BirdingNZ.net

These two were foraging on a playing field this evening. The one on the grass has a white nape and brown eyes, so an immature male ? The one that flew off and perched on a nearby soccer goal has scalloped brown belly feathers and dark brown wings, so a juvenile sex indeterminate ? They both have a saddle, incomplete in what I think is the older bird.

the one one the grass with a cream bill is adult male, juveniles have a black bill and grey breast. Perched bird looks a female Juvenile based on the grey side portion of the breast feathers i can see, hard to tell with out seeing the bill and breast fully.both may have some hybridization with back backed?

https://www.researchgate.net/figure/Dis ... _267230124

The first one is a female I would say. Males have pure white backs.

Larger White-backed magpies are common thought NZ, but smaller Black-backed magpies lives in east NZ.
They are very very wary & nervous and cannot be approached, expert breeding season when they become aggressive and attack humans and animals in defense of their chicks.

White-backed magpie are slightly bigger than feral pigeons.

Numbers and colors & pattern of immature plumages between first winter plumage and adult plumage, are unknown.
Adult Post breeding moult are complete, for 6 months from October to April, with all black flight feathers replaced once a year.

Not protected, due they had attacked native birds and blackbirds, and humans.

Lifespan: 20 years.

2-3 chicks per nest per year.

The first one on the grass is an adult, given the light blue beak colour. Juvenile's beaks are black. It is also a female given the mottled gray colouring on her back. Adult males are pure white around the nape, often extending through to their back.
The second magpie is unlikely to be an adult with the lighter coloured feathering on the underbelly. Adult's feathers are jet black, with the males carrying a slight blue sheen on their chest area. Magpies moult 4 times before they finally get their adult colouring. It is very difficult to pinpoint whether they are male or female prior to that.
They are mistakenly thought to be "corvids" like crows and NZ rooks, but are actually "artamids" like butcher birds in Aus.

Thanks everyone for your helpful comments. I will accept the first one as male, but not in final adult plumage due to the grey mantle, which makes it look female, but I think the white nape clinches it as male. They were in the same place this morning and a photo of the second bird shows a very pale grey bill with darker tip, so more adult than juvenile and with entirely grey nape and mantle likely to be female. I am concluding they are a young male and female pair, in this location for at least a fortnight.