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,
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)
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 or
at least resident, 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
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.
The female was initially tricky to locate on that day but she turned up in a small tree very close to the original nest site. Last year she spent approximately half the period while eggs were in the nest in that position (moving closer to the new nest site when the chicks emerged). Here she is:
