In my local park we have large numbers of Canada Geese and a smaller number of Greylag.
Looking at a group of this year's young I saw four adult Canada Geese looking after 13 goslings. Not unusual, I'm aware that pairs of parents will often band together in this way. However on closer inspection it appears that two of the youngsters are actually Greylags: yellow/orange bills and pink/orange legs and with white-tipped tail feathers starting to emerge.
Any suggestions as to how this has happened? Is it an unusual occurrence?