The top bird is almost in full breeding plumage; bottom is a non-breeding plumage adult. Juveniles are similar but have dark markings on the face and the white supercilium is incomplete. This is the widespread nominate race which occurs across the Palearctic. Two other races are found; one in Africa and another in Australia and New Zealand. Australian & New Zealand birds retain breeding plumage year round. African birds are smaller and lack the white supercilium. This species is famous for its elaborate courtship displays in which their head feathers are raised and extended. Digiscoped with Panasonic Lumix LX5 | Nikon Fieldscope III | 30X hand-held, no adapter (top). Canon PowerShot SX60 (bottom two).
References:
Llimona, F., del Hoyo, J., Christie, D.A., Jutglar, F. & Kirwan, G.M. (2019). Great Crested Grebe (Podiceps cristatus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/52489 on 25 March 2019).
Marchant, S. & Higgins, P.J. (co-ordinating editors) 1990. Handbook of Australian, New Zealand & Antarctic Birds. Volume 1, Ratites to ducks; Part A, Ratites to petrels. Melbourne, Oxford University Press. Pages 91-92, 115-124; plate 6.