Friday, 20 January 2012

Of Koels and bigger honeyeaters

In commenting on my post about the nesting Noisy Friarbirds Denis asked "Do you get Koels coming in to use these birds as host parents?"  The short answer is given in my response to his comment.  (An even shorter answer would be 'No.", but that is no fun for me.)   This post is an attempt to provide a fuller answer to he question, using information from the COG Garden Birds Survey (GBS).

NOTE: 
  1. To see the detail of Charts click on them and a larger version will appear. 
  2. In some of the Charts there are two vertical axes, where the intention is to demonstrate the similarity of shape rather than size of distribution.  Be aware of this!

A key point underlying the discussion is that Eastern Koels, other than as vagrants, are a relatively recent addition to the Canberra avifauna.   Writing in 1999, Steve Wilson in his authoritative book "Birds of the ACT: 2 Centuries of Change" noted "The ACT is just beyond the Southern edge of its normal distribution..." and records observations made in1946 and 1950 with a gap until 1981. He concludes his discussion with  "Since <1986> this species has been reported nearly every summer, but it has never been numerous."

They might now be approaching numerous.  The following Chart shows the Abundance (average number of birds per observer-week) and Frequency (percentage of sites reporting the species at least once per year) of Eastern Koels (hereafter 'Koel') over the 30 years of the GBS.
I have not included a trendline for A since the Excel trend function does bizarre things when dealing with values of '0'.   However it is broadly similar in shape to that of F (with an even better value for R2).

Much of the rest of this report concerns itself with interactions (or lack thereof)  between Koels, Red Wattlebirds and Noisy Friarbirds.  This is due to the host/parasite relationship between the Koel and the other two species.

Although the value of A for  has risen dramatically it is still small (A Year 30= 0.103) compared to that of the Red Wattlebird (A Year 30= 2.881) and Noisy Friarbird (A Year 30= 0.414).  In part this is due to the migration of the species  illustrated in the next Chart. 
Note:
  1. Week 1 begins 1 January even though the GBS Year runs from July to June.
  2. Separate A values were calculated for each week, thus adjusting for the low observer effort in weeks 25-27 due to vacation around Christmas.
For the next stage of analysis there seemed little point in considering the weeks when 2 of the 3 species were absent (or nearly so).  From eyeballing the original data the pronounced drop-off and regathering of numbers of Noisy Friarbirds was roughly equivalent to weeks in which the numbers of Friarbirds seen over 30 years dropped below 500 and that number was used as a cutoff.  This means that weeks 16 to 36 will be omitted from much of what follows.

In the next Chart I have reordered the weeks so that the peak period is not split by the year starting according to a Northern Hemisphere Winter period!  (It is in fact simply a different presentation of the information from the preceding Chart.)

Returning briefly to Koels alone, the next Chart shows how their period in the ACT has expanded over the recent past.  NOTE that the scale for the last 5 years (LHS) is approximately 10 times larger than that for the earlier period (RHS).  It is the shape of the graphs that is important as emphasised by the yellow areas illustrating the increased duration of the species' stay in the ACT.
Having exhaustively (exhaustingly?) covered the occurrence of the three species in the ACT let us move on to breeding activity.  As a first point, it should be noted that the system for recording breeding activity in the GBS was greatly enhanced in Year13 of the Survey and only records in years 13 - 30 will be considered in what follows.

The first records of Koels breeding in the ACT were in January -February 2009. and has been written up by Lenz, Haygarth and Oren in the COG Bulletin 'Canberra Bird Notes' (warning the link goes to a 3Mb file).  One of the records covered by that report was in a GBS site.  No GBS sites recorded Koel breeding in year 29 but in Year 30 (2010 - 11) of the GBS a further 4 Koel breeding events were reported from GBS sites.  4 of the records are of Dependent Young while one was of copulation.  All occur in the first quarter of the calendar year (ie GBS weeks 1 - 11) with the copulation record earlier than the dependent young records (at different sites).

The large honeyeater species are widely reported breeding in GBS sites.  As is the case with most species the commonest breeding records are the indicative 'Dependent Young' observations, rather than definitive "Nest with Egg" or "Nest with Young" records. The following Chart shows the number of Breeding Records for each of the three species as a proportion of the site weeks applicable to that week.
It appears that even in recent years most reports of the presence of Koels are made after the initial peak of Red Wattlebird breeding.  It would seem that the species arrives in time for the secondary peak of Red Wattlebird breeding reports and just before the peak of Noisy Friarbird breeding reports.  The (few) reports
thus far of Koel breeding (just visible in the enlarged image) occur right at the end of the breeding season for both honeyeaters.

Analysis (or at least hypothesis) Time
It is important to remember that even though Koels records in the ACT have increased dramatically in the recent past they are still not a common bird. It is also possible that many of the GBS observations duplicate records of a single bird in view of the far-carrying calls of the species.  Definitive breeding records for the host species are more common, but still not plentiful.   Thus observing the intersection of the more unlikely events is pretty much a lottery.

It would seem that the reliance, in the ACT, on Red Wattlebirds as hosts could be due to the greater number of Wattlebird breeding events recorded in comparison with events for Noisy Friarbirds: this applies even after the peak breeding period for Wattlebirds has passed.  It appears that this outweighs the preference, described in HANZAB (for areas where Koel breeding is more common) for Friarbird rather than Wattlebird nests.

As a totally heuristic hypothesis I wonder if it might also be the case that Wattlebirds are less protective of their nest sites for their second clutches?  Thus they make an easier target for the Koels?  From the obseravtion of the behaviour of Friarbirds around the nest which originally generated this enquiry they are very aggressive towards predators, driving off Brown Goshawks and Australian Magpies.  I would expect them to look with some prejudice at a Cuckoo in the neighbourhood!

No comments: