American goldfinch and prealternate molting

I photographed a small enclave of American goldfinches going about their business and later, in post, noticed that one of them had strange off-colour patterns. "Leucism, hermaphrodite, birth defect?", I wondered.  Then I noticed that others shared the same trait, although with less emphasis.  It was then that I learned of something called prealternate molting.

When a bird molts, it doesn't necessarily lose and replace every feather it possesses.  Many birds only go through a partial molt where body plumage is replaced but the major feathers in the wings and tail are not.  This is called a prealternate molt and tends to happen just prior to breeding.  

The prebasic molt often occurs after breeding and involves birds the shedding and replacement of all feathers.  I was involved in goose banding and egg addling one summer; this was done after the geese had lost their primary flight feathers and could no longer fly.  Given the fact that Canadian geese do not show sexual dimorphism as goldfinches do, they may not even undergo prealternate molting, which is why they lose their primaries even before their eggs hatch.

All birds molt, just not in the same way.  Interesting; I learned something today.

Thanks for reading.

Eric Svendsen     www.ericspix.com