This video from Israel is called Hoopoe Nest (HD) V1 – Part 1 of 2.
And this video is the sequel.
From Wildlife Extra:
The colour of a hoopoe’s eggs may signal the health and strength of the mother bird
Hoopoe females protect their chicks from ‘bad’ bacteria by coating their eggs in ‘good’ bacteria.
The substance that they spread on the eggs with their bills and belly feathers comes from their uropygial, or preen glands. This causes the eggs gradually to change colour when they are incubated, from bluish-grey to a more saturated greenish-brown.
Studies led by Juan J Soler of the Estación Experimental de Zonas Áridas, CSIC in Spain, and published in Springer’s journal Naturwissenschaften – The Science of Nature, have shown that the preen gland secretion of incubating hoopoes is brown in colour and holds antimicrobial properties.
The colour is due to a combination of symbiotic bacteria found in the uropygial gland that provides protection against pathogenic bacteria.
These symbiotic bacteria help to protect the bird’s embryos from trans-shell infections, and are highly effective against Bacillus licheniformis, a well-known feather-degrading bacterium.
The darker the colour of the secretion, the more of the ‘good’ bacteria are present, and therefore the better the protection against ‘bad’ bacteria so a bird’s embryos and feathers stay healthy.
The researchers speculated that the egg colouration might be a way through which the female hoopoe signals to the male that, for future reference, she is good breeding material.
It is thought that the colour informs the male of the abundance of the antimicrobial bacterial community found in the female’s glands – a quality that she will be able to carry over to their offspring should they mate in the future.
Although further experimental work is needed to establish the validity of this signalling hypothesis, Soler hopes that the new results will encourage such research in hoopoes as well as in other birds.
See also here.