Bilingual babies' vocabulary linked to early brain differentiation

Bilingual babies' vocabulary linked to early brain differentiation
ScienceDaily (Aug. 29, 2011)
quotes from this article:

"...the researchers report that the brains of babies raised in bilingual households show a longer period of being flexible to different languages, especially if they hear a lot of language at home".
"The bilingual brain is fascinating because it reflects humans' abilities for flexible thinking -- bilingual babies learn that objects and events in the world have two names, and flexibly switch between these labels, giving the brain lots of good exercise..."
"The researchers say the best way for children to learn a second language is through social interactions and daily exposure to the language....Learning a second language is like learning a sport [...] [t]he more you play the better you get".

Babies Raised In Bilingual Homes Learn New Words Differently Than Infants Learning One Language

Babies Raised In Bilingual Homes Learn New Words Differently Than Infants Learning One Language
ScienceDaily (Sep. 28, 2007)
a quote from this article:

"research has shown that bilinguals and monolinguals achieve language-learning milestones (such as speaking their first word) at similar ages and have vocabularies of similar sizes when words from both languages are taken into account".

My son is 20 months old

New words appeared:

- "катись" (to roll down) and he says "катись train" while pushing the train down the slide;
- "будешь", but I don't think that he understands that it means "you will".

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?