Combining colors

[Rosa moschata]

"Halfsiders" show bilateral asymmetry. The bird above is not a halfsider, so far as I can tell. Of course, the only way you could tell -- without taking tissue samples and running DNA tests -- would be if the genetic/chromosomal difference resulted in a visual difference.

 

[Dany12]

More ... !
Half male and half female.

 

[Rosa moschata]

Yes, that's a half-sider. Note that the difference is between left and right, not front and back. Something "happened" when the zygote first divided into two, leaving one cell different from the other. As the cells continued to divide and grow in number, that difference was maintained in all cells descending from the one that was changed, while the other cell's descendants remained unchanged. Because the first division into two cells becomes left and right, that's where the division will be.

There are also "quarter-siders" where the change happened at the 4-cell stage in just one cell, and then continued to develop.

These instances are probably more common than we assume, since the only times it's visual is when the change results in a unique appearance, such as male/female in dimorphic species, or when the difference is a color mutation. There are many other possible mutations that don't affect outward appearance, and a male/female half-sider wouldn't be noticed in a species that is not sexually dimorphic without some sort of lab or vet testing.