Newborn bird hatchlings are naked and do not have feathers until a few weeks.
The bath that drops down into the vestibule is clearly made of plastic, not cast iron. Cast iron would most likely have smashed on landing, you can see light shining through it, and when Ivy is fooling around in it, you can hear it is plastic when she touches it- which would give a characteristic 'ding' sound.
The story takes place in 1936, but while Sir Hallam Holland and his wife wait on the station platform to join the boat train a 'British Railways' poster can be seen behind them. British Railways, as an organisation, didn't come into being until 1st January 1948.
The paving stones outside 165 Eaton Place are a far more modern type than would have been used in 1936.