China wrote:

G,day Gerry,
I don't know for sure with Magpies, but in many birds
it's either a sign of reaching full maturity or, with older birds, the male
has become the senior of his clan.

I don't know either, but this suggestion sounds reasonable.

Magpies usually breed in family groups with one male and a number of
females (mum and daughter). Well that was the contents of a CSIRO paper
in the 70s.

The actual family size will depend on the patch they own.