Sunlight will darken many woods. Especially lighter colored woodsike birch. You have three things to consider.

1. Sunlight will darken the tannins in the wood, so the wood itself darkens.
2. Sunlight will have an affect on the finish unless you use a fade resistant, u-v resisitant poly finish.
3. Enviromental factors like smoking in the home, fireplaces and general kitchen air quality will all have an affect on cabinet finishes over time.

The most immediate effect is "direct" sunlight on wood. In some woods, it darkens all of the material, or it will lighten parts of it. If some of the wood contains heartwood, it will look dramatically different than surrounding areas.

An example of the suns affect on red oak floors, leave a mat on the floor for a year where the sun makes it way across the floor during the day and you will see a lightening of the dark red color. Your cabs ill be made of birch, but the process is the same.

Thank you Woodchuck for the clear explanation.

All woods age in color. As was said, sunlight will make them darken differently than the ones that get less light. When I had my cherry cabinets put in, I had a natural stain put on them to prevent this. Maybe you can do the same thing.