Noisy Miners in the COG Garden Bird Survey

As Coordinator of the COG Garden Bird Survey (GBS) I was recently asked to provide information about the changes in the information about Noisy Miner Manorina melanocephala collected in the Survey.   This request was related to current consideration of the Noisy Miner as a nuisance species[1], noting research which has shown that by ‘bullying’ insectivorous bird species they are not only reducing the numbers of those species but also permitting increased numbers of insect pests (Clarke et al).

The principle purpose of this article is to present some of the data available in the GBS in ways which may indicate some changes have occurred in the Noisy Miner population over the period of the GBS in the catchment area of that Survey.  It will conclude with some speculations on how these changes may relate to received knowledge about Noisy Miners.

There are a number of ways of presenting the annual summary results of the GBS of which the two most commonly used are:

• A (for Abundance): the aggregate number of birds of a given species reported each active survey site-week – typically, but not necessarily, covering all sites; and
• F (for Frequency): the proportion of sites in a given period - typically a year - in which the species was reported at least once.

The Chart below shows these two summary statistics for each of the 30 years of the GBS. (Note that each year begins on 3 July and Year 1 was 1981-82.) 

Figure 1: Noisy Miner, all sites : values of A and F

The smooth lines are the 4th order polynomials delivered by EXCEL, which I find a convenient way of smoothing out the 'noise' in the series.  In both cases the values of R² suggest the smoothed lines are a good fit to the raw data.   The broad picture offered by both series is similar: a reasonably constant level of reporting until about year 19 (1999-2000) followed by a strong increase since then.

Constant Sites

An issue of concern was that this pattern was in some way influenced by changes in the composition of the Survey panel in the later years.  Since the sites are self-selecting (ie anyone who wishes to participate is welcome to do so) it might be possible that something had happened to change the nature of the set of sites.

A simple way of compensating for such effects is to use a subset of sites which have participated in the survey for a large number of years.  29 sites have been in the GBS for more than 15 of the 30 years: restricting the analysis to these sites seemed a 'reasonable' way of ensuring that changes in the results should not be unduly affected by changes in the panel.  Typically they contributed between 25% and 30% of the set of sites active in a year, although this was higher (above 50%) when the overall survey participation went though a low patch in the late 1990s.

I calculated both numerators and denominators for A and F (as defined above) for the subset of sites active for more than 15.  The next two charts compare the values of these two summary statistics for all sites (as shown above) with the equivalent statistics for the >15 year sites.

Figure 2: Noisy Miner A values: all sites and sites reporting for >15 years

Figure 3: Noisy Miner F values all sites and sites reporting for >15 years

Not surprisingly the charts for the >15 year series are a little less stable than for the wider sample.  However allowing for the possible differences between the two series the degree of agreement is very good.  I therefore conclude that the pattern shown in the Figure 1 is not an effect of sample composition, but due to some other effect. 

Changes in Flock Size

Reviewing Figure 1 suggests that the value of A is rising faster than the value of F.  This implies that flock size is increasing as well as the frequency of observing the species.  Within the GBS Summary statistics, G (Group size) shows the average flock size by year.

Figure 4: Noisy Miner: Average Group size x GBS year

This chart suggests that, after a drop from high values in the first years of the Survey, group size has been increasing gradually over the last few years.  Knowing that the species is usually encountered in modest sized flocks (related to the Territory of a breeding male - Higgins et al) , but in Winter the groups can aggregate to quite large flocks (probably a Coalition within a single Coterie as described in Higgins et al) I charted the proportion of total flocks with >10 birds.  While the result is rather unstable in recent years the smoothing polynomial shows a reasonable increase in recent years (particularly during the drought years).

Figure 5: Noisy Miner: proportion of groups >10 birds

On examining the detailed information for each year, the first year of the GBS contained information for a site in Wanniassa with Noisy Miners in every week, often in groups of 10 or more and a site in Fraser with several groups of 10+.  The site in Wanniassa only reported in year 1 and while the site in Fraser also reported relatively high numbers in Year 2 the number of Miners decreased thereafter. 

