Why are these nesting magpies building another nest

Question

Pair if eurasian magpies occupy a nest visible from my balcony. It's the nesting season for them here, and I assumed they have laid eggs. I can't see the eggs directly, but one of the magpies has been regularly sitting inside the nest (at least for last two or three weeks) and they always attack other birds that come too close.

Today, they've started building a new nest on another tree. It's pretty close, the nests are about 20 meters apart. They still visit their original nest.

What does it mean? Is it a sign of nesting failure? Does it mean their original nest is no longer sufficient?

Answer 1

I found an article that explains secondary nests as backups in case of nest usurpation by other birds: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4858914/ This is for black sparrowhawks losing nests to Egyptian geese around Cape Town. They measured number of nests built per breeding pair of sparrowhawks and found it to correlate with number of Egyptian geese present and especially their nest usurpation behaviour.

That article mentions brood parasitism (e.g. cuckoos) as another possibility for other birds, and I've seen references to duplicate nests with eggs in each. ( https://www.tandfonline.com/doi/pdf/10.1071/MU939286a )