In this section
Sustainable society & policy
- Activating water-sensitive urban design for healthy resilient communities
- BEST: Biodiversity and ecosystems services for resource management
- Cities, settlements & communities
- Environmental Perceptions Survey
- Evidence empowering impact
- Garden Birds: The science behind the survey
- Freshwater values, monitoring and outcomes
- Moving the middle
- New Zealand Colony Loss Survey
- Responding to COVID-19
- Social Licence to Operate Framework
- Survey of Rural Decision Makers
The New Zealand Garden Bird Survey (NZGBS) is a national survey of garden birds carried out each year from late June into the first week of July. Volunteers are asked to record the type and number of birds they detect over a one hour period. The primary aim of the NZGBS is to answer the question of whether garden bird populations are increasing, decreasing or remaining stable.
However in order to reliably answer this question, the raw results need to be turned into meaningful summaries. In general, the raw data of bird counts per garden for a given year are averaged to give a value that represents the mean (i.e. average) number per garden for that year. A species that is increasing over time will have an average count that also increases over time, and vice versa.
In principal, calculating both the annual average and the trend is straightforward, however as we will explain, a number of challenges are present.
Is removing noise enough?
Even when we remove as much of the noise as we can (i.e. by accounting for region and garden-type) it still appears from the adjacent figure below as though tūī are increasing slightly. But are they really?
There are methods available to us that tell us how likely it is that any observed increase is actually due to chance.
We can investigate this by determining what patterns we could likely see if tūī were not in fact increasing but remaining stable.
Essentially we are including a degree of uncertainty with our trend.
Figure 10 below shows our best estimate of trend when we account for region and garden-type.
This time however we have included our uncertainty about this trend, as indicated by the grey shaded area.
Some of this grey shaded area is below the zero change line, indicating that there is a chance that tūī are in fact decreasing, despite our best estimate indicating a slight increase.
Our conclusion therefore is that we cannot be reliably sure as to whether tūī are increasing or decreasing.