Background
This measure relates to indicator 1.5.1 - Species composition & diversity.
DOC monitors status and trend in common and widespread terrestrial bird species across all public conservation land (PCL) in mainland Aotearoa New Zealand. Common species are important because they have considerable influence on the structure, biomass and function of most ecosystems. Their decline may be difficult to detect over short timeframes, particularly in the case of bird populations. However, even relatively small declines can reflect significant reductions in the number of individuals, which can impact on ecosystem structure, function and services. Such declines have been well documented for some of our rarer forest-dwelling birds, and there is increasing evidence of significant declines for common and widespread species.
Tomtit | miromiro, grey warbler | riroriro, bellbird | korimako and chaffinch | pahirini were the most common birds.
What did we measure?
DOC uses a National Monitoring Programme to assess state and trends of terrestrial biodiversity at nearly 1400 sites evenly spaced across public conservation land (Figure 1). Approximately 270 randomly selected sites are measured each field season (September–May) so that every site is measured on a 5-year rotation. Table 1 shows sample sizes for each season.
DOC staff conduct 5-minute bird counts at five stations at each site. Measurements of vegetation, acoustic records of pekapeka / bats and sign of introduced mammals including possums are also recorded. The bird data are used to estimate occupancy (presence adjusted for likely probability of actual presence or absence) and relative abundance for the most common species, using a statistical model that accounts for under-detection. Species richness (number of species per site) is stated for indigenous and introduced species separately.
What did we find?
- Species richness is variable across PCL, however indigenous dominance is generally high (Figure 1).
- Estimated average species richness has remained stable or declined slightly for indigenous and introduced species in woody and non-woody habitats (Figure 2).
- In general, places with higher numbers of indigenous bird species have fewer introduced species (Figure 3).
- The most common indigenous birds - tomtit | miromiro, grey warbler | riroriro, bellbird | korimako, silvereye | tauhou - are present at more than 60% of all PCL plots (Figure 4).
- With a few exceptions (like blackbird and chaffinch) the occupancy of introduced bird species is higher in non-woody habitats (Figure 5).
- For most species the relative abundance and occupancy have remained relatively stable (Figures 6, 7, 8). There have, however, been some changes, for example a significant increase in the occupancy of tūī in woody habitats (2% per year in national parks) and a decline in rifleman | tītitipounamu occupancy in woody habitats (3.6% per year) (Figure 6).
Figure 1: Bird species richness and estimated indigenous dominance at each monitored site. Each dot represents a sampling location on public conservation land (PCL). Richness is the number of different species present. Indigenous dominance is the percentage of the species present that are indigenous.
Figure 2: Estimated average indigenous and exotic bird species richness by habitat and conservation land status over all survey seasons. Click on the key to turn off individual environments.
Figure 3: Average observed bird species richness for areas of public conservation land. Enter a park name to highlight that area.
Figure 4: Estimated average occupancy for indigenous species across public conservation land over all survey seasons. Click on the key to see estimates for different habitat types.
Figure 5: Estimated occupancy for introduced species across public conservation land over all survey seasons. Click on the key to see estimates for different habitat types.
Figure 6: Trend in estimated occupancy for the most frequent indigenous bird species in woody and non-woody habitats. Solid lines are estimates for national parks, dotted lines are for other public conservation land. Click on the key to see trends for each species.
Figure 7: Trend in estimated occupancy for the most frequent introduced bird species in woody and non-woody habitats. Solid lines are estimates for national parks, dotted lines are for other public conservation land. Click on the key to see trends for each species.
Figure 8: Trend in relative abundance (5-minute bird counts) of the eight most common bird species by habitat and conservation land status. Click on the key to see trends for each habitat type and land status.
| Survey season | National Park - woody | National Park - non-woody | Other PCL - woody | Other PCL - non-woody | Total plots |
|---|---|---|---|---|---|
| 2011/12 | 35 | 0 | 35 | 0 | 70 |
| 2012/13 | 8 | 21 | 24 | 41 | 94 |
| 2013/14 | 59 | 18 | 151 | 55 | 283 |
| 2014/15 | 72 | 25 | 123 | 48 | 268 |
| 2015/16 | 72 | 23 | 115 | 56 | 266 |
| 2016/17 | 62 | 16 | 130 | 50 | 258 |
| 2017/18 | 64 | 26 | 128 | 55 | 273 |
| 2018/19 | 60 | 16 | 142 | 58 | 276 |
| 2019/20 | 63 | 22 | 122 | 50 | 257 |
| 2020/21 | 69 | 23 | 114 | 55 | 261 |
| 2021/22 | 65 | 15 | 128 | 50 | 258 |
| 2022/23 | 59 | 25 | 117 | 57 | 258 |
Data quality
This is a partial measure and complies with the data quality guidelines used in the Environmental Reporting Series. The data are nationally representative and cover more than 12 years of measurement. However, other species also need to be reported to fully assess the indicator.
It should be noted that the first year of monitoring (2011/12 survey season) included only forest sites, and that the first two years (2011/12 and 2012/13 survey seasons) had a smaller number of sites, as these were pilot years of the programme. This may have lead to some bias in the results for these early years (for example some of the species abundance trends in Figure 8).
This factsheet focuses on bird species diversity (richness), occupancy and relative abundance. Communities of diurnal (daytime) birds are surveyed at each plot location using a cluster of up to 5 different bird count stations (at least 150 m apart) between 1 hour after official sunrise and 1pm using the 5-minute bird count (5MBC) method. Only terrestrial birds were considered for analysis (number of species = 52). Aquatic birds such as gulls or ducks were removed. The occasional diurnal record of morepork | ruru was also removed as there are alternative acoustic detection and analysis methods for nocturnal (night-time) species (see [link to nocturnal birds factsheet]).
Presence / absence data for each species and habitat type were modelled using a zero-inflated binomial model to estimate occupancy in relation to time, park status and habitat type. Count data for each species were modelled using a negative binomial model to estimate relative abundance in relation to time, park status and habitat type.
Glossary of terms
5-minute bird counts (5MBC) are observer-based counts conducted at a fixed location. These counts require that all birds seen or from the point irrespective of distance are recorded over a 5-minute period. The 5MBC method is an index of relative abundance. Rather than measuring the actual density or abundance of birds directly, it records some of the individuals present and uses this measure to infer relative abundance without attempting to adjust for detection probability.
Confidence intervals (CI) indicate range of values that you can be 95% certain contains the true average of the population.
Indigenous species are those native species inhabiting a defined region as a result of natural processes with no human intervention.
Introduced species are species that occupy an area as a result of non-natural human processes, either deliberate or accidental.
Occupancy refers to the probability that a site is inhabited by a given species whilst also accounting for imperfect detection.
Richness is a count of the number of different species that occupy a given area.
Standard error indicates how different the population average is likely to be from the average taken from a sample. It tells you how much the sample average would vary if you were to repeat a study using new samples from within a single population.
Additional resources
Walker, Susan and Monks, Adrian and Innes, John: Status and change in native forest birds on New Zealand’s mainland, 1969-1979 to 1999-2004. Landcare Research, 2017.
Gaston, Kevin J: Common ecology. Bioscience, 2011.
Elliott, Graeme P and Wilson, Peter R and Taylor, Rowley H and Beggs, Jacqueline R: Declines in common, widespread native birds in a mature temperate forest. Biological Conservation, 2010.
Dorazio, RM and Royle, JA: Estimating size and composition of biological communities by modeling the occurrence of species. Journal of the American Statistical Association, 2005.