egg discoloration, day 28

[kapster]

Hi I am a teacher and usually hatch chicks I am doing ducks this year and am on day 28, I have had 3 hatch but a couple of the eggs are changing color around the air pocket, is this normal, I am attaching pic to show discoloration. Any advice would be greatly appreciated, thank you

 

[Amiga]

https://www.backyardchickens.com/t/861442/when-to-help-when-to-worry/300

There is also this

https://www.backyardchickens.com/t/491013/goose-incubation-hatching-guide-completed it seems to apply to ducks as well