2 'Polish' Mute swan cygnets (out of 7) - little brown in the plumage and pale bill and legs.
The gene responsible for melanin production is on the Z sex chromosome. A male bird's sex chromosomes are ZZ, while a female is ZW. (Contrast a mammal, where the FEMALE is XX and the male is XY).
A Polish swan does not have a normal melanin-producing gene - only deficient one(s).
These 2 birds are extremely likely to be females, since with only one Z chromosome they only 'need' one faulty gene.
A Polish male would have to inherit a faulty gene from each parent, and since the abnormality is not common, the chance of this happening is small.
Neither parent of these birds looks Polish, so one of the the male's two Z chromosomes must carry the Polish gene. Which he has given to his Polish daughters, while their mother supplied the W that made them female.