Please pardon my pedantry. A phase in this context isn't something they enter ( as in "She's going through a phase" ). You can see pale-phase Buzzards, dark-phase, and loads of in between birds - they're notoriously variable, but tend to stay their particular depth of colour. Due to the confusion, there's a trend to use "morph" instead of "phase", but this can lead observers to expect some sort of plasticine figure, apparently. See Wikipedia on
polymorphism for a more articulate and detailed explanation.
I blame the Victorians.