"Ageing" of Australasian grebes

[martin butterfield <>]

I have put quotes around the word ageing since this is whether it is possible to estimate the age of young Australasian Grebes, rather than their aging process.  On 25 November last year I observed 1 adult and 3 young Australasian grebes on our top dam.  Some pictures from that day are in http://franmart.blogspot.com/2010/11/australasian-grebes-on-top-dam.html  Today, 15 January, I was in the area again (primarily improving some blackberry regrowth) and found that there were still one adult and 3 young grebes: see attached image taken on full  zoom across a hazy dam.  

My expectation is that these are a second brood.  Does anyone know how quickly the chicks would grow?  Is it more likely that in two months the first brood:

1. became ex-grebes (the area is well supplied with raptors and foxes are not unknown in the area) and a second started from scratch; or
2. achieved independence and departed while the excellent season (the area around the dam is lush with vegetation full of frogs and insects) allowed the second brood?

Martin

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