Trends in the demographics of populations of kākā

Background

This measure relates to indicator 1.4.2 – Current and predicted trends in the demographics of threatened and at risk taxa under active management.

Reliable estimates of the bird population size and trends are fundamental to effective conservation management. With this information, inferences can be made about management interventions and impacts of environmental disturbances. Longer studies with repeated measures collected from the same survey points at the same time of year can be especially valuable, as they allow long-term trends to be separated from inter-annual fluctuations (Bellingham et al. 2020). As part of a national programme of research into the impacts of pest control on mainland kākā (Nestor meridionalis) populations, a monitoring programme was initiated in 2000, covering a 1150 hectare area of the Waipapa Ecological Area in Pureora Forest Park, central North Island (Figure 1).

North Island kākā

What did we measure?

A systematic survey grid was established, consisting of 131 points spaced 300 m apart. Every year, kākā were counted at each point in October (except for 2019, when the points were not surveyed due to operational resource constraints). At each point, a 10-minute distance sampling count was completed. This required an experienced observer to identify all kākā heard or seen within a 100 m radius and accurately measure the distance from the survey point to where the bird was first detected, using a range-finder. Some birds were ignored (e.g. those flying overhead), in accordance with the rules of the distance sampling method (see Greene et al. 2010).

What did we find?

• Despite the introduction of an intensive integrated pest control programme, between 2000 and 2011 the kākā population remained relatively stable at a density of around 0.5-0.6 birds per hectare (a total population of around 600-700 birds).
• From 2012 onwards there has been a steady increase in the size of the kākā population, with the 2020 density estimated at around 2.3 birds per hectare (a total population size of around 2650 birds). This suggests that the population size has increased four-fold since monitoring began in 2000 (Figure 2).
• The results demonstrate the importance of long-term monitoring for species such as kākā, which have low reproductive rates (they are episodic breeders due to reliance on mast-seeding events, and lay relatively few eggs) and are therefore slow to respond to the effects of pest control.

Figures

Figure 1: Location of the Waipapa Ecological Area.

Figure 2: Estimated kākā population size (\(\hat{N}\)) at Waipapa Ecological Area, between 2000 and 2020. Black dots represent mean population estimates based on results of distance sampling. The blue line is the fitted trend, modelled using a Generalised Additive Model (GAM), with confidence intervals represented by the shaded blue area above and below the line. The graph was plotted with a log scale on the y axis to show deviation from exponential (which would appear as a straight line) and to make the confidence intervals more comparable between years. The low estimate in 2002 is thought to be a result of the small number of birds detected in that year.

Data quality

This measure is classified as a case study, is considered a partial measure of high accuracy and complies with the data quality guidelines used in New Zealand’s Environmental Reporting framework.

The distance sampling method makes a number of assumptions, including: 1) species are correctly identified; 2) distances to birds are accurately measured; 3) the bird is detected prior to movement; 4) detection at or directly above the survey point is certain. Although steps are taken to minimise violations of these assumptions, in reality errors still inevitably occur which may introduce some bias to the results. Another limitation of this method is that the analysis requires a relatively high number of detections to produce a reasonable estimate of density or abundance. To ensure good quality data, DOC has employed highly-skilled field observers, all of whom have experience of kākā and the distance sampling method, and sufficient survey counts have been completed each year to ensure that the number of detections exceeds the recommended minimum.

Glossary of terms

Distance sampling is a widely used method for estimating animal density or abundance. Its name is derived from the fact that the data used for inference are the recorded distances to objects of interest (usually animals) obtained by surveying lines or points.

Density estimate is a metric produced by analysis of distance sampling data, that tells us how many animals there are on average within a unit of area (usually hectares), at our survey site.

Confidence intervals measure the degree of certainty of results. A 95% confidence interval is a range of values that you can be 95% certain contains the true value (such as density or number of birds).

Additional resources

Bellingham, P.J.; Richardson, S.J.; Gormley, A.M.; Allen, R.B.; Cook, A.; Crisp, P.N.; Forsyth, D.M.; McGlone, M.S.; McKay M.; MacLeod, C.J.; van Dam-Bates, P.; Wright E.F. 2020. Implementing integrated measurements of Essential Biodiversity Variables at a national scale. Ecological Solutions and Evidence 1: 1-11.

Greene, T.C.; Jones, A.; Dennis, G.; Sachtleben, T. 2010. Distance sampling to determine kākā (Nestor meridionalis septentrionalis) density within Waipapa Ecological Area, Pureora. New Zealand Journal of Ecology 34: 297-305.