At the other end of the time series a site in Ainslie also regularly reported flocks of 10+ birds in years 28 and 29.  The site reported few such flocks in year 30.  It is noteworthy that another site in much the same area reported an increase in numbers of Red Wattlebirds in year 30 as the nearby Noisy Miners had gone, allowing the Wattlebirds to move in (Haygarth pers comm).

Both cases of significant declines in the number could well reflect a factor leading to the coterie dropping below a critical size for group maintenance, as described in Clarke et al.

Seasonality
I have also examined the seasonality of reports tracking the aggregate number of birds reported each week.  As the number of birds reported ‘jumped’ significantly between years 23 (2003-04)[2] and 24 (2004 – 05) with no significant changes in panel composition I have plotted information for years 1 – 23 and 24+ separately in Figure 6.

Figure 6: Noisy Miner proportion of birds x GBS week – recent and early years.

For both series a higher proportion of the year’s birds occurs in late Autumn. This might coincide with:

• 1.      a migration out of the high country following the onset of colder weather; or
• 2.      simply an aggregation of breeding groups into Coalitions for cooperative feeding as with the mixed feeding flocks of other species. 

As HANZAB (Higgins et al) rates the species as sedentary[3] or at least resident[4], with most suggestions of seasonal movement dating from before 1950 it seems that the feeding flock hypothesis  is more likely.   In March 2012 it was reported on the COG chatline by Taws that in Goorooyaroo (ACT) “Noisy Miners were also abundant, roaming in mobs beyond their normal haunts to areas they are not normally recorded and feeding in the heavily-flowering mistletoe.”  This adds weight to the ‘feeding flock’ hypothesis.

Discussion

These data show that the numbers of Noisy Miners observed in an area can change over time without any deliberate human intervention.  The changes observed are almost certainly not a regular movement between breeding areas and non-breeding areas and could fit the cited definition of sedentary or resident.   

It should be noted that most research into Noisy Miners has been undertaken in rural sites with different vegetation and levels of human activity to the sites covered by the GBS

An online brochure (Grey and Clarke 2011) appears to imply that human removal of Noisy Miners is the best solution to the problems caused by their presence.  This brochure also states:

“Culling is the most humane, practical, cost-effective and time-efficient method of reducing the impact of Noisy Miners, as translocation simply moves the problem to a new locality and causes the displacement of other birds.”

However there has not been to my knowledge any organised  culling of Noisy Miners in the suburbs of Canberra suggesting that the birds may move out of such an area without culling.   

It is recommended in Clarke et al that culling be restricted to areas where either:

• 1.      The species being out-competed by the Noisy Miners is endangered (eg Regent Honeyeater); or
• 2.      There are endangered vegetative species being affected by the increase in insects following Noisy Miner bullying of smaller birds.

That report also shows that as Noisy Miners are far less of a problem when the understorey is dense, a better solution may be to restore a higher quality understorey.   I have been unable to establish whether the two areas in which the GBS shows Noisy Miners to have decreased dramatically have also developed such an understorey (eg through gardens maturing) making the sites less attractive to Miners.  (This does seem unlikely for the site in Ainslie as most gardens in that suburb were mature well before the GBS commenced, but possibly equivalent changes have occurred in the nearby Mt Ainslie component of Canberra Nature Park.)

References

Clarke M F et al,  1995, The Noisy Miner Manorina melanocephala and Rural dieback in Remnant Eucalypt Woodlands, RAOU Report No. 98

Grey M J and Clarke M F, 2011, The Noisy Miner: Challenges in managing an overabundant species, published on internet.

Higgins P J, Peter J M, and Steele W K, 2001, Handbook of Australian, New Zealand and Antarctic Birds.  Volume 5: Tyrant Flycatchers to Chats, OUP

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[1] See for example http://www.aceas.org.au/index.php?option=com_content&view=article&id=84:beating-the-bullies&catid=35:working-groups&Itemid=86

[2] It is interesting, but not provable by me, that the increase in numbers of miners occurs in the year following the January 2003 bushfires.  This may reflect birds being driven into suburbs with GBS sites due to the unavailability of Winter food in their previous haunts.

[3] HANZAB defines sedentary as “most individuals not moving more than 50km.”

[4] HANZAB defines resident as “most individuals non-migratory (ie don’t regularly move between breeding and non-breeding ranges) though some may move long distances.”