Sketchbook pages of chaffinches in the garden

I was not working with any particular objective in mind — simply observing and recording. I became interested in the different colours of the female chaffinches, some buff, others quite grey. There are colour variations elsewhere too, in the yellow of the primaries and rump. What does it all mean? Are the differences related to age, or perhaps to feather wear? And when did you last really look at the chaffinches in your garden? Maybe it's the scientist in me, but I find little details like this and the questions they pose interesting, even though I'm not at all interested in fussy detail in my art.