>are magpies getting more common?< Yes, it happens in periods of a few years, next year competiton will be great and lots won't be able to breed so the population will stabilise in tune with the environment.
>why do song thrushes have such bright blue eggs if they are not ment to be seen?< All things being equal, the eggs should never be seen, thrushes are tight sitters and will only leave the nest at the last minute.
>where do robins nest?< Presuming you mean the english red breasted robin, they nest in secluded places such as tree cavities and any place where they can build their nest in a container - upturned flowerpots, old watering cans, anywhere that they think they can use. They rarely build in the tree branches like blackbirds and finches.
>are skylarks rare?< No, but they mainly live in rural places and are well camouflaged so driving past them you won't see them. They usually nest in a scrape in on the ground.
>where do squirrels live?< In tree holes, the grey ones have been known to use any place such as your loft.
>can chickens fly?< Most definitely! How well they fly depends on the breed. The Old English Game Fowl is very similar to the ancestor of all chickens - the Jungle fowl - and can fly as well as a pheasant if it wants to. The Light Sussex breed is lucky if it can fly up 3'.
I have no idea about badgers at the three sisters.