My son 19 months old

My son  is 19 months old. I'm Russian (his mother), his dad is English. Even before he was born, we decided that he would be bilingual. It sounds fascinating - two languages and two cultures will be learnt from the very first day. And he would understand that a thing can have more that one name, or in academic terms one signified will have two signifiers (in case of English-Russian mind). Wow! I got it at university listening to the philosophy lecturer. And Jake should understand it much much earlier. A table, for example, will be стол, and moreover, not necessarily a table, might be a chair or a lamp; it depends on how we agree between each other, but it will still be that thing that we use for eating or writing or drawing...

Well, I've read about all this and decided to start.

The rules are
I speak Russian to my son
My husband speaks English
Between each other we speak English.

So, English is the dominant language and Russian, while we live in the UK, will always be catching up.

My son is not speaking yet, just several words... and to my disappointment, they are mostly English. He says "car", I say "машина" [mashina], and he repeats "car" :) He says "boat", "train", "ball", "bye-bye" and not "лодка" [lodka], "поезд" [poezd], "мяч" [myach], "пока" [poka]. There are 2 words that he says in Russian "птичка" [ptichka, small bird] pronouncing it as "kika" and "кран" [kran, crane] pronouncing it as "kan".

In case of  "машина" - "car", I understand that "car" is easier to pronounce, but how to explain the choice between "птичка" and "bird"? Or is it just arbitrary?