1 for sorrow, 2 for joy, 3 for a girl and 4 for a boy. 5 for silver, 6 for gold, 7 for a story never to be told.
8's a wish and 9 a kiss, 10 is the bird you must not miss
But that was just a lyric composed specifically for the ITV kid's programme Magpie, dreampt up in the 70s as a funky answer to Blue Peter.
this is also known as an augury
all I know of this particular one is
1 for sorrow 2 for myrth 3 for a wedding and 4 for a birth. Keep in mind auguries change just like folk tales the more they are passed on the more they change.
It could be that they are mourning one of there own as they are know to gather to mourn and they talk as if to talk the lost one back to life. The rhyme as it goes is just a myth and the magpie should be wished upon and not feared.
As your title says it is just a myth that the Magpie is an unlucky bird to see, however in answer to your question, Magpies are monogamus if one of the pair dies several magpies will gather in what is believed to be a mourning ceromony they can be heard chattering to each other in some ways it's as if they are trying to talk the lost one back to life.