Did a bit of research since I wrote last night.
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.
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!<<