Thursday, 27 November 2014

Where do Koels come from?

Anyone that has answered "Eggs" should report to the Headmaster's Study after the lesson!

This post has been stimulated by a couple of soundings of (what is called this month) Australian Koels.  The first was in Orange on our recent trip and the second in our front yard in Carwoola.

The latter observation was the second, in 8 years, on our property although the species has been reported somewhere in the Carwoola area in 6 of the 8 years.  Reports have tended to be in November or December.  Perhaps it is significant that we are 200m higher than Canberra and our plums are still way off ripe.

When I reported my latest sighting to the COG chatline a member commented that they regard Koels as an urban bird, including country towns such as Cowra and Goulburn.  He noted that there could be plenty of fruit - do Koels eat strawberries? - in a rural residential area like Carwoola to bring them in.  (They might also be aware - by whatever means Koels are aware of anything - that Noisy Friarbirds nest in this area, which a pair are doing in our yard as I write.)

I responded that a matter of interest to me is the route that the Koels travel to arrive in the Canberra Region.  Do they come from the North, travelling down the ranges, or from the South coast, perhaps coming up to the divide on the valleys (with the Shoalhaven being a strong candidate?  So I downloaded Koel data for NSW from ebird.  This gave me 1928 records.  (Note that I didn't do the separate selection for the ACT, because it is what happens before the birds get to that Territory that was of interest.)

What follows is pretty much of a mind game (or speculation) since I don't think there is enough data around to do the multi-dimensional analysis needed to try to tease out what is actually going on

A first point is to examine the frequency of reports.  I decided that month was a suitable time dimension.  As shown below they are very much a late Spring-Summer bird.
I then selected the latitude and Longitude of observations for the months of October (green icon) November (yellow icon) and December (red icon) and plotted them on Google Earth.
I shall deal with the letters A and B somewhat further below.

There are a mixture of red, yellow and green icons right down the Coast, rather than as i had hoped green icons (October) predominantly in the North and red ones (December) dominating in the South.   I have extended the process to look at the location of reports for September, and for week in September rather than the month as a whole, but again there is no clear picture, other than fewer reports South of Sydney.

There also seemed to be variation between years in the number of eBird reports of Koel in September  (from zero in some years up to 20 in 2010 and 2012) but no significant variation between the distribution through the month.

The conclusion I come to thus far is that when the Koels head South they pretty much come in a rush, arriving at more or less the same date all down the Coast.  One must presume that there are available, at more or less the same time throughout this range:
  • fruit of various sorts for them to eat, and 
  • host nests in which to deposit their eggs. 
It is about 750km from Brisbane to Sydney (in a straight line).  I haven't been able to find an estimate of the flying speed of a Koel (of any species) but did find one estimate of the flying speed of a European Cuckoo as 50mph (~80kph).  Thus, as a massive approximation, it might take a migrating Koel 10 hours to fly that journey.  This is obviously a benefit of avoiding the road works on the Pacific Highway.  The relevance of this is that it seems possible that if a bunch of Koels leave their Wintering grounds at the same time in response to some trigger, the ones that get to Sydney might only be a day after those that find satisfaction at Byron Bay and the adventurous ones get to beautiful downtown Moruya (or even Mallacoota) a day later.

Returning to the letters A and B.  These mark the position of a couple of areas in which the eBird reports are  dominated by the month of December.  A is roughly speaking between Moree and Inverell while B is the Blue Mountains West to about Bathurst. Perhaps this demonstrates the arrival of a few bold exploring Koels who finished their business on the Coast and followed a few food source over the ranges?  If that is the case it could lend support to the view that the ACT Koels come from the Coast rather than following down the Ranges.

It is of course possible that the route has changed over time.  Perhaps the first few Koels got here from the Coast route but have learnt over the years to travel by a hypotenuse (again, at 80kph, they could make it from Brisbane to Canberra in a hard day's flight, not being seen by observers on the ground).

An email comment on the first published version of this post commented that
"The data available are insufficient, as you noted, to really be able to tease anything out. And if they do indeed make the flight south in a 'rush', i.e. within say a 12-hour window, then opportunistic observations by birdwatchers will never be able to tell us how they move (i.e. where they come from and by what route). Only some sort of tracking device would do that effectively."
So there is a nice little research project for someone!

No comments: