Maps | Changes in species counts across regions 2023
Bar charts | Changes in species counts across regions 2023
