It is an offence to cut down a hedge or tree intentionally or recklessly whilst there are birds nesting in it.
Reckless is a term often used in legal definitions. In this context it means that if you cut the tree down knowing there was a chance that birds were nesting there and you didn't check, then you would have been reckless and therefore guilty of the offence.
The penalties are imprisonment and/or a fine. If there is more than one bird, nest or egg then each case will be treated as separate criminal offences.