Did a bit of research since I wrote last night.
Guess what?
The mark is a visual clue to the adults that the little tacker is hungry.
Thanks to Ashley over at https://aussiebirder.com for giving me a hint about what it all might mean. See his comment on the post last night.
https://birdsaspoetry.com/2021/02/06/saturday-evening-post-120-that-little-touch-of-love/
Here is a link to a good site explaining it
And a little clip of the info, thanks to David Craven.
>>So, what is this? A deformity? Some parasite? A magical third eye? A literature search was required. Wading through various papers there were lots of theories. The patch was natural, and present in nearly all grebe chicks. Some thought it helped control the chicks temperature while tucked on the parent’s back. Some thought it deterred predators. Others thought it was used in signalling parents.
It took a 1985 paper by Gary Nuechterlein to settle it. Hand-rearing some Western Grebe chicks, he used a series of experiments to determine that it was allied to begging for food. The more the bird begged, the brighter red the crown patch. Once fed, it faded to a lighter pink.
There we have it. Next time you spot grebe chicks, keep an eye out for the red patch!<<
Thanks David.
Birds as Poetry —— Just because I WANDER - --doesn't mean I'm LOST 
Fascinating!
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It is good that you were able to research this more David. We marvel at how very unique and individual the various forms of communication and display are for every species of creature, it is a marvelous Creation by a even more marvelous Creator.
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Thanks Ashley for the headsup on the likely reason. Not something I’ve ever seen before but then I don’t see that many Grebe young. Think this is a first for this pair. In fact, over the past 12 months, I’d have never picked them as a pair, rather just two of the same species in the same lakes, 🙂
I never to cease to be awestruck by both the variety and the complexity of the lives of birds
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Isn’t that interesting! Such a great way to let the parent know that the baby needs topping up.
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Thanks Eleanor, if you’d have not asked on #120, I probably wouldn’t have pursued it so it was good thing to learn.
I’m not sure I heard the young ones calling, but they are really only a day or two out at this stage, and flop about more than stand up.
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Isn’t science wonderful? Unless of course you’re a climate sceptic….
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Very interesting to know. It’s amazing how many ways there are to improve communication between parents and children in Nature.
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G’day Adam, I keep finding so many things that seem unimportant at first glance among the birds, only to discover to them it is a significant event or matter.
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