On the eve of the COG blitz 2015 I drew attention to a report of 22 GCGs on Cotter Reservoir. The report arose from research into birds in relation to native fish species. Photos made available at the time confirmed the identity of
the species and breeding activity
-----Original Message-----
From: Con Boekel [
Sent: Saturday, 29 April 2017 9:01 AM
To: canberrabirds chatline
Subject: Great Crested Grebe - diet #3
The other day I had an interesting discussion with Steve Wallace about prey items of Great Crested Grebe and why there was not a permanent population of Great Crested Grebes in Canberra while, at (relatively) nearby Pejar Reservoir (Just
west of Goulburn) they are seen reliably.
Steve has subsequently posted an image of a GCG with a Weatherloach prey item.
While reviewing my GCG images I noticed that one of the fish prey items had eyes considerably larger in proportion to body size than those of the Oriental Weatherloach. See image.
Unfortunately, the level of detail is insufficient to work out which species it might be. It may be a young of Redfin or Goldfish, both of which are known to inhabit the lake but the image fits neither very well. Other possibilities
are Mosquito Fish or Western Carp Gudgeon - both of which are common elsewhere in the ACT. But I don't know whether either of the latter are present in Yerrabi Pond.
I suggest that there is not a year-round supply of accessible small fish and/or that, at most times of the year, there are not enough prey items for GCGs.
I have occasionally seen Mosquitofish leap onto land in order to evade Redfin. It is quite common to see shoals of Mosquitofish leap out of the water to evade Redfin. Mosquitofish also use dense vegetation as cover.
(Quite a few COGites recently observed Egrets spp and White-faced Herons successfully hunting Mosquitofish from dense veg at Kelly's.) Mosquitofish also shoal in extremely shallow water - far too shallow for GCGs to hunt. In winter,
Mosquitofish tend to become inert and to sink to the bottom.
One reason for a lack of permanent GCGs may be that Redfin Perch are more efficient predators of small fish in open water and that Redfin and Mosquitofish interactions make the latter mostly inaccessible to GCGs.
I am not sure about how all this works for Western Carp Gudgeon. From personal observation, they tend to rest on or near the bottom.
Which brings us to the Yerrabi Pond GCG.
I have yet to see it dive in open water, or to dive into the very extensive Yerrabi waterplant beds. I am not sure about the name of the waterplants (they look a bit strap like) but I believe that they do not provide sufficient cover
for Mosquitofish from Redfin. I would be interested to know whether anyone has seen it feeding in the strap plants and whether or not it was successful.
The GCG essentially feeds along grassy edges or in and among Typha and debris. It will poke along grassy edges. It will clamber in, amongst and over debris. (I note in passing that many of Canberra's lakes have hardened edges unsuitable
for this form of GCG fishing.)
I believe that the Yerrabi Pond GCG may be the first to specialize in predating the Oriental Weatherloach.
The attached image (captured in the Pelican sculpture cove, Yerrabi
Pond) shows just how far along its learning curve the youthful Yerrabi Pond bird has gone.
regards
Con
