Responses to Friends of ANBG questions
When I gave a talk to the Friends of the ANBG I scored a couple of curly questions. Having delved a little into the data I hold I can now provide a little more information on them.
Which birds are declining?
This is a very big question. To try to get a handle on one approach to it I have restricted my investigation to the 50 species which were most abundant in the first decade (1981-82 to 1991-92) of the GBS. This will obviously not cover species which are relatively uncommon in an urban area nor address questions such as "When was the last Lesser-striped Whatsit (Nemo pallidus) seen in the Canberra area?"
To prevent single years having an undue influence I calculated the average abundance in the first decade and the third decade and ranked species according to the difference. For the top 10 species I then graphed the results for individual years to attempt to see what was going on. (More details on this are available if anyone is interested.)
A summary of the results follows. Where a dotted line is shown on a graph it is a polynomial trend.
Common Starling
As I suggested this species does show a steady decline throughout the series. Note that year 1 is omitted as it was overstated by a huge roost in one site, that wasn't used in following years. The fact that the trend 'kicks up' at the end can be ignored: I am always suspicious of end effects.
House Sparrow
Again a clear steady decline is evident.
Silvereye
A more 'interesting' graph. Disregarding minor 'ups and downs' for individual years there are plateaus from 1982- 2002 followed by a sharp drop to another, lower, plateau for 2004 -10. The data series and the trend both then tick up. This coincides with a drought and then recovery.
Yellow-faced Honeyeater and White-naped Honeyeater.
Both species are strongly migratory and it is likely that both series are affected by inconsistent application of the counting rules. No graphs prepared.
Silver Gull
I was surprised to see this species included. From looking at the detail of the data it appears to show two distinct phases.
This seems to have coincided with reduced operations at, and ultimately closure of, the tip.
Noisy Friarbird
The pattern for this species shows quite steady decline except for an increase in the late 1990's.
The pattern appears to be quite consistent across suburbs although reports from Kambah, the suburb which used to report the greatest numbers, have dropped off dramatically. As the species is associated with flowering eucalypts (especially ironbarks, common street trees in Kambah) perhaps this relates to some aspect of these species?
Pied Currawong
The species does show lower numbers in the third decade but the variation from year to year is such that no reliable trend is apparent.
As noted in my talk they have changed behaviour considerably but do not appear to be in decline.
Black-faced Cuckoo-shrike
Another species showing a general decline, but possibly with a recovery in the 2010s.
I have no idea why this is so.
Yellow-rumped Thornbill
This was another surprising inclusion in the list. The principle reason for inclusion of the species is very low numbers of the species reported in 2002-04. While numbers prior to, and following, that period are in general higher they do fluctuate considerably from year to year. It is also noteworthy that this is one of the few situations in which the series relating to all sites, and that for sites in the Survey for 15+ years, show different patterns, especially in the later years.
My conclusion is that the reports for this species are significantly affected by the composition of the panel of sites active in a year. This tends to be confirmed by reports for a number of long established suburbs showing a decline in the last half of the series while some sites in the rural hinterland have started up in the final few years and reporting large numbers of this species.
Common Myna
Although, as shown below, this species has dramatically reduced in abundance over the past decade it didn't fit my definition of a declining species as the average abundance in the third decade is still above the average of the first decade. However, that is my definition so I can break it if it suits me, and I have done so.
Exactly when the decline in Myna numbers commenced is a moot point. It is quite certain that the activities of CIMAG have accelerated the decline. It is possible that the data shows a decrease in the rate of decline in recent years but that is to be expected. However it does stress that the job isn't finished (and probably never will be).
Has the increase in Grey Butcherbirds affected the Abundance of Kookaburras?
Again, this was not something I had looked at in detail. However given their broadly similar appearance the question was very interesting.
This is really only applicable to the period since 2001, as that is when Butcherbirds began to be reported in the GBS in other than random instances.
The short answer would appear to be that the two species are not affecting each other as both are increasing (although the correlation coefficient associated with the trend for Kookaburra is not very strong).
Possibly this reflects their different food preferences. While both eat insects and reptiles, the Grey Butcherbird appears to have a greater preference for the latter and also predates birds (although I have seen a Kookaburra kill and eat a honeyeater).
Also they do not compete for nest sites, with the Kookaburra nestin in hollows while the Butcherbird makes a 'traditional twig nest..
Which birds are declining?
This is a very big question. To try to get a handle on one approach to it I have restricted my investigation to the 50 species which were most abundant in the first decade (1981-82 to 1991-92) of the GBS. This will obviously not cover species which are relatively uncommon in an urban area nor address questions such as "When was the last Lesser-striped Whatsit (Nemo pallidus) seen in the Canberra area?"
To prevent single years having an undue influence I calculated the average abundance in the first decade and the third decade and ranked species according to the difference. For the top 10 species I then graphed the results for individual years to attempt to see what was going on. (More details on this are available if anyone is interested.)
A summary of the results follows. Where a dotted line is shown on a graph it is a polynomial trend.
Common Starling
As I suggested this species does show a steady decline throughout the series. Note that year 1 is omitted as it was overstated by a huge roost in one site, that wasn't used in following years. The fact that the trend 'kicks up' at the end can be ignored: I am always suspicious of end effects.
Again a clear steady decline is evident.
A more 'interesting' graph. Disregarding minor 'ups and downs' for individual years there are plateaus from 1982- 2002 followed by a sharp drop to another, lower, plateau for 2004 -10. The data series and the trend both then tick up. This coincides with a drought and then recovery.
Both species are strongly migratory and it is likely that both series are affected by inconsistent application of the counting rules. No graphs prepared.
Silver Gull
I was surprised to see this species included. From looking at the detail of the data it appears to show two distinct phases.
- Up to the mid 1990s there were relatively frequent reports from sites in NW Belconnen of flocks flying to and from the West Belconnen landfill site (aka 'the tip'). Some reports from close to Lake Burley Griffin also included flocks that hang out on the Lake.
- Since 1995 these reports have ceased, although some of the reporting sites have continued.
This seems to have coincided with reduced operations at, and ultimately closure of, the tip.
Noisy Friarbird
The pattern for this species shows quite steady decline except for an increase in the late 1990's.
Pied Currawong
The species does show lower numbers in the third decade but the variation from year to year is such that no reliable trend is apparent.
Black-faced Cuckoo-shrike
Another species showing a general decline, but possibly with a recovery in the 2010s.
Yellow-rumped Thornbill
This was another surprising inclusion in the list. The principle reason for inclusion of the species is very low numbers of the species reported in 2002-04. While numbers prior to, and following, that period are in general higher they do fluctuate considerably from year to year. It is also noteworthy that this is one of the few situations in which the series relating to all sites, and that for sites in the Survey for 15+ years, show different patterns, especially in the later years.
My conclusion is that the reports for this species are significantly affected by the composition of the panel of sites active in a year. This tends to be confirmed by reports for a number of long established suburbs showing a decline in the last half of the series while some sites in the rural hinterland have started up in the final few years and reporting large numbers of this species.
Common Myna
Although, as shown below, this species has dramatically reduced in abundance over the past decade it didn't fit my definition of a declining species as the average abundance in the third decade is still above the average of the first decade. However, that is my definition so I can break it if it suits me, and I have done so.
Has the increase in Grey Butcherbirds affected the Abundance of Kookaburras?
Again, this was not something I had looked at in detail. However given their broadly similar appearance the question was very interesting.
This is really only applicable to the period since 2001, as that is when Butcherbirds began to be reported in the GBS in other than random instances.
Possibly this reflects their different food preferences. While both eat insects and reptiles, the Grey Butcherbird appears to have a greater preference for the latter and also predates birds (although I have seen a Kookaburra kill and eat a honeyeater).
Also they do not compete for nest sites, with the Kookaburra nestin in hollows while the Butcherbird makes a 'traditional twig nest..
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