White-throated Sparrows ~ two colour morphs

     Photo by BarrytheBirder

                                         Photo by Marshall Faintich/Cornell

The birds in these two photos are both White-throated Sparrows.   The sparrow at the top is often referred to as a 'White-striped' and the lower sparrow is referred to as a 'Tan-striped'. The stripes in each case are the stripes that run back from the yellow or gold spots, on the sides of the head, behind the beak.   All of my bird guides mention that there are two morphs (tan and white) but it is only Wikipedia that states when it comes to mating that 'white-stripes' mate with 'tan stripes' and vice versa. I was not aware of the anomalous mate selection these morphs apparently choose.   Maybe my bird guides are so old that this information was not deemed all that important in years gone by.   Maybe new guides do have this information.   Can someone clarify this a bit for me?   The three White-crowned Sparrows in the backyard today were two 'white-striped' birds and one 'tan-striped' bird.   I have no idea which were males or females, or who was mated-up with who.   

Please comment if you wish.

BtheB